the use of glDrawElementsBaseVertex()

[shapeare]

I am drawing an animated model of several frames. Say the model consists of 1000 vertices, and I want to render 5 frames.

I stored the vertex positions of the five frames in a large buffer, the size of which is sizeof(float) * 1000 * 3 * 5. During rendering I do not want to change the model color, so I stored the vertex color in another buffer which is of the size sizeof(float) * 1000 * 3.

But I have run into the problem of rendering it. I once thought of using glDrawElementsBaseVertex() that accepts an offset so I can render vertices of different frames, but the color array is shorter than the position array, so only the first frame has color associated with it. How can I solve this problem?

[Alfonse Reinheart]

I stored the vertex positions of the five frames in a large buffer, the size of which is sizeof(float) * 1000 * 3 * 5. During rendering I do not want to change the model color, so I stored the vertex color in another buffer which is of the size sizeof(float) * 1000 * 3.

This situation has nothing to do with base vertex. You can't add a base index to only some of the attribute indices. Just as you can't have different indices for different attributes.

[shapeare]

Thanks for your reply.
What should I do? Allocate the same size of space as the position buffer for the color buffer and repeat the color value five times?
Is there any better solution for this?

[Dark Photon]

Could use a diff block or buffer for vtx attribs that chg vs stay the same. Then share those that are the same.

[mhagain]

Just change the offset in your glVertexAttribPointer calls for position. Color remains the same - offset 0 - but make a glVertexAttribPointer call for position with offset sizeof(float) * 1000 * 3 * framenum. You'd need to separate position and color into different VBOs, and with only 5 frames you could even create 5 separate VAOs, one for each frame.

[Alfonse Reinheart]

Allocate the same size of space as the position buffer for the color buffer and repeat the color value five times?

It depends: how much does the added performance of base vertex matter to you? It's a memory-for-performance tradeoff.

If this is a real issue, profile it and see what the difference is.

[shapeare]

Thanks for the replies.

Just change the offset in your glVertexAttribPointer calls for position. Color remains the same - offset 0 - but make a glVertexAttribPointer call for position with offset sizeof(float) * 1000 * 3 * framenum. You'd need to separate position and color into different VBOs, and with only 5 frames you could even create 5 separate VAOs, one for each frame.

Thank you very much. But I did not separate position and color into different VBOs. it works by only changing the offset for glVertexAttribPointer() of the position array before the rendering of each frame.

[aqnuep]

There is also another alternative:

If you want to keep the color the same but change the rest, you can put your color into another buffer that you bind as a texture buffer and fetch the color separately using texture buffer fetches in your vertex shader.