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K a rtik T h a k o re

With c o n trib u tio n s fro m

https://github.com/PerlGameDev/SDL_Manual/contributors

http://%20https//github.com/PerlGameDev/SDL_Manual/contributors


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L a te x b a s e d o n th e P e r l6 b o o k : H T T P S://G itH ub . C o m/p e r l6 /bo o k


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SDL SDLx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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SDLx::App . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M Qimplut MM usic 9.94a M d09292 M09296 !


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Simple DirectMedia Layer (or libsdl) i s a c r os s- p la t fo rm C l i br a ry t h at p r ov id es a cc es s

t o s ev er al i np ut a nd o ut pu t d ev ic es . I ts m os t p op ul ar u sa ge i s t o p ro vi de a cc es s t o t he

v i de o f r am e bu f fe r a n d i n p u t d e vi c es f o r g a m e s . S D L a l s o h a s s e v e r al e x t en s io n l i b ra r i es t o

p r o v id e f e a tu r e s s u c h a s te x t d is p la y , s o u n d m ix in g , im a g e h a n d lin g , a n d g r a p h ic s e f f e c ts .

S DL P er l b i nd s s ev er a l o f t h es e l i br ar i es t og e th er i n t he SDL::* n a m e s pa c e . o r e o v er ,

S D L P e r l p r o v i d e s s e v e r a l h i g h - l e v e l l i b r a r i e s i n t h e SDLx::* n a m e s pa c e th a t e n c a p su la te

v a lu a b le g a m e - w r itin g a b s tr a c tio n s .


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SDL SDLx

T h e m a i n p u r p o s e o f t h e SDLx::* l a y e r i s t o s m o o t h o u t t h e d r u d g e r y o f u s i n g t h e SDL::*

la y e r d ir e c tly .

D o n’ t w o rr y a b ou t u n de r s ta n di n g t h e d e ta i l s o f t h i s c o d e r i g ht n o w. C o mp a r e t h e

c o m p l e x i t y a n d s i z e o f t h e c o d e l i s t i n g s .

U s in g th e SDL::* l a y e r t o d r a w a b l u e r e c t a n g l e l o o k s s o m e t h i n g l i k e :

1 u s e SD L

2 u s e S D L : :V i de o

3 use SDL::Surface

4 u s e S D L : :R e ct

5

6 # t he s iz e o f t he w in do w b ox o r t he s cr ee n r es ol ut io n i f f ul ls cr ee n

7 m y $ sc re en _w id th = 80 0

8 m y $ sc re en _h ei gh t = 6 0 0

9

10 SDL::init(SDL_INIT_VIDEO)

11

12 # s et ti ng v id eo m od e

13 my $screen_surface = SDL::Video::set_video_mode($screen_width,

14 $screen_height,

15 32,

16 SDL_ANYFORMAT)

17

18 # d ra wi ng a r ec ta ng le w it h t he b lu e c ol or

19 m y $ m a p p ed _ co l o r = S D L: : V id e o :: m ap _ R GB ( $ sc r ee n _ su r f ac e -> f o rm a t () ,

20 0 , 0 , 2 55 )

21 SDL::Video::fill_rect($screen_surface,

22 S D L :: R ec t - >n e w ($ s cr e e n_ w i dt h / 4 , $ s c re e n _h e ig h t / 4 ,

23 $ sc r ee n_ w id th / 2 , $ s cr ee n _h ei gh t / 2 ) ,

24 $mapped_color)


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25

26 # u pd at e a n a re a o n t he s cr ee n s o i t' s v is ib le

27 SDL::Video::update_rect($screen_surface, 0, 0,

28 $screen_width, $screen_height)

29

30 # just t o h av e t ime to se e i t

31 sleep(5)

. . . w h il e d r a w i ng a b l ue r e c ta n g le i n t h e SDLx::* l a y e r i s a s s i m p l e a s :

1 u s e s t ri c t

2 u s e w a r ni n gs

3

4 u s e SD L

5 u s e S D L x :: A pp

6

7 m y $ ap p = S DL x: :A pp -> ne w( w id th => 8 00 , h ei gh t = > 6 00 )

8

9 $ a pp -> dr a w_ re c t( [ $ a pp -> w id th / 4 , $ a pp -> h ei gh t / 4 ,

10 $ ap p- >w id th / 2 , $ ap p- >h ei gh t / 2 , ] ,

11 [ 0, 0 , 255, 255] )

12

13 $app->update()

14

15 sleep(5)

The SDLx::* m od u l es a l s o p r o v i de a n d m a na g e h i g he r -l e v el c o nc e rn s f o r u s e rs , s u c h a s

l a y e r s a n d g a m e l o o p s .


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T h is b o o k h a s a t w o- f o ld p u rp o se : f i r st , t o i n t ro d uc e g a me d e v el o pm e nt t o P e rl p r og r a m-

m e rs , a n d s e c o n d, t o i n t ro d uc e o d er n P e rl c o nc e p ts t h r ou g h g a m e d e ve l o pm e nt . W h i le

t he e xa mp le s a s su me s om e e x pe ri e nc e w it h P er l , n o e xp er i en ce w it h S DL i n P er l o r a s

libsdl its e lf is n e c e s s a r y .

T h e b o o k p r e s e n t s a p r o g r e s s i o n f r o m s i m p l e t o i n t e r m e d i a t e e x a m p l e s a n d p r o v i d e s s u g -

g e st i o ns f o r m o re a d va n c ed e n de a vo r s . T h e c h a p t er s o f t h i s b o o k i n c re a s e p r o g r es s i ve l y

i n c o m p l e x i t y , b u t e a c h c h a p t e r h a s a s i n g u l a r g o a l ( s u c h a s c h a p t e r f i v e ’ s Making Pong)

w h ic h s t a n d s a l o n e a s a n i n di v i du a l t u t o r ia l . S o ur c es a n d d a t a f i l e s a r e a l l a v a il a b le f r om

http://sdl.perl.org/.

W e a s s u m e t h e p r e s e n c e o f a r e c e n t v e r s i o n o f t h e P e r l l a n g u a g e ( a t l e a s t P e r l 5 . 1 0 ) a n d s u p -

p o rt i n g p a c k ag e s . We a l s o a s s u m e t h a t y o u c a n i n s ta l l p a c ka g es f r o m t h e C PA N , i n c l u di n g

S D L P e r l i t s e l f .

Alien::SDL w il l i ns ta ll b in ar ie s f or 3 2b it a nd 6 4b it s o t he re i s n o n ee d t o c om pi le a ny -

thing.

http://sdl.perl.org/


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F i nk h a s p a c k a ge s f o r S D L P e rl a v ai l a bl e . H o we v er , t h e y d o n o t s u p p o rt P a n go , a l i b ra r y

w h ic h p r o v id e s in te r n a liz a tio n s u p p o r t f o r te x t h a n d lin g .

Installing Alien::SDL f r o m th e C P A N w ill c o m p ile S D L a n d its d e p e n d e n c ie s , p r o v id e d y o u

h a v e in s ta lle d s e v e r a n n e c e s s a ry d e p e n d e n c ie s. We r e c o m m e n d th a t y o u in s ta ll libfreetype6,

libX11, libvorbis, libogg, libpng, a n d t h e i r h e a d e r s .

o s t c u r r e n t G N U / L i n u x d i s t r i b u t i o n s i n c l u d e a l l t h e p a r t s n e e d e d f o r t h i s t u t o r i a l i n t h e

d e fa u l t i n s t a ll a n d i n t h e ir p a ck a g e m a n a ge m en t s y st e m . I t i s a l s o a l w a ys p o ss i b le t o i n st a l l

o n G NU /L in ux u si n g t he a va i la bl e o pe n s ou r ce c od e f ro m t he p r op er r ep o si t or i es . T he

Alien::SDL p e rl m o du l e a u to m at e s m u ch o f d o wn l o ad i ng , c o mp i l in g , a n d i n s ta l l in g t h e

n e e d e d lib r a r ie s .

Yo u c a n p r ob a bl y u s e y o ur d i s tr i b ut i o n’s p a ck a ge s . O n U b un t u a n d D e bi a n t r y :

$ s u do a p t -g e t i n st a l l l i b sd l - ne t 1. 2 - de v l i b sd l - mi x er 1 . 2- d e v \libsdl1.2-dev libsdl-image1.2-dev libsdl-ttf2.0-dev \libsdl-gfx1.2-dev libsdl-pango-dev

T o c o m p i l e f r o m s c r a t c h , y o u m u s t i n s t a l l a c o m p i l e r , s y s t e m h e a d e r p a c k a g e s , a n d s o m e

lib r a r ie s a r e r e q u ir e d .

$ s u do a p t -g e t i n st a l l b u i ld - e ss e nt i a l x o r g- d ev l i bx 1 1 -d e v l i bx v - de v \libpango1.0-dev libfreetype6-dev libvorbis-dev libpng12-dev \libogg-dev


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B e f o r e i n s t a l l i n g S D L P e r l , e n s u r e t h a t y o u h a v e t h e m o s t r e c e n t v e r s i o n s o f t h e m o d u l e s

n e c e s s a r y t o b u i l d S D L :

$ s ud o c pa n C PA N

$ s ud o c p an Y A ML M o du le :: B ui ld

A f t e r t h e s e t w o s t e p s C P A N w i l l b e a b l e t o i n s t a l l S D L :

$ s ud o c pa n S DL

F o r m o s t p l a tf o r ms a C PA N i n s t a l l w i l l s u ff i c e. S u pp o r te d a n d t e s t e d p l a t f or m s a r e l i s te d

at http://pass.cpantesters.org/distro/S/SDL.html.

H o pe f u ll y t h i s b o o k a n sw e rs m o st o f y o ur q u es t i on s . F o r a d di t i on a l a s s is t a nc e , c o n ta c t t h e

p r o je c t v ia :

• the web, b y v i s i t i n g t h e S D L P e r l h o m e p a g e a t http://sdl.perl.org/.

• IRC , i n t h e #sdl c h a n n e l o n irc.perl.org. T h is i s a v e ry a c ti v e a n d h e l p fu l r e s o u rc e .

• email, t h r o u g h t h e [email protected] m a ilin g lis t.

http://sdl.perl.org/http://pass.cpantesters.org/distro/S/SDL.html


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T h e c o d e e x a m p l e s i n t h i s b o o k a r e a v a i l a b l e f r o m https://github.com/PerlGameDev/SDL_

Manual/tree/master/code_listings.

