OpenGL Selection

Three Selection Methods

• Color coding (OpenGL)

• Selection mode (OpenGL)

• Selection ray (generic)

Method 1: Color Coding

• To make a selection at (x, y):• Draw a color-coded image in the mouse

callback function,• Read the pixel value from the back frame

buffer,• The pixel color tells you the selection.

(x, y)

Method 1: Color Coding

• To read the pixel value:

• Only the visible object will be selected• The nearest object (if using the depth test)• The latest object (otherwise)

• To avoid the color-coded image from display• Remember to use double buffering• Don’t swap the frame buffer

glReadPixels(x, y, width, height, format, type, *data)

glReadBuffer(GL_BACK)

default

Method 2: Selection Mode

• The idea is similar to color coding. You will still need to pretend to draw something.

• However,• No need to assign unique colors.• No need to use double buffering.• The selection can ignore the visibility and pick

multiple objects. • OpenGL can help you do it.

Method 2: Selection Mode• To start the fake drawing process:

• To specify the selection region:

• To store the selections:

• To give each object a name:

glRenderMode(GL_SELECT)

gluPickMatrix(x, y, width, height, viewport)

glGetIntegerv(GL_VIEWPORT, viewport)

glSelectBuffer(buffer_length, buffer)

glInitNames();glPushName(…);glLoadName(…);

An ExampleglRenderMode(GL_SELECT);

int selectBuffer[100];glSelectBuffer(100, selectBuffer);

glMatrixMode(GL_PROJECTION);glLoadIdentity();int vp[4];glGetIntegerv(GL_VIEWPORT, vp);gluPickMatrix(x, y, 5, 5, vp);gluPerspective(10, 1, 1, 100);

glMatrixMode(GL_MODELVIEW);glLoadIdentity();//Change camera view

glInitNames();glPushName(0);glLoadName(1);//Draw A, A has a name: 1glLoadName(2);//Draw B, B has a name: 2

int hits=glRenderMode(GL_RENDER);//The selections are stored in the buffer

In the Selection Buffer

Some Tips

• The fake drawing process is the same as the regular drawing process, except for those selection functions.

• An object can have multiple names.

• If you don’t reset the projection matrix in your display function, remember to restore it after you finish the selection projection (using glPushMatrix and glPopMatrix).

• When multiple objects are selected, you can use the depth information to know which one is visible.

Method 3: Selection Ray

• It is a generic method, not limited to OpenGL.

• It is a basic component in ray tracing (an advanced rendering algorithm).

• Relies on the projection matrix and ray-object intersection tests. So it needs some effort to implement.

Basic Idea• A pixel on the image is actually a ray. Any point

on this ray will be projected onto this pixel.

Image Plane

Center of Projection

An image(640*640)

Basic Idea• So to find the selection, we can just test the

intersection between the ray and any object.

Image Plane

Center of Projection

An image(640*640)

Basic Idea• Two questions:• How to get this ray• How to do the intersection test

Image Plane

Center of Projection

An image(640*640)

To Get the Selection Ray• Given a projection matrix M, which can be obtained by glGetFloatv(GL_PROJECTION_MATRIX, M)

• Given a selection pixel (x, y), for -1<x, y<1, which can be obtained by:

w ⋅xw⋅yw⋅zw

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=M

rxryrz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

x=mouse_x×2/screen_width-1y=1-mouse_y×2/screen_height

• We know:

u ⋅rxu⋅ryu⋅rzu

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=M−1

xyz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

Any point on the ray Any vector

To Get the Selection Ray• So we can find two points on the ray:

• They need be transformed from the eye to the world coordinate system (how?):

• Finally, the ray is:

wp ⋅pxwp ⋅pywp ⋅pzwp

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

=M−1

xy−11

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

wq ⋅qxwq ⋅qywq ⋅qzwq

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

=M−1

xy11

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

rx

ry

rz

1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=s

pxpypz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

+ (1−s)

qxqyqz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

px

py

pz

1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=M eye2world

pxpypz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

qx

qy

qz

1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

=M eye2world

qxqyqz1

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

To Find The Intersection

• The ray-sphere intersection

r

C

P

Q

R =Ps+Q(1−s), R−C 2 =r2

spx + (1−s)qx −cx( )2+

spy + (1−s)qy −cy( )2+

spz + (1−s)qz −cz( )2=r2

s is the unknown. Solve it!The smallest s (s>0) gives the earliest intersection.

To Find The Intersection• The ray-triangle intersection

Q

N =(V1−V0 )×(V2−V0 )(V1−V0 )×(V2−V0 )

V0

V2

V1

P

N

Let N be the triangle normal:

R =Ps+Q(1−s), R−V0( )⋅N=0

R

This is a linear equation. Solve s and find R.

b1 =AV0RV2AV0V1V2

, b2 =AV0V1RAV0V1V2

,

b0 =1−b1 −b2 , Aijk = j−i( )×(k−i)( )⋅Nfor

R is in the triangle, if barycentric Coordinates 0≤b0, b1, b2≤1.

Method 3: Selection Ray

• It does not need to read the frame buffer.• It is flexible, independent of the rendering system.

• Each primitive type needs a ray intersection algorithm.

• Needs multiple matrix computation steps, so it is relatively slow.

Method 3: Selection Ray• When the scene contains multiple objects?• Test each of them and find the nearest

intersection.• It is slow, how to make it faster? Use

Bounding Volume Hierarchy.