David Eger wrote:
> Dear Mesa Users,
>
> I have what I expect is a typical usage scenario, but I'm having a
> hard time getting all of the pieces to fall into place. I would like
> to use Mesa to load up many textures and models and then use them to
> render images of various resolutions on user request. My first
> attempt to order things was as follows:
>
> [A] context = OSMesaCreateContextExt(OSMESA_RGBA, 0, 0, 0, NULL);
>
> [B] glGenTextures(1, &tex_id1); glBindTexture(GL_TEXTURE_2D, tex_id1); ...
> glGenTextures(1, &tex_id2); glBindTexture(GL_TEXTURE_2D, tex_id2); ...
>
> [C] OSMesaMakeCurrent(
> context, render_buffer, GL_UNSIGNED_BYTE,
> w, h),
>
> [D] glMatrixMode(GL_PROJECTION);
> glLoadIdentity();
> gluOrtho2D(0.0, (GLfloat)w, 0.0, (GLfloat)h);
> glMatrixMode(GL_MODELVIEW);
> glLoadIdentity();
> glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
> glBegin(QUADS);...glEnd(); glFlush();
> glReadPixels()..
>
> However, I found that repeated calls to glGenTextures() all yielded
> texture id's of 0.
Correct, you must have a currently bound context before calling any gl
function.
> I then moved the ordering to the following:
>
> [A] context = OSMesaCreateContextExt(OSMESA_RGBA, 0, 0, 0, NULL);
>
> [C1] OSMesaMakeCurrent(
> context, render_buffer, GL_UNSIGNED_BYTE,
> 1, 1),
>
> [B] glGenTextures(1, &tex_id1); glBindTexture(GL_TEXTURE_2D, tex_id1); ...
> glGenTextures(1, &tex_id2); glBindTexture(GL_TEXTURE_2D, tex_id2); ...
>
> [C2] OSMesaMakeCurrent(
> context, render_buffer, GL_UNSIGNED_BYTE,
> w, h),
>
> [D] glMatrixMode(GL_PROJECTION);
> glLoadIdentity();
> gluOrtho2D(0.0, (GLfloat)w, 0.0, (GLfloat)h);
> glMatrixMode(GL_MODELVIEW);
> glLoadIdentity();
> glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
> glBegin(QUADS);...glEnd(); glFlush();
> glReadPixels()..
>
> This "fixed" the repeated calls to glGenTextures - they produce
> distinct texture ids as advertised. However, C2 appears to have only
> partially worked. The glClear() does indeed wipe all pixels in the
> newly enlarged render buffer, but the output from glBegin()...glEnd()
> is confined to the original 1 pixel! Is it intended that one cannot
> re-bind a context created with OSMesaCreateContext(Ext) to multiple
> buffers over time?
The first time a rendering context is bound, the viewport and scissor
bounds are set to the window/drawable size. So the first call to
OSMesaMakeCurrent() is setting the viewport/scissor to 1x1. The second
call to OSMesaMakeCurrent() does not change the viewport/scissor.
The solution to your problem is to call glViewport(0,0,w,h) after the
2nd OSMesaMakeCurrent(). Or, pass w,h to the first call, instead of 1,1.
-Brian
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