T h a n k s t o c o n t r i b u t o r s a n d r e v i e w e r s f r o m t h e #sdl c h a n n e l, in c lu d in g :

Alias

bobross

Blaizer

cfedde

chromatic

FROGGS

garu

jamesw

perlpilot

PerlJam

Pip

waxhead

https://github.com/PerlGameDev/SDL_Manual/tree/master/code_listingshttps://github.com/PerlGameDev/SDL_Manual/tree/master/code_listings


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a n d m a n y m o r e

( Ap ol og ie s i f I h av e m is se d y ou l et m e k no w a nd I w il l a dd y ou .)


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S D L’s p r im a ry p u r po s e i s t o d i s pl a y g r a p h ic s . I t d o es s o b y p r ov i d in g a n a b st r a ct i o n c a l l e d

a screen, w h i c h r e p r e s e n t s a video device. T hi s v i de o d e v ic e i s a n i n t e rf ac e p ro v id ed b y

y o u r o p e r a t i n g s y s t e m , s u c h a s X 1 1 o r D i r e c t X . B e f o r e y o u c a n d i s p l a y a n y t h i n g , y o u m u s t

c r ea t e a s c r ee n . T h e SDLx::App c l a s s d o e s s o f o r y o u :

1 u s e s t ri c t


3 u s e SD L


5

6 m y $ a pp = S D L x: : Ap p - >n e w( )

7

8 s le ep ( 2 )

T h is e x am p le c a u se s a n e m pt y w i n d o w t o a p p ea r o n t h e d e s k to p . o st s y s te m s w i l l f i l l t h at

w i nd o w w i t h t h e c o lo r b l a ck . O t he r s y s te m s m i g h t d i s pl a y a t r a ns p a re n t w i nd o w. S D L’s


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d ef au l t b eh av i or i s t o f i ll t he s cr ee n w i th b l ac k. To e n fo rc e t h is b eh av io r o n a ll s ys t em s,

y o u m u s t update() t h e a p p t o d r a w t o t h e w i n d o w :

$app->update()

SDLx::App

SDLx::App a l lo w s y o u t o s p ec i f y s e v e ra l o p t io n s f o r t h e s c r e e n a n d y o ur a p pl i c at i o n. F i r st

a re t he p hy si ca l d im en si on s o f t he s cr ee n i ts el f. To m ak e t he s cr ee n o f t he SDLx::App

w i n d o w a 4 0 0 × 4 0 0 p i x e l s q u a r e , c h a n g e t h e i n i t i a l i z a t i o n l i n e t o :

m y $ ap p = S DL x: :A pp -> ne w( w id th = > 4 00 , h ei gh t = > 4 00 )

A n ot h e r i m p o rt a nt o p ti o n i s t h e w i n d o w’s t i t le . S o me s y st e ms d i sp l a y t h e p a t h t o t h e r u n -

n i n g p r o g ra m . O t he r s l e a v e t h e t i t le b l a nk . Yo u c a n c h a ng e t h e d i sp l a ye d t i t le w i t h a n o t he r

a r g u m e n t t o t h e SDLx::App constructor:

m y $ ap p = S DL x: :A pp -> ne w( w id th = > 4 00 ,

h ei g ht = > 4 00 ,

tit le => 'Pong - A clone' )

A t t h i s p o i n t y o u r s c r e e n w i l l b e :

A b br e v ia t i on s f o r t h es e p a r am e te r s a r e a v ai l a bl e . I n st e a d o f width, height, a n d title, y o u

m a y u s e w, h, a n d t r e s p ec t i ve l y. T h e p r e vi o us e x am p le c o ul d a l s o b e w r i tt e n :

m y $ ap p = S DL x: :A pp -> ne w( w = > 4 00 ,

h => 400,

t => 'Pon g - A clone ' )


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F i g u r e 2 . 1 : Y o u r f i r s t S D L s c r e e n !


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S D L p r o v i d e s s e v e r a l w a y s t o d r a w g r a p h i c a l e l e m e n t s o n t h e s c r e e n i n t h r e e g e n e r a l c a t -

e go ri e s: p r im it i ve s , i ma ge s, a nd t e xt . A ll d r aw in g o cc ur s o n a s u rf a ce , r ep r es en t ed b y

the SDLx::Surface c l as s . E v en t h e SDLx::App i s a n SDLx::Surface. T h ou g h t h i s m e a n s i t ’sp o ss i b le t o d r a w d i r e ct l y t o t h e a p p ’s s u r fa c e, t h er e a r e s e ve r a l a d v a nt a g es t o d r a wi n g o n

m u ltip le s u r f a c e s .

S D L’s s u r fa c e c o or d i na t e s y s t e m h a s i t s o r i gi n ( w he r e b o th t h e x a n d y c o o rd i n at e s h a v e

t he v a lu e o f z e ro ) i n t h e u p p er l e ft c or n er. A s t he v al u e o f x i n cr ea s es , t he p o si t io n m ov e s

t o t h e r i g ht o f t h e o r i gi n . A s t he v a lu e o f y i nc r ea se s , t h e p os it i on m ov es d ow nw ar d f r om

t h e o r i g i n. T h e A P I a l wa y s l i s t s c o o r d in a te s i n x , y o r de r.


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T h e S D L l i b r a r y d o c u m e n t a t i o n h a s a n e x t e n d e d d i s c u s s i o n o n c o o r d i n a t e s : http:

//sdltutorials.com/sdl-coordinates-and-blitting.

Y o u c a n p r o d u c e o r i g i n a l p i c t u r e s k n o w i n g l i t t l e m o r e t h a n h o w t o d r a w t o a s u r f a c e w i t h

SDL:

F i g u r e 3 . 1 : A f i e l d o f f l o w e r s

http://sdltutorials.com/sdl-coordinates-and-blittinghttp://sdltutorials.com/sdl-coordinates-and-blitting


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A s m en ti o ne d e a rl i er, a l l d ra wi n g i n S DL r e qu ir e s a s u rf ac e . T he SDLx::Surface object

p r o v i d e s a c c e s s t o m e t h o d s i n t h e f o r m o f :

$surface->draw_{something}( .... )

P a r a m e t e r s t o t h e s e m e t h o d s a r e g e n e r a l l y c o o r d i n a t e s a n d c o l o r s , p r o v i d e d a s a r r a y r e f e r -

ences.

S o m e p a r a m e ter s a r e s e ts o f c o o r d in a te p o s itio n s a n d d im e n s io ns . F o r e x a m p le , p a r a m e ter s

t o d e s c r i b e a r e c t a n g l e o f 40x40 p i x e l s p l a c e d a t (2 0, 2 0) p i x e l u n i t s o n t h e s c r e e n m a k e a

f o u r - e l e m e n t a r r a y r e f e r e n c e o f x , y , w i d t h , h e i g h t :

m y $ re ct = [ 20 , 2 0, 4 0, 4 0]

S D L c o lo r p a r a m e te r s r e q u ir e f o u r- e le m e nt a r r a y r e f e r e n c e s . T h e f ir s t th r e e n u m b e r s d e f in e

t h e R e d , G r ee n , a n d B l ue i n t en s i ty o f t h e c o l o r. T h e f i n a l n u mb e r d e f i n es t h e t r a n s pa r e nc y

o f t h e c o l o r .

m y $ co lo r = [ 25 5, 2 55 , 2 55 , 2 55 ]

T h e m a g n i t u d e o f e a c h c o l o r v a l u e d e t e r m i n e s h o w m u c h o f t h a t c o l o r c o m p o n e n t w i l l b e

m i x ed i n t o t h e r e s u l t in g c o l o r. A 0 v a l ue s p ec i f ie s t h a t n o ne o f t h e c o l or c h an n el s h o ul d b e

u s e d w h i l e 2 5 5 s p ec i f ie s a m a xi m um i n t en s i ty f o r a p a r ti c u la r c h an n el . T h e f i r s t v a l ue c o r-

r e s p o n d s w i t h t h e R e d c h a n n e l , s o a h i g h e r n u m b e r t h e r e m e a n s m o r e r e d w i l l b e m i x e d i n t o

t h e r e s u l ti n g c o l o r. I t i s a c o mm o n p r a c t ic e t o a c hi e v e a g r ay s ca l e o f v a r yi n g i n t e n si t y b y


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s p ec i f yi n g t h e s a me v a lu e f o r e a ch o f t h e R e d, G r ee n , a n d B l ue c o l or c h an n el s . T h e f o u r th

a n d f i n a l v a l u e d e s i g n a t e s t h e t r a n s p a r e n c y ( o r A l p h a c h a n n e l ) w h e r e a 0 v a l u e m a k e s t h e

r e su l t in g c o lo r f u l ly t r a ns p a re n t a n d 2 5 5 m a ke s i t e n ti r e ly o p aq u e. A t r a ns p ar e n cy v a l ues o m e w h e r e i n b e t w e e n w i l l a l l o w u n d e r l y i n g ( p i x e l d a t a o f s u r f a c e s b e l o w t h e c u r r e n t o n e )

c o l o r s t o b e b l e n d e d w i t h t h e s p e c i f i e d R G B v a l u e s i n t o t h e f i n a l c o l o r o u t p u t .

Yo u m ay a l so r e pr es en t a c ol o r a s h ex ad ec i ma l v al u es , w he re t he v al u es o f t he n um be rs

r a n g e f r o m 0 - 2 5 5 f o r 3 2 b i t d e p t h i n R G B A f o r m a t :

m y $ co lo r = 0 x FF FF F FF F

m y $ wh it e = 0 x FF FF F FF F

m y $ bl ac k = 0 x 00 00 0 0F F

m y $ re d = 0x FF 00 00 FF

m y $ gr ee n = 0 x 00 FF 0 0F F

m y $ b lu e = 0 x0 00 0F FF F

. . . o r a s f o ur - by t e h e x a d ec i m al v a l ue s , w h e r e e a c h t w o- d i gi t b y te e n c od e s t h e s a me R G BA

values:

m y $ g ol d e nr o d = 0 x D AA 5 20 F F

N O T E : D e p t h o f S u r f a c e

T h e c o l o r d e p t h o f t h e s u r f a c e – h o w m a n y b i t s a r e a v a i l a b l e t o d e s c r i b e c o l o r s – i s a

p r o p e r t y o f t h e r e l e v a n t SDLx::Surface or SDLx::App. S et i t i n i t s c on st r uc t or :

m y $ ap p = S DL x: :A pp -> ne w( d ep th = > 3 2 )

T h e d e f a u l t b i t d e p t h i s 3 2 , s u c h t h a t e a c h c o l o r c o m p o n e n t h a s 2 5 6 p o s s i b l e v a l u e s .

O t h e r o p t i o n s a r e 2 4 , 1 6 , a n d 8 .


