OpenGL ES 2.0, drawing using multiple vertex buffers

Question

I can't find much info on whether drawing from multiple vertex buffers is supported on opengl es 2.0 (i.e use one vertex buffer for position data and another for normal, colors etc). This page http://developer.apple.com/library/ios/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/TechniquesforWorkingwithVertexData/TechniquesforWorkingwithVertexData.html and listing 9.4 in particular implies you should be able to, but I can't get it to work on my program. Code for the offending draw call:

glBindBuffer(GL_ARRAY_BUFFER, mPositionBuffer->openglID);
glVertexAttribPointer(0, 4, GL_FLOAT, 0, 16, NULL);
glEnableVertexAttribArray(0);

glBindBuffer(GL_ARRAY_BUFFER, mTexCoordBuffer->openglID);
glVertexAttribPointer(1, 2, GL_FLOAT, 0, 76, NULL);
glEnableVertexAttribArray(1);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, mIndexBuffer->openglID);
glDrawElements(GL_TRIANGLES, 10788, GL_UNSIGNED_SHORT, NULL);

This draw call will stall or crash with EXC_BAD_ACCESS on the simulator, and gives very weird behavior on the device (opengl draws random triangles or presents previously rendered frames). No opengl call ever returns an error, and I've inspected the vertex buffers extensively and am confident they have the correct sizes and data.

Has anyone successfully rendered using multiple vertex buffers and can share their experience on why this might not be working? Any info on where to start debugging stalled/failed draw calls that don't return any error code would be greatly appreciated.

Answer 1

Access violations generally mean that you are trying to draw more triangles than you have allocated in a buffer. The way you've set up buffers is perfectly fine and should work, I would be checking if your parameters are set properly:

http://www.opengl.org/sdk/docs/man/xhtml/glVertexAttribPointer.xml http://www.opengl.org/sdk/docs/man/xhtml/glDrawElements.xml

I think your issue is either that you've switched offset and stride in your glVertexAttribPointer calls, or you've miscounted the number of indices you're drawing

Answer 2

Yes, you can use multiple vertex buffer objects (VBOs) for a single draw. The OpenGL ES 2.0 spec says so in section 2.9.1.

Do you really have all those hard-coded constants in your code? Where did that 76 come from?

If you want help debugging, you need to post the code that initializes your buffers (the code that calls glGenBuffers and glBufferData). You should also post the stack trace of EXC_BAD_ACCESS.

It might also be easier to debug if you drew something simpler, like one triangle, instead of 3596 triangles.