Yes, you're doing it wrong. You don't pass OpenGL textures for sampler2Ds, you're supposed to pass texture units. So your code should look a little like this:

glActiveTexture(0);
glBindTexture(bmp1);
glActiveTexture(1);
glBindTexture(bmp2);
glUniform1i(blah, 0);
glUniform1i(blah2, 1);

If you had used the shader addon, this would be done for you .

Someone might wanna tell that to whoever wrote ex_opengl_pixel_shader.c to go with the A5 examples.

However, even after making this change it still isn't working. It appears as though texture 0 is always the bitmap Allegro is drawing from and anything beyond that is cleared away. If I try assigning a texture to ID 1, nothing goes through, yet even if I assign it to ID 1 then pass ID 0 to glUniform1iARB(), it goes through fine, but then I'm still left only able to get one texture through the pipeline and not two.

I'm using 5.0.5 until 5.1 is considered a stable release with pre-compiled MSVC10 binaries and such, so I don't have access to the shader addon.

EDIT: *looks through Dario's situation* So... waitaminute... is the solution nothing more than just manually drawing the texture and skipping on Allegro's drawing functions?

*tries this, will be back momentarily*

After much tweaking, I finally have it working, WITH the built-in Allegro functions!

Basically, once I enable the fragment shader, I have to do this:

  glActiveTexture(GL_TEXTURE0);
  glBindTexture(GL_TEXTURE_2D,al_get_opengl_texture(_bitmap_1));
  loc = glGetUniformLocationARB(sceneblend,"Bmp1");
  glUniform1iARB(loc,0);
  glActiveTexture(GL_TEXTURE1);
  glBindTexture(GL_TEXTURE_2D,al_get_opengl_texture(_bitmap_2));
  loc = glGetUniformLocationARB(sceneblend,"Bmp2");
  glUniform1iARB(loc,1);
  glActiveTexture(GL_TEXTURE0);

Then I can call the Allegro function to draw the bitmap to the screen and it will process both textures through the pipeline.

After a whole bunch of tweaking, the real trick to this entire mess was, no matter what texture IDs you use, to specifically call glActiveTexture(GL_TEXTURE0) after setting up both textures. Failing to do this throws the entire thing out of whack, even when trying to handle everything manually through OpenGL calls.

At first it didn't work and I was spending nearly half an hour trying everything until I realized I forgot to turn the fragment shader off afterwards, thus it was getting used to render the scene elements themselves, which obviously wasn't going to work properly.

And of course, my sceneblend shader isn't perfect and needs tweaking. Ah well, the fact that it's working means tweaking it will be simple enough.

Thanks, guys!