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All SDLx::Surfaces a r e c o l le ct i on s o f p i xe ls . Yo u c an r ea d f r om a n d w r i te t o t h es e p i xe ls

b y t r e a t i n g t h e s u r f a c e a s a n a r r a y r e f e r e n c e :

$ a p p- > [$ x ] [$ y ] = $ c o lo r

. .. w he r e $color i s a n u n s i g n e d i n t e g e r v a l u e u s i n g t h e h e x a d e c i m a l f o r m a t ( 0xRRGGBBAA)

or a n a n o n y m o u s a r r a y o f t h e f o r m [ $ re d , $ g re e n, $ b lu e , $ a lp h a].

D r a w i n g p r i m i t i v e s a r e s i m p l e s h a p e s t h a t S D L s u p p o r t s n a t i v e l y .

F i g u r e 3 . 2 : D r a w i n g a l i n e


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A r e c ta n gl e i s a f o u r- s i de d , f i l l e d p o l y go n . R e ct a ng l e s a r e a c o mm o n b u i l di n g b l o c k f o r

g am es . I n S DL , r ec ta ng le s a re t he m os t c os t e ff ec ti ve o f t he p ri mi ti ve s t o d ra w. T he

d r a w r e c t m e t h o d d r a w s a r e c t a n g l e o n a s u r f a c e :

$ a pp -> dr a w_ re c t( [ 1 0, 2 0, 4 0, 4 0 ] , [ 25 5, 2 55 , 2 55 ,2 5 5] )

T h i s d r a w s a w h i t e s q u a r e o f s i z e 40x40 o n t o t h e s c r e e n a t t h e p o s i t i o n (10,20).

F i g u r e 3 . 4 : D r a w i n g a C i r c l e


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F i g u r e 3 . 5 : D r a w i n g a f i l l e d C i r c l e

A c i r c l e i s a p r i mi t i ve a f i x ed r a d iu s a r o u n d a g i ve n p o i n t . C i r cl e s m a y b e f i l le d o r u n fi l l ed .The d r a w c i r cl e and d r a w c i r c l e f i l l ed m e t h o d s d r a w t h e s e t o a s u r f a c e :

$app->draw_circle( [100, 100], 20, [255, 0, 0, 255] )

$ ap p- >d ra w_ ci rc le _f il le d( [ 10 0, 1 00 ], 1 9, [ 0, 0 , 2 55 , 2 55 ] )

T h e s e d r a w a n u n f i l l e d r e d c i r c l e a n d a f i l l e d b l u e c i r c l e .

S D L p r o v i d e s m o r e c o m p l e x p r i m i t i v e s i n SDL::GFX::Primitives.


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I t ’ s e a s y t o c o m b i n e s e v e r a l p r i m i t i v e s t o d r a w a n i n t e r e s t i n g i m a g e s .

1 u s e s t ri c t


3 u s e SD L


5

6 m y $ ap p = S DL x :: Ap p -> ne w(

7 w => 500,

8 h => 500,

9 d => 32,

10 t it l e = > ' Pr et t y F lo w er s'

11 )

12

13 # A dd t he b lu e s ki es

14 $a pp->dr aw_re ct( [ 0, 0 , 500 , 500 ], [ 20, 5 0, 1 70 , 255 ] )

15

16 # D ra w a g re en f ie ld

17 $ ap p- >d ra w_ re ct ( [ 0 , 4 00 , 5 0 0, 1 00 ] , [ 5 0, 1 70 , 2 0, 1 00 ] )

18

19 # M ak e a s ur fa ce f or t he f lo we r

20 m y $ fl ow er = S DL x: :S ur fa ce -> ne w( w id th = > 5 0, h ei gh t = > 1 00 ) 21

22 # W it h a b la ck b ac kg ro un d

23 $flower->draw_rect( [ 0, 0, 50, 100 ], [ 0, 0, 0, 0 ] )

24

25 # D ra w a p re tt y g r ee n s t em

26 $ fl ow er -> dr aw _r ec t( [ 2 3, 3 0, 4 , 1 00 ] , [ 0 , 2 55 , 0 , 2 55 ] )

27

28 # A nd a s im pl e f l ow er b ud

29 $ fl ow er -> dr aw _c ir cl e_ fi ll ed ( [ 2 5, 2 5 ] , 1 0, [ 1 50 , 0 , 0 , 2 55 ] )

30 $flower->draw_circle( [ 25, 25 ], 10, [ 255, 0, 0, 255 ] )

31

32 # D ra w f l ow er o n $ a pp

33 $ fl ow er -> bl it ( $ ap p, [ 0 , 0 , 5 0, 1 00 ] )


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34

35 $app->update()

36

37 sleep(1)

F i g u r e 3 . 6 : L o o k s s o l o n e l y t h e r e a l l a l o n e

T he e xa mp le s s o f ar h av e d r aw n o n o nl y a s in gl e s u rf ac e , t h e d i sp l ay. S DL m ak es i t p os -

s ib l e t o w ri t e o n m ul t ip l e s ur f ac es . T he s e o th er s u rf ac e s e xi st o nl y i n m em or y u nt i l y ou

d r a w t h e m t o t h e d i s p l a y .


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T h e r e a r e s e v e r a l w a y s t o c r e a t e a n SDLx::Surface f o r u s e . T he m os t c om mo n i s t o c r e at e

o n e m a n u a l l y w i t h a c o n s t r u c t o r c a l l :

$ s ur fa ce = S D Lx :: Su r fa ce - >n ew ( w id th = > $ w id th , h ei gh t = > $ h ei gh t )

SDL::Image and SDL::Video c a n l o a d i m a g e s a s s u r f a c e s t o o . SDL::Image p r o v id e s s u p p o r t

f o r a l l t y p e s o f i m a g e s , p r o v i d e d t h a t t h e u n d e r l y i n g S D L i m a ge l i b r a r y s u p p o r t s t h e i m a g e

t yp e y ou w an t t o l o ad . F or e xa mp le , S D L i m a ge m u s t s u p p o r t P N G i m a g e s t o u s e :

$ s u rf a ce = S D L :: I ma g e :: l o ad ( ' p i ct u r e. p ng ' )

I n t h e e v e n t t h a t t h e d e s i r e d S D L i m a ge l i b r a r y i s u n a v a i l a b l e , y o u c a n f a l l b a c k t o t h e b u i l t - i n

s u p p o r t f o r t h e .bmp format.

$ s u rf a ce = S D L :: V id e o :: l o ad _ BM P ( ' p i ct u re . b mp ' )

The SDLx::Sprite m o d u l e p r o v i d e s a n o t h e r o p t i o n t o m a n i p u l a t e s u r f a c e s .

T he f l ow er e x am pl e u s ed a m et ho d c al l ed blit to d ra w a s ur fa ce t o t he d is pl ay. T hi s

m et h od c op ie s d at a f ro m o ne s ur f ac e t o a no th e r. I t ’s a f un da me nt a l o pe r at i on , b ut i t ’s a

l o w l e v el o p er a t io n . SDLx::Sprite p r ov i d es h i gh e r l e ve l o p t io n s . B e si d es m a ki n g d r a w-

in g s im p le r , SDLx::Sprite a d ds s e ve r a l o t h e r f e at u r es u s ef u l f o r m o vi n g i m ag e s . H e r e’s a

r e v is e d e x a m p le u s in g SDLx::Sprite f o r f lo w e r s :

1 u s e s t ri c t


3 u s e SD L



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5 use SDLx::Sprite

6


8 w => 500,

9 h => 500,

10 d => 32,

11 t it l e = > ' Pr et t y F lo w er s'

12 )

13

14 # A dd in g b lu e s ki es

15 $a pp->dr aw_re ct( [ 0, 0 , 5 00 , 5 00 ], [ 20, 50 , 1 70, 255 ] )

16

17 # D ra w a g re en f ie ld

18 $ ap p- >d ra w_ re ct ( [ 0 , 4 00 , 5 00 , 1 00 ] , [ 5 0, 1 70 , 2 0, 1 00 ] ) 19

20 m y $ fl ow er = S DL x: :S pr it e- >n ew ( w id th = > 5 0, h ei gh t = > 1 00 )

21

22 # U se - > su rf a ce () t o a cc e ss a s pr it e 's S DL x :: Su r fa ce

23

24 # M ak e t h e b a ck gr ou n d b l a ck

25 $flower->surface->draw_rect( [ 0, 0, 5 0, 1 00 ] , [ 0, 0, 0 , 0 ] )

26

27 # N ow f or a p re tt y g r ee n s t em

28 $ fl ow er -> su rf ac e- >d ra w_ re ct ( [ 2 3, 3 0, 4 , 1 00 ] , [ 0 , 2 55 , 0 , 2 55 ] )

29

30 # A dd t he s im pl e f l ow er b ud

31 $ fl ow er -> su rf ac e- >d ra w_ ci rc le _f il le d( [ 2 5, 2 5 ] , 1 0, [ 1 50 , 0 , 0 , 2 55 ] )

32 $f lower- >surf ace->d raw_ci rcle( [ 25, 25 ], 10 , [ 255, 0, 0, 25 5 ] )

33

34 $ f lo we r- > dr aw _ xy ( $ a pp , 0 , 0 )

35

36 $app->update()

37

38 sleep(1)

F l o we r s u s u a ll y d o n ’ t g r o w i n t h e s k y. F l ow e rs m a ke m o re s e ns e o n t h e g r o u nd . I t ’s e a s y

t o i n s er t p l en t y o f i d e nt i c al f l o we r s f r o m a s i ng l e s p r i t e. R e pl a c e t h e l i n e:


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$ f lo we r- > dr aw _ xy ( $ a pp , 0 , 0 )

. . . w i t h:

1 for (0 .. 500) {

2 my $y = 425 - rand( 50)

3 $ fl o we r- > dr aw _x y ( $a p p, r a nd (5 00 ) - 2 0 , $y )

4 }

. . . t o m ak e a n e nt i re f i el d o f f l ow er s.


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T h e c o r n er s t on e o f a n S D L a p pl i c at i o n i s e v en t h a n d l i ng . T h e u s e r p r e s s es a k e y o r m o ve s

t h e m o u s e. T h e o p e r at i n g s y s t em s w i tc h es t h e f o c u s o f t h e a c t i v e w i n do w. T h e u s e r s e l e c ts

t h e q u i t o p t io n f r om t he m en u o r t he o p er at i ng s y st em . T he s e a r e a ll e ve nt s . H ow d o y ou

h a n d le th e m ?

S DL p ro vi d es a n e ve n t q ue ue w hi ch h o ld s a l l e ve n ts t h at o c cu r u nt i l t h ey a re r em ov ed .

