Efficiency of multple glDrawElements commands

[letslearn]

Ah, did not know about that. I was wondering why uniforms cannot be changed within a draw call. I'm assuming the OpenGL implementation takes care of that post-T&L buffer optimized triangle list itself, without exposing it to the user.

I'm guessing glDrawElementsInstanced cannot take advantage of the post-T&L buffer, as it reads a different gl_InstanceID for each draw.

[Alfonse Reinheart]

I was wondering why uniforms cannot be changed within a draw call.

How could you change them within a draw call? The only functions that change uniforms are API functions. And a draw call is a single, atomic API call.

And glBegin/glEnd expressly forbid calling most functions between them, glUniform included.

I'm guessing glDrawElementsInstanced cannot take advantage of the post-T&L buffer, as it reads a different gl_InstanceID for each draw.

It can't use it for vertices between instances. But it certainly can use it for vertices within an instance.

[letslearn]

How could you change them within a draw call? The only functions that change uniforms are API functions. And a draw call is a single, atomic API call.

I was wondering why shaders themselves could not modify the uniform values they read.

It can't use it for vertices between instances. But it certainly can use it for vertices within an instance.

Ah that makes a lot more sense than what how I thought glDrawElementsInstanced works.

[Alfonse Reinheart]

I was wondering why shaders themselves could not modify the uniform values they read.

Because then they wouldn't be uniform

[thokra]

I was wondering why shaders themselves could not modify the uniform values they read.

A more pressing question is: Why would you want to modify values that are inherently constant across a primitive, across multiple objects, across the whole frame or over the whole runtime?

[letslearn]

Thokra, I can't precisely remember the reason why I wanted such functionality. I think I wanted to emulate glDrawElementsInstaced's capabilities using glDrawElements (in case OpenGL 3.x is not available on the target system) and this involved me using uniforms as "loop counters". Looking back, this entire idea doesn't even make sense in my head, so it's probably best to forget about it.

That said, I don't even know if shaders being able to modify uniforms is even needed for that.

[mbentrup]

You should read the pseudo instancing paper from NVidia: http://developer.download.nvidia.com...udo_instancing

[Alfonse Reinheart]

It should be noted that this paper is from the pre-DX10 days, and it is not known how efficient this is for hardware that isn't a GeForce FX or GeForce 6xxx.

Someone should get an OpenGL performance test suite together.

[mbentrup]

Hardware that is OpenGL 3 capable should use native instancing, so pseudo instancing is only a fallback for those old chips anyway.

This is especially nice as a fallback for GL_ARB_instanced_arrays (pseudocode):

<bind non-instanced attribs>
if( GL_ARB_instanced_arrays_available) {
  glEnableVertexAttribArray(<instanced attr>);
  glVertexAttribPointer(<instanced attr>, 4, GL_FLOAT, 0, 0, attribs);
  glVertexAttribDivisorARB(<instanced attr>, 1);
  glDrawElementsInstancedARB(GL_TRIANGLES, num_instances, ...);
} else {
  glDisableVertexAttribArray(<instanced attr>);
  for(i = 0; i < num_instances; i++) {
    glVertexAttrib4fv(<instanced attr>, attribs[i]);
    glDrawElements(GL_TRIANGLES, ...)
  }
}

You application can use the identical shaders, VBOs etc.

+1 for the performance test suite.