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Assert when glDrawElements cannot draw anything because of missing vertex bounds #26525

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Assert when glDrawElements cannot draw anything because of missing vertex bounds #26525

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2 changes: 1 addition & 1 deletion src/lib/libglemu.js
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Original file line number Diff line number Diff line change
Expand Up @@ -3036,7 +3036,7 @@ var LibraryGLEmulation = {
// upload based on the indices. If they are in a buffer on the GPU, that is very
// inconvenient! So if you do not have an array buffer, you should also not have
// an element array buffer. But best is to use both buffers!
assert(!GLctx.currentElementArrayBufferBinding);
assert(!GLctx.currentElementArrayBufferBinding, 'must use array buffers when using element buffer');
#endif
for (var i = 0; i < numProvidedIndexes; i++) {
var currIndex = {{{ makeGetValue('ptr', 'i*2', 'u16') }}};
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4 changes: 4 additions & 0 deletions src/lib/libwebgl.js
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Expand Up @@ -575,6 +575,10 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
var cb = GL.currentContext.clientBuffers[i];
if (!cb.clientside || !cb.enabled) continue;

#if ASSERTIONS
assert(count || !GLctx.currentElementArrayBufferBinding, 'must use array buffers when using element buffer');
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@sbc100 sbc100 Mar 23, 2026

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Does this assert belong here inside this for loop? Could it not go outside the loop?

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@sbc100 sbc100 Mar 23, 2026

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Should this be !count || !GLctx.currentElementArrayBufferBinding ? (i.e. I'm not sure what the count || part is doing here.

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@paradust7 paradust7 Mar 25, 2026

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It is written as intended, but there may be an edge case I didn't consider.

preDrawHandleClientVertexAttribBindings is called in two places. It is called by glDrawArrays, where the vertex count is always passed by the user. And in glDrawElements, where the vertex count is computed if it can be, or otherwise passed as 0.

The error case is only for glDrawElements, when the vertex count cannot be computed, so that the count is passed as 0, even though the real count is non-zero.

I added the "count ||" to make sure the assert doesn't fire for glDrawArrays. But it could still fire if glDrawArrays is called with a count of 0.

I'll get a fix for this ready.

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@juj juj Mar 30, 2026 •
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Pondering on this a bit, it does seem that

assert(count == 0 || !GLctx.currentElementArrayBufferBinding, 'must use array buffers when using element buffer');

would work well.

With

assert(count || !GLctx.currentElementArrayBufferBinding, 'must use array buffers when using element buffer');

the assert would only ever fire if count == 0 && index buffer bound (i.e. we are not drawing anything and there is an index buffer bound)

The assert logic in general that verifies that there cannot be a GPU-side index buffer bound when the rendering has any CPU-side vertex attribute pointers seems sound. (we'd have to track the min/max vertex index at buffer upload time, like browsers internally also do).

Does this assert belong here inside this for loop?

Yeah, it must be inside the loop, since the assert only applies if there is at least one vertex attribute stream found that has a client-side buffer pointer (i.e. attribute stream index for which cb.clientside == true)

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@paradust7 paradust7 Mar 31, 2026

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@juj The original issue explains the case I'm trying to fix:

#26460

The problem case occurs when the element buffer is not client-side, but one or more array buffers is. In this situation, glDrawElements always passes count=0 to preDrawHandleClientVertexAttribBindings, because it doesn't know the real vertex count. This results in nothing being drawn.

I think it would be cleaner to pass count=null when the vertex bounds are unknown, and check for null explicitly in the loop. The only downside of this is that the null check has to occur even when assertions are disabled, but the check is pretty cheap.

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@juj juj Mar 31, 2026

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In this situation, glDrawElements always passes count=0 to preDrawHandleClientVertexAttribBindings, because it doesn't know the real vertex count. This results in nothing being drawn.

Ohh, gotcha. Looks good to me then. Might be good to drop this note as a comment before the assert?

#endif

GL.resetBufferBinding = true;

var size = GL.calcBufLength(cb.size, cb.type, cb.stride, count);
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