E v er y t i m e a n e v en t o c c u r s, S D L p l a ce s i t i n t o t h e q u e u e. T h e SDL::Event o b je c t r e p r e s e n ts

t h i s q u e u e i n P e r l , a l l o w i n g y o u t o a d d a n d r e m o v e e v e n t s c o n s t a n t l y :

1 u s e s t ri c t

2 u s e w a rn i n gs

3 u se S DL

4 use SDL::Event

5 use SDL::Events

6 u s e S D Lx : : Ap p

7


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8 my $app = SDL x::App ->new ( w => 200, h => 200 )

9 m y $ e v en t = S D L :: E ve n t -> n e w( )

10

11 my $quit = 0

12

13 w hi l e ( !$ q ui t) {

14 # U pd at es t he q ue ue t o r ec en t ev en ts

15 SDL::Events::pump_events()

16

17 # p ro ce s s a l l a va i la bl e e ve nt s

18 w h i le ( S D L: : E ve n t s: : po l l _e v e nt ( $e v e nt ) ) {

19

20 # c he ck b y E ve nt t yp e

21 d o _k e y () i f $ e ve n t -> t y pe = = S D L_ K E YD O W N22 }

23 }

24

25 sub do_ke y { $q uit = 1 }

E v er y e v en t h a s a n a s so c i at e d t y p e w h ic h r e p r e s en t s t h e c a te g or y o f t h e e v e n t. T h e p r e v io u s

e x a m p l e l o o k s f o r a k e y p r e s s e v e n t 1 . T h e S D L l i b r a ry d e f in e s s e v e ra l t y p e s o f e v en t s , a n d

S D L p e rl m a ke s t h e m a v a il a b le a s c o n s t a nt s w i t h n a me s s u ch a s S D L K E Y DO W N and S D L Q U IT.

See p e rl d o c S D L :: E v e nt s f o r a l i s t o f a l l e v e n t t y p e s .

C h ec k i ng f o r e v e r y p o s s i bl e e v e n t t y p e w i th i n t h a t e v en t l o o p c a n b e t e d io u s . T h e SDLx::-Controller a v a il a b le f r o m t h e SDLx::App o ff e rs t he u s e o f e ve nt c al l ba ck s w it h w hi ch t o

h a nd le e v en ts . P r oc es si n g e ve nt s i s a m at t er o f s e tt i ng u p t he a pp r op ri a te c al l ba ck s a nd

l e t t i n g S D L d o t h e h e a v y w o r k .

SDL Events Types

A d d i t i o n a l E v e n t t y p e s t h a t c a n b e c a p t u r e d b y S D L a r e :

Keyboard

1 S D L s e p a r a t e s t h e e v e n t o f p r e s s i n g a k e y f r o m t h e e v e n t o f r e l e a s i n g a k e y , w h i c h a l l o w s y o u t o i d e n t i f y

c o m b i n a t i o n s o f k e y p r e s s e s , s u c h a s C t r l + P t o p r i n t .


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S D L K E YD O WN S D L K E YU P - K e y b o a r d b u t t o n p r e s s e d

ouse

S D L M O U S EM O T I ON - o u s e m o t i o n o c c u r e d

S D L M O U S EB U T T ON D O WN S D L M O U SE B U T TO N U P - o u s e b u t t o n p r e s s e d

Joystick

SDL JOYAXISMOTION - J o y s t i c k a x i s m o t i o n

SDL JOYBALLMOTION - J o y s t i c k t r a c k b a l l m o t i o n

S D L J O Y H AT M O T IO N - J o y s t i c k h a t p o s i t i o n c h a n g e

S D L J O Y B UT T O N DO W N S D L J O Y B UT T O NU P - J o y s t i c k b u t t o n p r e s s e d

W i n d o w & S y s t e m

S D L A C T I VE E V E NT - A p p lic a tio n v is ib ility

S D L V I D E OR E S I ZE - W i n d o w r e s i z e d

S D L V I D E OE X P O SE - W in d o w e x p o s e d

S D L Q U IT - Q u i t r e q u e s t e d

S D L U S E R EV E N T - A u s e r - d e f i n e d e v e n t t y p e

S D L S Y S W ME V E N T - P la tf o r m - d e p e n d e n t w in d o w m a n a g e r e v e n t

F o r m o r e i n f o r m a t i o n l o o k a t :


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perldoc SDL::Event

T he e xa mp l e a pp li c at i on s s o f a r h av e n ot e x it e d c le an l y. H an dl i ng q ui t e ve nt s i s m uc h

better:

1 u s e s t ri c t


3 u s e SD L

4 u s e S D L : :E v en t


6


8 w => 200,

9 h => 200,

10 d => 32,

11 t it l e = > " Qu it E ve nt s "

12 )13

14 $app->add_event_handler( \& q ui t _ ev e n t ) 15 $app->run()

16

17 s u b q u i t _e v en t

18 {

19 # t h e c a ll ba ck r e c ei v es t h e a p pr o pr ia t e S DL : :E ve n t

20 m y $ ev en t = s hi ft

21

22 # .. . a s w el l a s t he ca ll in g S DL x: :C on tr ol le r

23 m y $ c on tr ol l er = s hi ft

24

25 # s t op pi n g th e c on tr o ll er w i ll e x it $ a pp -> r un () f o r u s


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26 $ c on t r ol l er - > st o p i f $ e v en t - >t y pe = = S D L _Q U IT

27 }

SDLx::App c al l s t h e e v e nt h a nd l e rs , f r om a n i n t er n al SDLx::Controller. W he n t h is e ve nt

h a n dl e r r e c e i ve s a q u it e v en t , i t c a l ls SDLx::Controller::stop() w h ic h c a u s e s SDLx::App

to e x it g r a c e f u lly .

E x it i n g o n r e c ei v i ng t h e S D L Q U I T e v e n t i s s u c h a c o m m o n o p e r a t i o n t h a t SDLx::App pro-

v i d e s i t a s a c o n s t r u c t o r o p t i o n :

1 u s e s t ri c t


3 u s e SD L


5


7 w => 200,

8 h => 200,

9 d => 32,

10 title => "Quit Events",

11 e xi t _o n_ q ui t = > 1

12 )

13

14 $app->run()


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S DL e ve nt s a ls o a ll o w i n p ut h an dl i ng . C on si d er a s i mp l e p ai n t p r og ra m. I t w il l p ro vi d e

a s ma l l b la ck w i nd ow. ov i ng t h e m o u se d ra ws o n t h is w in do w. P r es si n g a n um be r k e y

c h oo s es a p a i nt c o l or. P r es s i ng q or Q e x i ts t h e p r o g r am . P r es s i ng c or C c l e a r s t h e s c r e e n .

Pressing Ctrl-S s a v e s t h e i m a g e t o a f i l e n a m e d painted.bmp.

F i gu r e 4 . 1: S i mp l e P a in t : S m il e

S t a r t b y d e f i n i n g t h e s a v i n g f u n c t i o n :

1 s u b s a ve _ im ag e {

2 i f ( S DL :: V id eo : :s av e_ B MP ( $ a pp , ' p ai nt e d. bm p ' ) = = 0

3 & & - e ' p a in t ed . b mp ' )


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4 {

5 w ar n ' Sa ve d p ai nt ed .b mp t o ' . c wd ()

6 }

7 else

8 {

9 w ar n ' Co ul d n ot sa ve pa in te d .b mp : ' . S D L: :g e t_ er r or s( )

10 }

11 }

K e y b o a r d h a n d l i n g r e q u i r e s s o m e c o l o r d a t a a s w e l l a s a k e y p r e s s c a l l b a c k :

1 m y $ br us h_ co lo r = 0

2

3 sub keyboard_event

4 {

5 m y $ ev en t = s hi ft

6

7 i f ( $ ev e nt -> ty p e = = SD L_ K EY DO WN )

8 {

9 # c on v er t t h e k e y_ sy mb o l ( in t eg er ) t o a k ey na m e

10 m y $ k e y_ n am e = S D L :: E v en t s: : g et _ k ey _ na m e ( $ e v en t -> k e y_ s y m ) 11

12 # if $key_n ame is a di git, use it as a color

13 $ br us h _c ol o r = $ k ey _n a me i f $ ke y_ n am e = ~ / ^\d$/14

15 # g et th e k e yb oa r d m od i fi er (s ee pe rl do c S DL : :E ve n ts )

16 my $mod_state = SDL::Events::get_mod_state()

17

18 # we ar e u si ng an y C TR L s o K MO D_ CT RL is fi ne

19 s av e_ i ma ge ( ) i f $k ey _n a me =~ / ŝ $ / & & ( $ mo d_ s ta te & KM OD _ CT RL )

20

21 # c le ar t he s cr ee n

22 $ ap p- >d ra w_ re ct ( [ 0, 0, $a pp -> w, $a pp -> h ] , 0 )

23 i f $ ke y _n am e = ~ / ^ c$ /


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24

25 # e xi t

26 $ ap p- > st op ( ) i f $ ke y_ n am e = ~ / ^q $ /

27 }

28

29 $app->update()

30 }

31

32 $app->add_event_handler(\&quit_event)33 $app->add_event_handler(\&keyboard_event)

NOTE: W h e n a d d i n g a c a l l b a c k t o SDLx::App w h i c h u s e s v a r i a b l e s d e c l a r e d o u t -

s i d e o f t h e f u n c t i o n ( $ b r us h c o l or and @colors i n t h i s c a s e ) , b e s u r e t o d e f i n e t h e mb e fo r e d e c l a r in g t h e s u br o ut i n e. N o rm a l P e r l s c op i n g a n d i n i ti a l iz a t io n r u l es a p -

ply.

H a nd l i ng m o us e e v en t s i s a l mo s t a s s t r ai g h tf o r wa r d a s k e yb o ar d e v en t s : = b eg i n p r og r a m-

listing

# t ra ck t he d ra wi ng s ta tu s

m y $ dr aw in g = 0

s u b m ou s e_ ev e nt {

m y $ ev en t = s hi ft

# d et ec t M ou se B ut to n e ve nt s a nd c he ck i f u se r i s c ur re nt ly d ra wi ng

i f ( $ e ve n t -> t yp e = = S D L _M O U SE B UT T O ND O W N | | $ d ra w i ng )

{

# set drawi ng to 1

$ dr aw in g = 1

# get the X and Y valu es of the m ouse


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m y $ x = $ ev e nt -> b ut to n _x

m y $ y = $ ev e nt -> b ut to n _y

# d ra w a r ec ta ng le a t t he s pe ci fi ed p os it io n

$ ap p- > dr aw _ re ct ( [ $ x, $ y, 2 , 2 ] , $ co l or s[ $b r us h_ c ol or ] )

$app->update()

}

# d i sa bl e d ra wi n g w he n u se r r el e as es m o us e b ut to n

$ dr a wi ng = 0 i f ( $e ve n t- >t y pe = = S DL _M O US EB U TT ON U P )

}

$app->add_event_handler( \& m ou s e _e v e nt )

T h i s i s a l l o f t h e c o d e n e c e s s a r y t o m a k e a s i m p l e d r a w i n g a p p l i c a t i o n .

