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#4109 Fix inside-out geometry from negative scale transforms in GLTF loader #4156

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Merged
merged 2 commits into from
May 25, 2025
Merged

#4109 Fix inside-out geometry from negative scale transforms in GLTF loader #4156

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#4109 Fix inside-out geometry from negative scale transforms in GLTF …
marchcat May 24, 2025
f8e702b
#4109 Fix GLTF model extents calculation
marchcat May 24, 2025
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23 changes: 21 additions & 2 deletions indra/newview/gltf/llgltfloader.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -262,6 +262,9 @@ bool LLGLTFLoader::populateModelFromMesh(LLModel* pModel, const LL::GLTF::Mesh&
// Combine transforms: coordinate rotation applied to hierarchy transform
const glm::mat4 final_transform = coord_system_rotation * hierarchy_transform;

// Check if we have a negative scale (flipped coordinate system)
bool hasNegativeScale = glm::determinant(final_transform) < 0.0f;

// Pre-compute normal transform matrix (transpose of inverse of upper-left 3x3)
const glm::mat3 normal_transform = glm::transpose(glm::inverse(glm::mat3(final_transform)));

Expand Down Expand Up @@ -429,9 +432,22 @@ bool LLGLTFLoader::populateModelFromMesh(LLModel* pModel, const LL::GLTF::Mesh&
vertices.push_back(vert);
}

for (U32 i = 0; i < prim.getIndexCount(); i++)
// When processing indices, flip winding order if needed
for (U32 i = 0; i < prim.getIndexCount(); i += 3)
{
indices.push_back(prim.mIndexArray[i]);
if (hasNegativeScale)
{
// Flip winding order for negative scale
indices.push_back(prim.mIndexArray[i]);
indices.push_back(prim.mIndexArray[i + 2]); // Swap these two
indices.push_back(prim.mIndexArray[i + 1]);
}
else
{
indices.push_back(prim.mIndexArray[i]);
indices.push_back(prim.mIndexArray[i + 1]);
indices.push_back(prim.mIndexArray[i + 2]);
}
}

// Check for empty vertex array before processing
Expand Down Expand Up @@ -572,6 +588,9 @@ bool LLGLTFLoader::populateModelFromMesh(LLModel* pModel, const LL::GLTF::Mesh&
}
}

// Call normalizeVolumeFaces to compute proper extents
pModel->normalizeVolumeFaces();

// Fill joint names, bind matrices and prepare to remap weight indices
if (skinIdx >= 0)
{
Expand Down