Re: [Mesa3d-users] Mesa/GL Contexts, glGenTextures and more

[Brian P. <bri...@tu...>]

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