Ta ke n ot e o f t wo t hi ng s. F ir st , S DL p er l i nv ok es t he e ve nt h an dl er s i n t he o rd er o f a t-

t a c hm e nt . I f t h e u s e r p r e s s es Q a n d t h e n m o v e s t h e m o u s e , t h e a p p l i c a t i o n w i l l q u i t b e f o r e

p r o c e s s in g th e m o u s e m o v e m e n t.

S e c o n d , t h e a p p l i c a t i o n m a k e s n o d i s t i n c t i o n b e t w e e n r i g h t , m i d d l e , o r l e f t m o u s e c l i c k s .

S D L p r o v i de s t h i s i n f o r ma t i on . S e e t h e button button() m e t h o d i n SDL::Event.

Hey! The above document had some coding errors, which are explained below:

A r o u n d l i n e 3 1 7 :

= e n d p r o g r a m listin g w ith o u t m a tc h in g = b e g in . ( S ta c k : [ e m p ty] )


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J u s t a s a n i n t e r a c t i v e S D L a p p b u i l d s a r o u n d a n e v e n t l o o p , a g a m e b u i l d s a r o u n d a g a m e

l o op . T h e s i m p le s t g a m e l o o p i s s o me t h in g l i k e:

1 w h i le ( ! $ q u it )

2 {

3 get_events()

4 calculate_next_positions()

5 render()

6 }

T h e n a m e s o f t h e f u n c t i o n s c a l l e d i n t h i s l o o p h i n t a t t h e i r p u r p o s e s , b u t t h e s u b t l e t i e s o f

e ve n t hi s s i mp l e c od e a re i mp or t an t . g e t e v e n t s () o b vi o u sl y p r oc e s se s e v e nt s f r om t h e

r e le v a nt i n pu t d e vi c e s ( k e y b oa r d , m o u s e , j o y s t ic k ) . P r oc e s si n g e v e nt s a t t h e s t a rt o f e v er y

g a m e l o o p i t e r a t i o n h e l p s t o p r e v e n t l a g .


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c a l cu l a t e n e x t p o s it i o n s u pd at e s t h e g am e s t at e a cc or di n g t o u se r i np u t a s w el l a s a ny

a c t i v e a n i m a t i o n s ( a p l a y e r w a l k i n g , a n e x p l o s i o n , a c u t s c e n e ) . render() f in a lly u p d a te s

a n d d i s p l a y s t h e s c r e e n .

C o n s i d e r a g a m e w i t h a m o v i n g l a s e r b o l t :

1 u s e s t ri c t


3 u s e SD L

4 u s e S D L : :E v en t

5 use SDL::Events


7


9 widt h => 200,

10 h ei g ht = > 2 00 ,

11 t it le = > ' Pe w P ew '

12 )

13

14 my $quit = 0 15

16 # s ta rt l as er o n t h e l ef t

17 my $lase r = 0

18

19 s u b g e t_ e ve nt s {

20 m y $ e v en t = S D L :: E ve n t -> n e w( )

21

22 SDL::Events::pump_events

23

24 while( SDL::Events::poll_event($event) )

25 {

26 $ qu it = 1 i f $ ev en t- >t yp e = = S DL _Q UI T

27 }


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28 }

29

30 sub calculate_next_positions {

31 # m ov e t he l as er

32 $laser++

33

34 # if th e l as er go es of f t he sc re en , b ri ng it ba ck

35 $ la se r = 0 i f $l as er > $ ap p- >w ()

36 }

37

38 s ub r e nd er {

39 # d r aw t h e b ac k gr ou nd f ir s t

40 $ ap p- >d ra w_ re ct ( [ 0 , 0 , $ ap p- >w , $ ap p- >h ] , 0 )

41

42 # d ra w th e la se r ha lf wa y up t he s cr ee n

43 $ ap p- >d ra w_ re ct ( [ $ la se r, $ ap p- >h / 2 , 1 0, 2 ] , [ 2 55 , 0 , 0 , 2 55 ] )

44

45 $app->update()

46 }

47

48 w h i le ( ! $ q u it )

49 {

50 get_events()


52 render()

53 }

T h i s g a m e l o o p w o r k s v e r y w e l l f o r c o n s o l e s a n d o t h e r d e v i c e s w h e r e y o u k n o w e x a c t l y

h ow m uc h C PU t i me t he g a me w il l g et f or e ve ry l o op i t er at i on . T ha t h ar d wa r e s t ab il i ty

i s e as y t o p r ed ic t : e ac h a n im at i on a n d c a l cu l at i on w il l h ap p en a t t he s am e t i me f o r e ac h

m a c h ine . U n f o r tun a te ly, th is is not t r u e f o r m o d e r n o p e r a t i n g s y s t e m s a n d g e n e r a l p u r p o s e

c o m p u tin g h a r d w a r e . C P U s p e e d s a n d w o r k lo a d s v a r y, s o f o r th is g a m e to p la y c o n s is te n tly

a c r o s s m u l t i p l e m a c h i n e s a n d m y r i a d c o n f i g u r a t i o n s , t h e g a m e l o o p i t s e l f n e e d s t o r e g u l a t e

its u p d a te s .


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O ne w ay t o s o lv e t h is p ro bl e m i s t o r eg ul a te t h e n u m be r o f f r am es p er s ec on d t h e g a m e

w il l p r o du ce . A frame i s a c o m p l e t e r e d r a w o f t h e s c r e e n r e p r e s e n t i n g t h e u p d a t e d g a m e

s t at e . I f e a c h i t e ra t i on o f t h e g a me l o o p d r a ws o n e f r a m e, t h e m o r e f r a m es p e r s e c o nd , t h e

f as t er t h e g a m e i s r un n in g. I f t he g am e l oo p l i mi t s t h e n um be r o f f r am es p er s e co nd , t h e

g a m e w i l l p e r f o r m c o n s i s t e n t l y o n a l l m a c h i n e s f a s t e n o u g h t o d r a w t h a t m a n y f r a m e s p e r

second.

Y o u c a n s e e t h i s w i t h t h e e x a m p l e p r o g r a m game fixed.pl. W h en r u n w i t h n o a r g u me n ts :

$ perl game_fixed.pl

. .. . t he F PS r at e w il l b e e rr at ic . T he l as er s ee ms t o c ha ng e i ts s pe ed r an do ml y. W he n

r u n w i t h a s in g le a rg um en t , t he g am e s et s a n u pp er b ou nd o n t he n um be r o f f r am es p er

second:

$ perl game_fixed.pl 1

T hi s w il l p re v en t t h e l a s er f r om g oi n g f a s te r t h an 6 0 f r am es p e r s e c on d. W h en r un w it h a

s e c o n d a r g u m e n t , t h e g a m e w i l l s e t a l o w e r b o u n d o f f r a m e s p e r s e c o n d :

$ perl game_fixed.pl 1 1

A t t h i s p o i n t t h e F P S s h o u l d h o l d s t e a d y a t 6 0 f r a m e s p e r s e c o n d .

1 u s e s t ri c t


3 u se S DL

4 use SDL::Event

5 use SDL::Events


7

8 m y $ a pp = S DL x: : Ap p- > ne w(

9 widt h => 200,


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10 h ei g ht = > 2 00 ,

11 t it le = > ' Pe w P ew '

12 )

13

14 m y ( $ st ar t, $ en d, $ de lt a_ ti me , $ FP S, $ fr am es ) = ( 0 , 0 , 0 , 0 , 0 )

15

16 # a im f or a rat e of 6 0 f ra mes p er s eco nd

17 m y $ fi xe d_ ra te = 6 0

18

19 # c o mp en s at e f or t i me s s to r ed i n m ic ro s ec on d s

20 m y $ fp s_ ch ec k = ( 10 00 / $ f ix ed _r at e )

21

22 my $ quit = 0

23

24 # st ar t l as er on th e l ef t

25 my $ laser = 0

26

27 s ub g et _e v en ts {

28 m y $ e ve n t = S D L: : E ve n t- > n ew ( )

29

30 SDL::Events::pump_events

31

32 w h i le ( S D L: : E ve n t s: : po l l _e v e nt ( $e v e nt ) ) {

33 $ qu it = 1 i f $ ev en t- >t yp e = = S DL _Q UI T

34 }

35 }

36

37 sub calculate_next_positions {

38 $laser++

39

40 $ la se r = 0 i f $ la se r > $ ap p- >w

41 }

42

43 s ub r en de r {

44 # d ra w t he b a ck gr o un d f i rs t

45 $ ap p- >d ra w_ re ct ( [ 0 , 0 , $ ap p- >w , $ ap p- >h ] , 0 )

46

47 # d ra w t he l as er


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48 $ ap p- >d ra w_ re ct ( [ $l as er , $ ap p- >h / 2, 10 , 2 ], [ 2 55 , 0 , 0 , 2 55 ] )

49

50 # draw the F PS

51 $ ap p- >d ra w_ gf x_ te xt ( [ 10 , 1 0 ] , [ 25 5, 0, 25 5, 25 5 ] , " FP S: $F PS " )

52

53 $app->update()

54 }

55

56 # C a ll ed a t t he e nd o f e ac h f ra me , w he th er w e d ra w o r n ot

57 sub calculate_fps_at_frame_end

58 {

59 # T ic ks a re m ic ro s ec on ds s in c e l oa d t im e

60 $ e n d = S D L: : g et _ ti c k s( )

61

62 # s m oo th t h e f ra me r a te b y a v er ag in g o ve r 1 0 f ra me s

63 if ( $frames < 10 ) {

64 $frames++

65 $ de l ta _t im e + = $ e nd - $ st ar t

66 }

67 M M e M e M e M e M M 6M ( $ ] extes

64 - ;

65 $delt_tit $start

66 }

56 }

59

$end = SDL::get_ticks() h avermoot; ;

3


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86 {

87 # i f d el ta t im e i s g oi ng t oo s lo w f or f ra me c he ck

88 i f ( $ de lt a_ ti me > $ fp s_ ch ec k ) {

89

90 calculate_fps_at_frame_end()

91

92 # s ki p r en de r in g a n d c ol l is io n d et e ct io ns

93 # ( he av y f un ct io ns i n t he g am e l oo p)

94 next

95 }

96 }

97


99 render()100

101 # a n or m al f r am e w i th r e nd er i ng a ct u al ly p er fo r me d

102 calculate_fps_at_frame_end()

103

104 # i f f ix in g t he u pp er b ou nd s o f t he f ra me r at e

105 i f ( $A RGV[0 ] ) {

106

107 # i f d el ta t im e i s g oi ng t oo f as t c om pa re d t o t he f ra me c he ck

108 i f ( $ de lt a_ ti me < $ fp s_ ch ec k ) {

109

110 # d el ay f or t he d if fe re nc e

111 S D L :: d e la y ( $ f p s_ c h ec k - $ d el t a _t i m e )

112 }

113 }

114 }

T hi s m et ho d i s g e ne ra l ly s uf fi ci e nt f or m os t c om pu t er s . T he a ni ma t io n s w il l b e s mo ot h

e n o u g h t o p r o v i d e t h e s a m e g a m e p l a y e v e n o n m a c h i n e s w i t h d i f f e r e n t h a r d w a r e .

