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[Fix] (DirectVulkan): report the index the arbitrary-restart rewrite cannot represent instead of silently drawing a different vertex

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Swung0x48
2026-08-27 03:18:10 -04:00
parent 90b7a689c5
commit eb9e4fdac1
1 changed files with 15 additions and 4 deletions
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MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.cpp
+15 -4
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@@ -3916,10 +3916,15 @@ void main() {
// Copies index data, replacing every occurrence of the application's arbitrary restart
// index with the fixed all-ones value of the index type - the only one Vulkan restarts
// on. An index that already equals the fixed value would then be indistinguishable from
// a restart, so it is nudged to the next-lowest value: it can only be a real index (the
// application's restart index is a different number), and the vertex it selects is
// outside any well-defined draw anyway, whereas leaving it alone would tear the
// primitive in two.
// a restart, so it is nudged to the next-lowest value, which silently draws the wrong
// vertex. That is a real (if narrow) loss and it is reported once rather than left
// invisible; DirectGLES avoids it for 8- and 16-bit indices by widening the copy instead,
// and the same treatment here is follow-up work.
//
// The caller guarantees applicationRestartIndex fits the index type, so no truncating
// cast is needed - and none may be used: truncating turns glPrimitiveRestartIndex(0x100)
// over 8-bit indices into "restart on index 0", which shreds every primitive that
// references vertex 0.
void RewriteRestartIndices(const void* source, SizeT sizeBytes, VkIndexType indexType,
Uint32 applicationRestartIndex, Vector<Uint8>& output) {
output.resize(sizeBytes);
@@ -3933,6 +3938,12 @@ void main() {
if (indices[i] == static_cast<decltype(fixedMax)>(applicationRestartIndex)) {
indices[i] = fixedMax;
} else if (indices[i] == fixedMax) {
MGLOG_E_ONCE("GL_PRIMITIVE_RESTART with restart index %u over index data that also uses "
"the all-ones index %u: both cannot be spelled at this index width, so every "
"all-ones index is drawn one vertex lower. Use "
"GL_PRIMITIVE_RESTART_FIXED_INDEX, or keep the all-ones value out of the "
"index data.",
applicationRestartIndex, static_cast<Uint32>(fixedMax));
indices[i] = fixedMax - 1;
}
}