OpenGL - How to draw each nth triangle using glVertexAttribPointer

Question

I have a vertex buffer. There are situations when I don't want to render the whole mesh, but let's say each nth triangle of the mesh. I am using VAOs and VBOs.

My data in the buffer is like this:

first triangle indx        seciond triangle indx
0 1 2   3 4 5   6 7 8    | 9 10 11   12 13 14   15 16 17 ...

What I want to do is - render first triangle, then skip second, then render 3d and so on. Can it be done with adjusting glVertexAttribPointer() or are there some other gl methods that can manipulate the data like that, or I have to create a new buffer with skipped triangles ?

Answer 1

I have no idea why would you want to draw each n-th triangle, and I cannot imagine a situation in which you would want to do that.

...but supposing you do have a legitimate reason to do that...

I would say you can't, because glVertexAttribPointer is used to set the layout for the data at the vertex level, not the triangle level. In fact, OpenGL has no idea of how are you going to use the vertices inside the vertex buffer until you make a draw call.

I suppose you were thinking about playing with the stride parameter, but once again, glVertexAttribPointer works at the vertex level, so the most you can do is make triangles out of each n-th vertex, which is not what you want.

Short of creating a new vertex buffer, you can handle this when making the draw call. glDrawArrays takes the amount of triangles you want to draw and the offset from which you want to draw them, so you would have to do something like

for (int k = 0; k < totalTriangleCount; k += n)
    glDrawArrays(GL_TRIANGLES, k * 3, 3);

You might be tempted to think that this is a performance killer, but it's most likely not. You will most likely not be issuing an OpenGL flush on each glDrawArrays call, so I wouldn't say performance would drop noticeably. At least compared with creating and uploading a new vertex buffer each frame.

Now, the real question is "why would you want to do such a thing?", but we may never know...

Answer 2

you can step away from glDrawArrays and instead use glDrawElements and use a GL_ELEMENT_ARRAY_BUFFER​ buffer that hold the indexes of the vertexes you want to draw

the element buffer would then contain {0,1,2,9,10,11,...} and so on.

In you comments I see that you want this to draw a "light" object for quick updates, you can add all other triangles after the first third and draw all triangles that way