H ow ev er, t h is m et h od s t il l h a s s o m e s e r io us p r ob le ms . F ir s t, i f a c om pu te r i s t oo s l ow

t o s u s t a i n a r a t e o f 6 0 F P S , t h e g a m e w i l l s k i p r e n d e r i n g s o m e f r a m e s , l e a d i n g t o s p a r s e

a n d j i t t e r y a n i m a t i o n . i t w i l l s k i p a l o t o f r e n d e r i n g , a n d t h e a n i m a t i o n w i l l l o o k s p a r s e a n d


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jittery. It might be better to set a lower bounds of 30 FPS, though it’s difficult to predict

t h e b e s t f r a m e r a t e f o r a u s e r .

T h e w o r st p r ob l e m i s t h at t h i s t e c hn i q ue s t i ll t i e s r e n d er i n g s p e ed t o t h e C P U s p e e d: a v e r y

f a s t c o m p u t e r w i l l w a s t e C P U c y c l e s d e l a y i n g .

T o f i x t h e p r o b l e m o f a c o m p u t e r b e i n g c o n s i s t e n t l y t o o f a s t o r t o o s l o w f o r t h e h a r d - c o d e d

F PS r at e i s t o a dj us t t he F PS r at e a cc or di ng ly. A s lo w C PU m ay l im it i ts el f t o 3 0 F PS ,

w h i l e a f a s t C P U m i g h t r u n a t 3 0 0 F P S . A l t h o u g h y o u m a y a c h i e v e a c o n s i s t e n t r a t e t h i sw a y ( c o n s i s t e n t f o r a n y o n e p a r t i c u l a r c o m p u t e r ) , t h i s t e c h n i q u e s t i l l p r e s e n t s t h e p r o b l e m

o f d if f e r in g a n im a tio n s p e e d s b e tw e e n d if f e r e n t c o m p u te r s .

B e tte r s o lu tio n s a r e a v a ila b le .

T he p ro bl e m c au se d b y c o up li n g r en d er i ng t o t he C PU s pe ed h as a c on ve ni e nt s ol u ti o n.

I n s t e a d o f u p d a t i n g o b j e c t p o s i t i o n s b a s e d o n h o w f a s t t h e c o m p u t e r c a n g e t t h r o u g h t h e

g am e l oo p, d er i ve t he i r p os i ti on s f r om a p hy si c al m od el b as ed o n t h e p as s ag e o f t i me .

O bj e ct s m ov in g a cc or d in g t o r e al w or ld t i me w il l h av e c on s is t en t b eh av i or a t a l l C PU

s p e e d s a n d s m o o th in te r p o la tio n b e tw e e n f r a m e s . SDLx::App p r o v id e s th is b e h a v io r th r o u g h

m o v e m e n t a n d s h o w h a n d le r s .

C o n s i d e r a s i m p l e p h y s i c s m o d e l f o r t h e l a s e r h a s a c o n s i s t e n t h o r i z o n t a l v e l o c i t y i n p i x e l s

p e r t i m e s t e p a t t h e w i n d o w ’ s m i d - p o i n t :

X = V el oc it y * t im e s te p,

Y = 100


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A s s u m i n g a v e l o c i t y o f 1 0 , t h e l a s e r w i l l p a s s t h r o u g h t h e c o o r d i n a t e s :

0 , 1 00

1 0, 1 00

2 0, 1 00

3 0, 1 00

...

2 0 0, 1 00

N ot e t ha t t h e s p e ed o f p ro ce s si n g t h e g am e l oo p n o l o ng er m at t er s . T he p os i ti on o f t h e

l a s e r d e p e n d s i n s t e a d o n t h e p a s s a g e o f r e a l t i m e .

T h e b i g g e s t p r o b l e m w i t h t h i s a p p r o a c h i s t h e r e q u i r e d b o o k k e e p i n g f o r t h e m a n y o b j e c t sa n d c a l l b ac k s. T h e i m p l e me n t at i o n o f s u ch c o mp l e x m o d e ls i s n o n- t r iv i a l; s e e t h e l e ng t h y

d i s c u s s i o n i n t h e d o c u m e n t a t i o n o f t h e SDLx::Controller module.

SDLx::App u s in g t h e SDLx::Controller m o du l e p r o vi d e c a ll b a ck s t o h a n dl e b o t h a s p ec t s

o f t hi s t yp e o f g am e l oo p . O ne i s t h e t h e m ov em en t h an dl e r, w hi c h i s a c a ll b ac k w he r e

c a l cu l a ti o n s o f t h e n e x t s t e p f o r e a ch r e l ev a nt d a ta p o i nt i s c a l cu l at e d . I n t h e a b o v e e x a m pl e

t h e m o v e m e n t h a n d l e r w o u l d c a l c u l a t e t h e X and Y v a l u e s , f o r e a c h t i m e s t e p b e t w e e n t h e

f r a m e s o f a n im a tio n s .

W he n w e a re r ea dy t o r en de r t he f ra me i t i s h an dl ed b y t he s ho w h an dl er. I n t he a bo ve

e x a m p l e t h a t w o u l d m e a n t h e s h o w h a n d l e r w o u l d p r i n t o r r e n d e r t h e X, Y values.

T h i s v e r s i o n o f t h e l a s e r e x a m p l e d e m o n s t r a t e s t h e u s e o f m o v e m e n t , s h o w h a n d l e r s , a n d a

s i mp l e p h y s i cs m o de l . T h i s e x a m pl e a l s o s h o w s h o w SDLx::App c a n d o m o r e o f t h e w o r k ,

e v e n p r o v i d i n g t h e e n t i r e g a m e l o o p :

1 u s e s t ri c t


3 u se S DL


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4 use SDL::Event


6

7 m y $ a pp = S DL x: : Ap p- > ne w(

8 width => 2 00,

9 h ei gh t = > 2 0 0,

10 t it le = > ' Pe w P ew '

11 )

12

13 my $laser = 0

14 m y $ ve lo ci ty = 1 0

15

16 $app->add_event_handler( \& q ui t _ ev e nt )

17

18 # t e ll a p p t o h an dl e t he a p pr op ri at e t im es t o

19 # c a ll b o th r e nd er in g a nd p h ys ic s c al c ul at i on

20

21 $app->add_move_handler( \&calculate_laser )22 $app->add_show_handler( \& r e nd e r _l a se r ) 23

24 $app->run()

25

26 s ub q ui t_ e ve nt {

27 my $event = shift

28 m y $ c on tr ol l er = s hi ft

29

30 $ c on t r ol l er - > st o p i f $ e v en t - >t y pe = = S D L _Q U IT

31 }

32

33 s u b c a lc u l at e _l a s er {

34

35 # T he s te p is t he d if fe re nc e in T im e ca lc ul at ed f or t he n ex t ju mp

36 my ( $step, $app , $t ) = @_

37 $ la se r + = $ v el oc i ty * $ st ep

38 $ la se r = 0 i f $ la se r > $ ap p- >w

39 }

40

41 s ub r en de r _l as e r {


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42 my ( $delta , $ap p ) = @_

43

44 # Th e d el ta ca n b e u se d t o r en de r b lu rr ed fr am es

45

46 # d ra w t he b ac kg r ou nd f ir st

47 $ ap p- >d ra w_ re ct ( [ 0, 0, $a pp -> w, $a pp -> h ] , 0 )

48

49 # d ra w t he l as er

50 $ ap p- >d ra w_ re ct ( [ $ la se r, $ ap p- >h / 2 , 1 0, 2 ] , [ 2 5 5, 0 , 0 , 25 5 ] )

51 $app->update()

52 }

T o l e a r n m o r e a b o u t t h i s t o p i c p l e a s e , s e e a n e x c e l l e n t b l o g p o s t b y GafferOnGames.com:

HTTP://GafferOnGames.Com/game-physics/fix-your-timestep.

http://http//GafferOnGames.Com/game-physics/fix-your-timestep


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P o n g i s o n e o f t h e f i r st p o p ul a r v i d e o g a me s i n t h e w o r l d. A l la n A l c o r n c r e at e d i t f o r A t a r i,

I n c. I t s r e l ea se i n 1 97 2 w as b ot h A ta ri ’s f i rs t g am e e ve r a n d t h e s p ar k w hi c h b e g an t h e

v id e o g a m e in d u s tr y .

P o n g s i m u l a t e s a t a b l e t e n n i s m a t c h (“p i n g p o n g”) . E ac h p l ay er c on tr o ls a p a dd le w hi c h

m o v e s v e r t i c a l l y o n t h e s c r e e n t o h i t a b a l l b o u n c i n g b a c k a n d f o r t h b e t w e e n t h e p l a y e r s .

Y o u e a r n a p o i n t i f y o u r o p p o n e n t i s u n a b l e t o r e t u r n t h e b a l l t o y o u r s i d e o f t h e s c r e e n .

Y o u c a n r e c r e a t e P o n g y o u r s e l f w i t h P e r l a n d S D L .


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S ta rt b y m ak i ng a s im pl e s cr e en f o r P o ng . O pe n a f i le i n y ou r f av ou r it e t e xt e di t or a nd

type:

1 #!/usr/bin/perl

2 u s e s t ri c t


4

5 u s e SD L


7

8 # c re at e t he m ai n s c re en


10 width => 500,

11 height => 500,

12 title => 'My Pong Clone!',

13 d t => 0.02,

14 e xi t _o n_ q ui t = > 1 ,

15 )

16

17 # l et 's r ol l!

