[visionegg] Re: GL state responsibility (warning: may contain theory)

Sorry about the paragraphs getting messed up...

Tony Arkles wrote:


The bug I discovered last week was that changes to the modelview matrix
state would, against the above stated policy, cause other stimuli to
draw wrongly. So, I updated those builtin stimuli which modified it to
restore it after use.  If we want to live  by the documentation as
written, my approach is just a workaround, and I need to fix all stimuli
to be insensitive to the state of the modelview matrix when their draw()
method is called.



The modelview matrix state is an aspect that I just stumbled onto, and I found something somewhat worrisome.

http://www.opengl.org/resources/faq/technical/viewing.htm (section 8.030)
http://sjbaker.org/steve/omniv/projection_abuse.html

These both seem to say that all of the camera movements should be taking place in the modelview matrix and not the projection matrix. This came up when I started using my custom-made viewport that does the mapping for our screen. It involves several camera modifications to capture views at different perspectives. I put a Model3DS into the scene and put it on a circular path to make sure that it properly moves from camera to camera, only to find that it showed up in every camera at the same time.


I *think*, after preliminary analysis, that a simple yet tedious modification would fix this problem without modifying existing behaviour. I'm willing to put the time into making this modification assuming that it's deemed a legitimate concern. I think that if we were to modify every stimulus and remove the glLoadIdentity(), and then in the Viewport class wrap stimulus.draw() with a glPushMatrix() and glPopMatrix(), it would be possible to do everything as we are right now without having to modify the GL_PROJECTION matrix to point the camera.


Any thoughts? This may not be a complete analysis... I've just spent an hour or two tracing through (mostly my) code to figure out what the problem is. As I said before though, I'm not adverse to testing and fixing this problem myself. I've used the library a fair bit this summer already and I'd love to contribute back :)

Tony
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