# Composing Transformations

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16-Jan-2016Category

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Composing TransformationsComposing Transformations the process of applying several transformations in succession to form one overall transformation If we transform a point P using matrix M1 first, and then transform using M2, and then M3, we have:

(M3 x (M2 x (M1 x P ))) = M3 x M2 x M1 x P

Composing Transformation Matrix multiplication is associative M3 x M2 x M1 = (M3 x M2) x M1 = M3 x (M2 x M1)Transformation products may not be commutative A x B != B x A Some cases where A x B = B x A A B translation translation scaling scaling rotation rotation uniform scaling rotation (sx = sy)

Transformation order matters!Example: rotation and translation are not commutativeTranslate (5,0) and then Rotate 60 degree

OR

Rotate 60 degree and then translate (5,0)??Rotate and then translate !!

How OpenGL does it? OpenGLs transformation functions are meant to be used in 3D No problem for 2D though just ignore the z dimensionTranslation: glTranslatef(d)(tx, ty, tz) -> glTranslatef(d)(tx,ty,0) for 2D

How OpenGL does it? Rotation: glRotatef(d)(angle, vx, vy, vz) -> glRotatef(d)(angle, 0,0,1) for 2D (vx, vy, vz) rotation axis xyYou can imagine z is pointing outof the slide

OpenGL Transformation CompositionA global modeling transformation matrix (GL_MODELVIEW, called it M here) glMatrixMode(GL_MODELVIEW)The user is responsible to reset it if necessary glLoadIdentity() -> M = 1 0 0 0 1 0 0 0 1

OpenGL Transformation CompositionMatrices for performing user-specified transformations are multiplied to the current matrixFor example, 1 0 1 glTranslated(1,1 0); M = M x 0 1 1 0 0 1 All the vertices defined within glBegin() / glEnd() will first go through the transformation (modeling transformation) P = M x P

Transformation Pipeline Modeling transformation

Something noteworthy Very very noteworthy OpenGL post-multiplies each new transformation matrix M = M x Mnew Example: perform translation, then rotation 0) M = Identity 1) translation T(tx,ty,0) -> M = M x T(tx,ty,0) 2) rotation R(q) -> M = M x R(q) 3) Now, transform a point P -> P = M x P = T(tx, ty, 0) x R(q) x P

Example Revisit We want rotation and then translationGenerate wrong results if you do: You need to specify the transformation in the opposite order!!

How Strange OpenGL has its reason It wants you to think of transformation in a different way Instead of thinking of transforming the object in a fixed global coordinate system, you should think of transforming an object as moving (transforming) its local coordinate system

When using OpenGL, we need to think of object transformations as moving (transforming) its local coordinate frameAll the transformations are performed relative to the current coordinate frame origin and axes OpenGL Transformation

Translate Coordinate FrameTranslate (3,3)?

Translate Coordinate Frame (2)Translate (3,3)?

Rotate Coordinate Frame Rotate 30 degree?

Scale Coordinate FrameScale (0.5,0.5)?

Compose Transformations(7,9)45oAnswer:

Translate(7,9)Rotate 45Scale (2,2) Transformations?

Another example How do you transform from C1 to C2?Translate (5,5) and then Rotate (60)

OR

Rotate (60) and then Translate (5,5) ???Answer: Translate(5,5) and then Rotate (60) C1C2

Another example (contd)If you Rotate(60) and then Translate(5,5) 55C1C2You will be translated (5,5)relative to C2!!

Transform ObjectsWhat does coordinate frame transformations have to do with object transformations? You can view transformations as tying the object to a local coordinate frame and moving that coordinate frame

Put it all together When you use OpenGL Think of transformation as moving coordinate frames Call OpenGL transformation functions in that order OpenGL will actually perform the transformations in the reverse order Everything will be just right!!!