18 $app->run

S a v e t h i s f i l e a s pong.pl a n d r u n i t b y t y p i n g o n t h e c o m m a n d l i n e :

perl pong.pl

Y o u s h o u ld s e e a 5 0 0 x 5 0 0 b la c k w in d o w e n title d “ My Pong Clone!”. T h e o n ly n e w f e a t u re

y ou m ig ht n ot h av e s ee n b ef or e i s t he dt p ar a me t er t o t h e SDLx::App co n s tr u c to r. T h is

r e p r e s e n t s t h e l e n g t h , i n s e c o n d s , o f a m o v e m e n t s t e p a s m a n a g e d b y a n SDLx::Controller

o bj ec t . B ec au se t h e SDLx::App o bj ec t i s a ls o a n SDLx::Controller o bj e ct , i t c a n h an dl e

S D L Q U IT events.


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Game Loop Granularity

T h e g a m e l o o p r u n s u s i n g a p r o c e s s t h a t p e r f o r m s c a l c u l u s o n t h e t i m e p r o g r e s s i o n .

T o d o t h i s i t u s e s t h e dt p a r a m e te r o f SDLx::App. dt b y d e f a u l t i s s e t t o 0.01 granu-

l a r it y. I n s i mp l e t e r ms dt d e t e r m i n e s h o w m a n y s m a l l s t e p s i n m o v e m e n t ( c a l l s t o

t h e m o v e h a n d l e r ) s h o u l d b e m a d e p e r e a c h r e n d e r ( c a l l t o t h e s h o w h a n d l e r ) .

W h a t i s i m p o r t a n t t o r e m e m b e r i s t h a t t h e dt g r a n u l a r i t y m u s t n e v e r b e s e t s o b i g

t h at i t f a i ls t o c a pt u r e c o l l i si o n s a n d i n t er a c ti o n b e tw e en m o vi n g o b j e c ts . T h i s c a n

e s p e c i a l l y h a p p e n w i t h f a s t m o v i n g o b j e c t s s u c h a s b u l l e t s , b u t t h e p r i n c i p l e a p p l i e s

t o m an y s it u at i on . O n t h e o t h er h an d h av in g a t o o f i n e g r an ul a ri t y t a x es t he C PU

resources.

T h er e a r e t h r ee m a in g a me o b j ec t s i n P o ng : t w o p l a y er p a dd l e s a n d t h e b o u nc i ng b a l l. P a d-

d l es a r e r e c ta n gl e s m o vi n g w h i ch m o ve v e rt i c al l y. T h ey ’ r e e a s y t o r e p re s e nt w i t h S D L x : :-

R e ct o b j ec t s . F i r st , p u t SDLx::Rect in y o u r m o d u le ’ s d e c la r a tio n s :

1 u s e SD L


3 use SDLx::Rect;

N e xt , a d d a h a sh r e fe r e nc e t o s t o re t h e f i r s t p l a y e r ’s p a dd l e . U s i ng a h a s h r e f e re n ce a l l ow s

t h e p o s s i bi l i ty o f a d di n g m o r e i n fo r ma t i on l a t er. I n a m o re c o mp l ex g a me , c o ns i d er u s i ng

a n a c t u a l o b j e c t w h i c h contains an SDLx::Rect. F o r n o w, t h i s w i l l s u f f ic e :

1 my $player1 = {

2 paddle => SDLx::Rect->new( 10, $app->h / 2, 10, 40 ),

3 };

4


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5 # l et 's r ol l!

6 $app->run

T h i s c r e a t e s a 1 0 x 4 0 p a d d l e r e c t f o r t h e f i r s t p l a y e r o n t h e l e f t s i d e o f t h e s c r e e n ( x = 10)

a n d s o m e w h a t i n t h e c e n t e r ( y = $app->h / 2) . T h e s e c o n d p l a y er ’ s p a d d le i s s i mi l a r:

my $player2 = {

paddle => SDLx::Rect->new( $app->w - 20, $app->h / 2, 10, 40),

};

# l et 's r ol l!

$app->run

T h e s e c o n d p a d d l e n e e d s t o a p p e a r o n t h e r i g h t s i d e o f t h e s c r e e n , s o i t s x p o si t i on i s t h e

s cr ee n ’s w id th m in us 2 0. A s t h e p ad dl e h a s a w id t h o f 1 0 a n d t h e x p o s i t i o n r e f e r s t o t h e

r e c t ’ s t o p - l e f t c o r n e r , t h e p a d d l e h a s a 1 0 p i x e l m a r g i n f r o m t h e r i g h t e d g e o f t h e s c r e e n .

F i n a l l y , t h e b o u n c i n g b a l l i s a 1 0 x 1 0 r e c t i n t h e m i d d l e o f t h e s c r e e n :

my $ball = {

rect => SDLx::Rect->new( $app->w / 2, $app->h / 2, 10, 10 ),

}; # let's roll! $app-run

J u s t l i k e t h e o r i g i n a l P o n g , t h i s b a l l i s s q u a r e .

W i t h t h e g a m e o b j e c t s c r e a t e d , a d d a s h o w h a n d l e r t o r e n d e r t h e m t o t h e s c r e e n :

1 $app-add_show_handler(>

2 sub {

3 # first, clear the screen

4 $app->draw_rect( [ 0, 0, $app->w, $app->h ], 0x000000FF );

5


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6 # then render the ball

7 $app->draw_rect( $ball->{rect}, 0xFF0000FF );

8

9 # ... and each paddle

10 $app->draw_rect( $player1->{paddle}, 0xFF0000FF );

11 $app->draw_rect( $player2->{paddle}, 0xFF0000FF );

12

13 # finally, update the screen

14 $app->update;

15 }

16 );

17

18 # l et 's r ol l!

19 $app->run

T hi s a p pr oa ch i s r at h er s i mp l e. T he c od e c l ea r s t h e s c re en b y p ai n ti n g a b la c k r ec t an gl e

t he s iz e o f t h e s c r ee n, t he n p ai n ti n g o p a qu e r ed (0xFF0000FF) r e ct a n gl e s i n e a c h o b je c t ’s

position.

T h e r e s u l t c a n b e s e e n o n t h e s c r e e n s h o t :

I t ’s t i m e t o l e t t h e p l a ye r m o ve t h e l e f t p a dd l e ! R e me m be r t h a t m o ti o n i s m e re l y c h a n gi n g

a n o b j e c t’s p o s it i o n w i t h r e s p e ct t o t i me . I f t h i s m o t i on i s , i n t h e g a m e , a m a gi c a l t e l e po r -

t at i on , y ou c an c ha ng e t he ( x , y ) c o or di na t es a nd b e d on e w it h i t . I f t h e m o t io n n ee ds t o

r e p r e s e n t s o m e s o r t o f r e a l i s t i c p h y s i c s , t h e o b j e c t n e e d s t o m o v e a t a n u n d e r s t o o d s p e e d .

P on g p ad dl e s h a v e a c o ns ta n t s p e ed , s o t he r e’s n o n ee d t o m od el a cc e le r at i on . A ls o, a s

p ad dl es m ov e o nl y v er t ic a ll y, t he g am e o nl y n ee ds t o t ra c k v er t ic al v el oc i ty. A dd a v y

e l e m e n t t o e a c h p a d d l e s t r u c t u r e :

m y $ pl ay er 1 = {

p ad dl e = > S DL x: :R ec t- >n ew ( 1 0, $ ap p- >h / 2 , 1 0, 4 0 ) ,


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F i g u r e 6 . 1 : F i r s t v i e w o f o u r P o n g c l o n e

v_y => 0,

}

N ow w ha t? H ow d o es t h is n e w a t t ri bu t e h e l p m od if y t he p o si t io n o f a p a dd le ? Ve l oc it yr e p r e s en ts th e displacement h ow m uc h d is p la ce me nt h ap pe n s i n a u ni t o f t im e, a s i n 2 0

k m/ h o r 4 m /s . I n t h i s P o n g c l on e, t h e u n it o f t i m e i s t he a p p’s dt. T h e v e l o c i t y o f a p a d d l e

is v y p i x e l s p e r dt. H er e i s w he re t h e m o t io n h an dl e rs c om e i n h an dy :

# handles the player's paddle movement

$app->add_move_handler( sub {

my ( $step, $app ) = @_;

my $paddle = $player1->{paddle};

my $v_y = $player1->{v_y};

$paddle->y( $paddle->y ( $v_y * $step ) );

});


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I f y o u r e c a l l p r ev i o us c h ap t e rs , t h e c o de s h ou l d b e s t r ai g h tf o r wa r d. W h en v y i s 0 a t a n y

g iv en r un c yc le , t h e p ad dl e w on ’t c ha ng e i t s y p os i ti o n. I f, h ow ev e r, t he r e i s a v e rt i ca l

v e lo c i ty, t h e c o de u p da t e s t h e y p o si t io n b as ed o n h ow m uc h o f t he e xp ec t ed c yc l e t i me( th e a p p ’ s dt) h a s p a s s e d . A v a l u e o f 1 i n $step i n d i c a t e s a f u l l c y c l e h a s o c c u r r e d , s o t h a t

$v y * $step i s t h e s a m e a s $v y * 1, w h i c h s i m p l i f i e s t o $v y – t h e d e s i r e d s p e e d f o r o n e

c y c le . I f t h e h a n d l er g e t s c a l l e d m o r e f r e q u en t l y, t h e p a d d l e w i l l m o ve a r e l at i v el y s h o rt e r

amount.

T h e s e c o n d p l a y e r ’ s p a d d l e w i l l u s e t h e s a m e m o t i o n m e c h a n i c s , s o i t w o n ’ t h u r t t o p r e p a r ef o r i t s m o t i o n :

1 m y $ pl ay er 2 = {

2 p ad dl e = > SD Lx :: Re ct -> ne w( $ a pp -> w - 2 0, $ a pp -> h / 2 , 1 0, 4 0 ),

3 v_y => 0,

4 }

A n d a d d a n o t h e r m o t i o n h a n d l e r , j u s t l i k e o u r p l a y e r ’ s :

1 # handles AI's paddle movement2 $app->add_move_handler( sub {

3 my ( $step, $app ) = @_;

4 my $paddle = $player2->{paddle};

5 my $v_y = $player2->{v_y};

6

7 $paddle->y( $paddle->y ( $v_y * $step ) );

8 });

F or t h e s a ke o f s im pl i ci t y o f e xp l an at i on , t hi s c od e h as r ep et i ti on a r ea l p r og ra m

w ou l d n o t w an t . T hi s r e pe ti t io n c ou l d g o a wa y i n s ev er a l w a y s. Yo u c ou l d u s e a na r ra y t o h o ld a l l m o v i ng e l e me n ts . Yo u c o u l d u s e a h e l pe r f u n c t io n t o c r ea t e a n e w


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c l os u r e f o r e a ch p a dd l e . Yo u c o ul d t u r n t h e g a me o b je c t h a s h r e fe r e nc e s i n t o r e al

o b j e c t s a n d a d d a move() or update position() method.

