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View Full Version : Reason why opengl transform in reverse?

Sarah22
02-07-2010, 12:58 AM
Is it because when I have a point P in space. If I rotate, then scale it. OpenGl reads it like this

P' = Rotate()P
P'' = Scale()P'
P'' = Scale()Rotate()P

So starting from left it scale then rotate point P.

Can anyone here enlighten me.

Thank you very much.

zeoverlord
02-07-2010, 08:20 AM
Yea, It has to do how openGL deals with matrix multiplication.
In your example you should remember that the first line essentially does this
ModelviewMatrix=RotationMatrix*ModelviewMatrix
then that also means that the second line does this
ModelviewMatrix=ScaleMatrix*ModelviewMatrix
put these two together and you will have
ModelviewMatrix=ScaleMatrix*(RotationMatrix*Modelv iewMatrix)

Now this might be confusing at the beginning, but that's the way it works, fortunately starting with openGL 3.0 you can do it anyway you like.

overlay
02-07-2010, 08:27 AM
OpenGL 3.2 compatibility profile spec: http://www.opengl.org/registry/doc/glspec32.compatibility.20091207.pdf

page 62:
"If C is the current matrix and M is the matrix pointed to by MultMatrix's argument, then the resulting current matrix, C' , is
C' = C · M.
"

page 63:
"Rotate, Translate, Scale, Frustum, and Ortho manipulate the current matrix. Each com-
putes a matrix and then invokes MultMatrix with this matrix."

marshats
02-07-2010, 12:00 PM
So starting from left it scale then rotate point P.

Sound like semantics but for me it makes more sense to say openGL matrix operations occur from right-to-left (or bottom-to-top in C-code).

For instance say you have code like

glMatrixMode(GL_MODELVIEW);
glLoadIdentity(); // load Matrix => MV=I
glTranslatef(car_x_pos, car_y_pos, car_z_pos); // times Matrix T => MV=I*T
glRotatef(RAD_TO_DEG * car_direction, 0.0, 0.0, -1.0); // times Matrix R => MV=I*T*R
glScalef(Sx,Sy,Sz); // times Matrix S => MV=I*T*R*S
draw(P); // draw points P'= MV*P = I*T*R*S*P

The effective operation order is of scaling first, then rotation, then translating. I think of operations going from bottom-to-top in openGL code which corresponds to right-to-left in matrix operations as

P' = I*T*R*S*P