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Matrix Stacks


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Posted 16 December 2001 - 02:46 PM

I''m having trouble understanding this concept, does someone know of a good tutorial that might explain this graphically? I know glPushMatrix() puts the matrix on the stack and glPopMatrix() takes it off, but in the examples of code I''m reading, I don''t understand why they are pushing an object onto the stack, then calling PopMatrix right after that. Wouldn''t that just take it right back off?

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Posted 16 December 2001 - 04:19 PM

Yes it would. Push saves the matrix and pop restores it. If you see a push followed by a pop with no code between was probably some object removed but the push/pop was forgotten.

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Posted 16 December 2001 - 04:36 PM

Actually, they call a push, create the object, then call a pop. I must not understand the matrix stack, b/c in my mind, this would put it on the matrix stack, then take it off. So, I''m just not getting something.

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Posted 16 December 2001 - 04:39 PM

Yes the matrix stack would be unchanged but they are temporary saving the current matrix. Pop does not just remove it from the stack but also load the current matrix with the top of the stack.

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Posted 16 December 2001 - 07:20 PM

ok, one last question to really help me understand this. I''m typing the Robot example in Gamedev''s OpenGL book if your wondering. Anyway, it calls a push, then it creates a head, torso, then it calls another push, creates one arm, then calls a pop. Why isn''t it calling a pop after each object?

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Posted 16 December 2001 - 07:54 PM

They do that so the hand will move with the arm, but not the legs. Make since?

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Posted 16 December 2001 - 08:42 PM

From http://www.cs.rutgers.edu/~decarlo/442-F99/opengl.html

Matrix stacks and transformations

OpenGL maintains several matrix stacks. The top matrix on each stack is used in the transformations performed by OpenGL. In this course, we will be using the modeling/viewing stack (GL_MODELVIEW) and the camera projection stack (GL_PROJECTION). Which one of these to use is specified using the glMatrixMode function. Other matrix stack functions that are commonly used include: glPushMatrix Adds a matrix to the current matrix stack (duplicating the previous matrix at the top of the stack)
glPopMatrix Discards the current (topmost) matrix on the current stack
glLoadIdentity Replaces the current matrix with the Identity matrix
Geometric transformations modify the current matrix. These functions include: glTranslate, glRotate and glScale.

Viewing transformations (orthographic and projective) modify the current matrix (typically the mode will be GL_PROJECTION in this case). These functions include: glOrtho, glFrustum, gluOrtho2D and gluPerspective.

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Posted 16 December 2001 - 08:58 PM

quote:
Original post by Floppy
They do that so the hand will move with the arm, but not the legs. Make since?


In the sequence I listed, there is no hand. Only a head, a torso, and a arm. Do you mean arm with the torso? If so, after they call that pop, they call a push, create arm #2 and call another pop, so that doesn''t seem to have anything to do with the torso. But the entire robot creation is nested inside a push and pop, so maybe that ties the whole thing together or something.

Is this easy for everyone to understand? I understand every line of code except this push and pop thing. I am just not visualizing it right or something.


subnet_rx

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Posted 16 December 2001 - 09:08 PM

Okay.

The whole reason for matrix stacks is so you can save your transformation matrix, alter it, render something, then restore it.

Push. This makes a copy of the current matrix on the stack, so you can alter it and restore it later.

Pop. This restores the old matrix.

Imagine drawing an arm. You want the forearm to be drawn RELATIVE to the upper arm.

Draw upper arm.
Push.
Rotate by the forearm rotation.
Draw the forearm.
Pop.

It really is that simple.




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