P a d d le v e lo c ity v y h as a d e fa ul t v al ue o f z e ro , s o p ad dl e s b e g in b y n o t m ov i ng . T ha t ’s

g o od , u n ti l t h e p l a y e r w a n t s t o m o ve t h e p a d d l e. To d i v in e t h e p l a y e r ’s i n t en t , t h e p r o gr a m

m us t b in d t he u p a nd d ow n a rr ow k ey s o f t h e k e y bo ar d t o m an ip ul a te t h e p os i ti v e a nd

n eg at i ve v el o ci t y o f t h e p ad dl e t h ro ug h a n e v en t h oo k . T hi s m ea n s l oa d in g t h e S DL :: -

E v e n ts m o d u le :

1 u s e S DL

2 use SDL::Events;

3 u s e S D L x: : Ap p

4 u s e S D L x: : Re c t

. . . a n d c r e a ti n g a n e v en t h o o k :

1 # h an dl e s k e y bo ar d e ve nt s

2 $app-add_event_handler(>3 sub {

4 my ( $event, $app ) = @_;

5

6 # user pressing a key

7 if ( $event->type == SDL_KEYDOWN ) {

8

9 # up arrow key means going up (negative velocity)

10 if ( $event->key_sym == SDLK_UP ) {

11 $player1->{v_y} = -2;

12 }

13

14 # down arrow key means going down (positive velocity)

15 elsif ( $event->key_sym == SDLK_DOWN ) {


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16 $player1->{v_y} = 2;

17 }

18 }

19 # user releasing a key

20 elsif ( $event->type == SDL_KEYUP ) {

21

22 # up or down arrow keys released, stop the paddle

23 if (

24 $event->key_sym == SDLK_UP

25 or $event->key_sym == SDLK_DOWN

26 ) {

27 $player1->{v_y} = 0;

28 }

29 }30 }

31 )

A g ai n , t h e re ’s n o t hi n g n e w. W h en e ve r t h e u s er p r e ss e s t h e u p a r ro w k e y, t h e p a d d l e s h o u l d

m ov e u p . K ee p i n m in d t h at t he o ri gi n p oi nt o f 0 , 0 i n S DL i s t he t op -l ef t c or ne r, s o a

negative v y w i l l d e cr e as e t h e p a dd l e ’s y a nd s en d i t up t he s c re en . S i mi l ar l y, a dd in g a

p o s i t i v e v a l u e t o v y w h e n e v e r t h e u s e r p r e s s e s t h e d o w n a r r o w k e y w i l l m o v e t h e p a d d l e

d o wn . W h en t h e u s e r r e le a s es e i t he r a r r ow k e y, a s s ig n i ng z e ro t o v y s t o p s t h e m o t i o n .

T h e b a l l ’ s m o v e m e n t i s s i m i l a r t o t h a t o f e i t h e r p a d d l e , e x c e p t t h a t i t a l s o h a s a h o r i z o n t a l

v e lo c ity c o m p o n e n t o f v x. A dd t h at t o t h e b al l s t ru ct u re :

1 my $ball = {

2 r ec t = > S DL x: :R ec t- >n ew ( $ ap p- >w / 2 , $ ap p- >h / 2 , 1 0, 1 0 ) ,

3 v_x => -2.7,

4 v_y => 1.8,

5 }


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T h e b a l l w i l l h a ve a n i n i ti a l v e l o c it y o f - 2 . 7 h o r iz o nt a l ly a n d 1 . 8 v e r ti c a ll y. J u s t a s a n e ga -

t i v e v e r t i c a l v e l o c i t y m o v e s t h e o b j e c t u p , a n e g a t i v e h o r i z o n t a l v e l o c i t y m o v e s i t t o w a r d s

t he l ef t s id e o f t he s cr ee n. T he b al l a ls o n ee ds a m ot io n h an dl er t o u pd at e i ts p os it io na c c o r d i n g t o i t s v e l o c i t y :

1 h a nd le s t h e b a ll m o v em en t

2 a p p -> a dd _ m ov e _ ha n dl e r ( s u b {

3 my ( $step , $app ) = @_

4 my $b all_r ect = $ball- >{rect }

5

6 $ b al l _ re c t -> x ( $ b al l _r e c t- > x + ( $ b al l -> { v _x } * $ s t e p) )

7 $ b al l _ re c t -> y ( $ b al l _r e c t- > y + ( $ b al l -> { v _y } * $ s t e p) )

8 )

A l l o f t h e s e m o t i o n h a n d l e r s l o o k s i m i l a r s o f a r , b u t i f y o u ’ r e p a y i n g c l o s e a t t e n t i o n , y o u

c a n p r o b ab l y s p o t a b u g c a u s ed b y m i s s i ng c o de . T ry r u nn i n g t h e g a m e . Yo u ’ ll s e e t h e b a ll

g o i n g , g o i n g , a n d g o n e !

T hi s h an dl e r n e e ds t o c o nf in e t he b al l t o t he s cr ee n . W he ne ve r t he b al l r ea ch es a t op o r

b o tt o m e d g e o f t h e s c r e e n, i t n e ed s t o b o un c e. T h at ’s e a sy e n ou g h t o a d d:

1 # h a n dl e s t he b a l l m ov em e nt

2 $ a p p- > a dd _ mo v e _h a n dl e r( s u b {

3 my ( $st ep, $a pp ) = @_4 my $ball _rect = $bal l->{re ct}

5

6 $ b a ll _ re c t -> x ( $ b al l _ re c t -> x + ( $ ba l l -> { v_ x } * $ st e p) )

7 $ b a ll _ re c t -> y ( $ b al l _ re c t -> y + ( $ ba l l -> { v_ y } * $ st e p) )

8

9 # collision to the bottom of the screen

10 if ( $ball_rect->bottom >= $app->h ) {

11 $ball_rect->bottom( $app->h );

12 $ball->{v_y} *= -1;

13 }

14

15 # collision to the top of the screen

16 elsif ( $ball_rect->top


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17 $ball_rect->top( 0 );

18 $ball->{v_y} *= -1;

19 }

20 })

I f t h e n e w y ( "bottom" or "top") v a l u e w o u l d t a k e t h e b a l l o f f t h e s c r e e n i n p a r t o r i n w h o l e ,

t h e h a n d l e r u p d a t e s t h e b a l l ’ s p o s i t i o n w i t h t h e f u r t h e s t p o s i t i o n p o s s i b l e w h i l e r e m a i n i n g

o n t h e s c r e e n , s o t h a t t h e b a l l w i l l o n l y e v e r touch t h a t e d g e . T h e h a n d l er a l s o r e v e rs e s y y

s o t h a t t h e b a l l w i l l b o u n c e b a c k o n t o t h e s c r e e n g o i n g t h e o p p o s i t e d i r e c t i o n a t t h e s a m e

speed.

T h a t f i x e s o n e b u g , b u t w h a t s h o u l d h a p p e n w h e n t h e b a l l h i t s t h e l e f t o r r i g h t e d g e s o f t h e

s c re e n ? A c co r d in g t o t h e r u l e s o f P o ng , t h i s m e an s t h e p l a ye r o n t h e o p p o si t e s i d e s c or e d

a p oi nt , a nd t he b al l s ho ul d g o b ac k t o t he c en te r o f t he s cr ee n. S ta rt b y a dd in g a score

a t t r i b u t e f o r e a c h p l a y e r :

1 m y $ pl ay er 1 = {

2 p ad dl e => S DL x: :R ec t- >n ew ( 10 , $a pp -> h / 2, 1 0, 4 0) ,

3 v_y => 0,

4 score => 0,

5 }

6

7 m y $ pl ay er 2 = {

8 p ad dl e = > SD Lx :: Re ct -> ne w( $ a pp -> w - 2 0, $ a pp -> h / 2 , 1 0, 4 0 ),

9 v_y => 0,

10 score => 0,

11 }

T he n u pd a te t he b a ll ’s m ot i on h an dl er t o h an dl e t h e o ut o f b ou nd s c on di t io n f o r t h e l e ft

a n d r i g h t b o r d e r s :

1 # h a n dl e s t he b a l l m ov em e nt

2 $ a p p- > a dd _ mo v e _h a n dl e r( s u b {


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3 my ( $st ep, $a pp ) = @_

4 my $ball _rect = $bal l->{re ct}

5

6 $ b a ll _ re c t -> x ( $ b al l _ re c t -> x + ( $ ba l l -> { v_ x } * $ st e p) )

7 $ b a ll _ re c t -> y ( $ b al l _ re c t -> y + ( $ ba l l -> { v_ y } * $ st e p) )

8

9 # c ol li si on t o th e bo tt om o f th e sc re en

10 i f ( $b al l_ re ct -> bo tt om > = $a pp -> h ) {

11 $ball_rect->bottom( $app->h )

12 $ b al l - >{ v _ y} * = - 1

13 }

14

15 # c ol li si on t o th e to p of t he s cr ee n

16 e ls if ( $ ba ll _r ec t- >t op t op ( 0 )

18 $ b al l - >{ v _ y} * = - 1

19 }

20

21 # collision to the right: player 1 score!

22 elsif ( $ball_rect->right >= $app->w ) {

23 $player1->{score}++;

24 reset_game();

25 return;

26 }

27

28 # collision to the left: player 2 score!

29 elsif ( $ball_rect->left {score}++;

31 reset_game();

32 return;

33 }

34 })

I f t h e b a l l h i t s t h e r i g h t e d g e o f t h e s c r e e n ( t h e a p p ’ s w i d t h ) , w e i n c r e a s e p l a y e r 1 ’ s s c o r e ,

call r e s e t g a m e( ), a nd r e tu r n w i t ho ut u pd at i ng t he b al l ’s p os i ti o n. I f t h e b a l l h it s t he l e ft

e d g e o f t h e s c r e e n , d o t h e s a m e f o r p l a y e r 2 .


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The r e s e t g a m e( ) f u n c t i o n m u s t r e t u r n t h e b a l l t o t h e c e n t e r o f t h e s c r e e n :

1 sub reset_game {

2 $ball->{rect}->x( $app->w / 2 );

3 $ball->{rect}->y( $app->h / 2 );

4 }

T he g a me ’s e xi s ti ng c ol l is i on d et e ct i on i s v er y s i mp le b ec a us e t h e p ad dl e s a nd b a ll c an

o nl y c ol l id e w it h t he f i xe d e dg es o f t h e s c r ee n

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