glDrawElements draws nothing when ELEMENT_ARRAY_BUFFER is bound, but ARRAY_BUFFER is not.

[paradust7]

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.3 (285c424)
clang version 23.0.0git (https:/github.com/llvm/llvm-project e5927fecf8a6ce89e1a4eac5b828e7d42676452a)
Target: wasm32-unknown-emscripten
Thread model: posix

Minimal example to demonstrate the problem visually

https://gist.github.com/paradust7/cd6610b76939b9756b6ee561a929164d

Compile with:

emcc -g -O0 -s USE_SDL=2 -s FULL_ES2=1 -s WASM=1 -s ALLOW_MEMORY_GROWTH=1 -o index.html minimal.c

And then serve with python3 -m http.server 8080 (or other).

Explanation

The problem originates from here:

emscripten/src/lib/libwebgl.js

Lines 3853 to 3908 in cb060ca

	glDrawElements: (mode, count, type, indices) => {
	#if FULL_ES2
	var buf;
	var vertexes = 0;
	if (!GLctx.currentElementArrayBufferBinding) {
	var size = GL.calcBufLength(1, type, 0, count);
	buf = GL.getTempIndexBuffer(size);
	GLctx.bindBuffer(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/, buf);
	GLctx.bufferSubData(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/,
	0,
	HEAPU8.subarray(indices, indices + size));
	// Calculating vertex count if shader's attribute data is on client side
	if (count > 0) {
	for (var i = 0; i < GL.currentContext.maxVertexAttribs; ++i) {
	var cb = GL.currentContext.clientBuffers[i];
	if (cb.clientside && cb.enabled) {
	let arrayClass;
	switch(type) {
	case 0x1401 /* GL_UNSIGNED_BYTE */: arrayClass = Uint8Array; break;
	case 0x1403 /* GL_UNSIGNED_SHORT */: arrayClass = Uint16Array; break;
	#if FULL_ES3
	case 0x1405 /* GL_UNSIGNED_INT */: arrayClass = Uint32Array; break;
	#endif
	default:
	GL.recordError(0x502 /* GL_INVALID_OPERATION */);
	#if GL_ASSERTIONS
	err('type is not supported in glDrawElements');
	#endif
	return;
	}
	vertexes = new arrayClass(HEAPU8.buffer, indices, count).reduce((max, current) => Math.max(max, current)) + 1;
	break;
	}
	}
	}
	// the index is now 0
	indices = 0;
	}
	// bind any client-side buffers
	GL.preDrawHandleClientVertexAttribBindings(vertexes);
	#endif
	GLctx.drawElements(mode, count, type, indices);
	#if FULL_ES2
	GL.postDrawHandleClientVertexAttribBindings(count);
	if (!GLctx.currentElementArrayBufferBinding) {
	GLctx.bindBuffer(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/, null);
	}
	#endif
	},

When !currentElementArrayBufferBinding, "vertexes" (the maximum vertex count) is calculated using the indices data. Otherwise, vertexes is never calculated and remains 0, so vertex data is never copied.

[kripken]

Thanks! I think you're right, though I'm not a GL expert.

Would you like to open a PR with a fix + this test?

[paradust7]

Thanks! I think you're right, though I'm not a GL expert.

Would you like to open a PR with a fix + this test?

Unfortunately I think the complete fix is unreasonably expensive.

The library would have to track the maximum index present in every element buffer, updating on calls to glBufferData/SubData. Since there are three index formats (byte/short/int), and the format is not known until the call to glDrawElements, each maximum would have to be calculated separately. But worse, since the new maximum after a glBufferSubdata can depend on values in the buffer outside the updated range, to maintain the correct answer would require keeping a copy of the entire buffer in javascript, even after it has been uploaded to the GPU.

I would be happy with a verbose assertion/GL error, since that saves future developers from having to debug this again. I will submit a PR for that if you agree?

[kripken]

Makes sense. Yes, a warning or an assert (not sure offhand which is better) behind the GL_DEBUG flag sounds good.