v0.21

This release contains improvements to the meshoptimizer library and many gltfpack enhancements! Notably, the introduction of sparse and absolute error simplification options in meshopt_simplify as well as per-vertex locking in meshopt_simplifyWithAttributes improves Nanite-style processing pipelines, meshlet optimization increases rasterization performance further on NVidia hardware, and gltfpack now preserves most data compression / optimization extensions which makes it easier to compose with other glTF processing tools.

Library improvements

• Introduce experimental meshopt_optimizeMeshlet algorithm that should be used to optimize individual meshlets for rendering efficiency (mostly affects NVidia hardware but can be used for other vendors as well)
• meshopt_simplifyWithAttributes now takes an additional optional array, vertex_lock, which allows specifying 1 for individual vertices that should not be moved during simplification (note that this is an interface change and will require source changes; the API continues to be experimental and may see further changes in future releases)
• meshopt_simplify options now support a new flag, meshopt_SimplifySparse, which significantly increases simplification performance when simplifying small subsets of a larger mesh (note that the error in this mode is relative to the subset, as such use of meshopt_SimplifyErrorAbsolute is recommended for consistency)
• meshopt_simplify options now support a new flag, meshopt_SimplifyErrorAbsolute, which treats target error and the result error as absolute (in mesh coordinate space) instead of relative to mesh extents
• Simplifier now tries harder to prevent triangle flips at a minimal cost to simplification performance and efficiency
• meshopt_optimizeVertexCache now needs ~5% less memory and is ~7% faster on Windows
• MESHOPT_SOVERSION CMake option can be used to customize .so version for shared library builds

gltfpack improvements

• Implement support for KHR_texture_basisu and EXT_texture_webp extensions in the input files; the extensions and images will be preserved as is regardless of compression flags
• Implement support for EXT_mesh_gpu_instancing extension in the input files; instanced meshes will be preserved regardless of -mi flag
• Implement support for EXT_meshopt_compression extension in the input files; files using this extension will be automatically decompressed (and recompressed if -c/-cc is specified)
• Implement support for KHR_materials_dispersion extension
• Implement attribute aware simplification: when -sv flag is specified, vertex colors and normals are used during simplification to improve appearance
• Image names are now preserved when present in the input file
• Improve texture compression when using UASTC format: textures are now a little smaller by default, and using low quality levels via -tq allows to push compressed size further down at the expense of some quality loss
• Improve behavior of simplification with extremely low factors (eg -si 0)
• Improve behavior of quantization when the input file is already quantized
• Improve behavior for textures that are missing on disk when the output file is a GLB
• Automatically fix incorrect MIME type for PNG/JPEG input textures
• Normal attributes are removed for meshes that use unlit materials to reduce size
• Normals can be optionally stored without quantization by using -vnf flag (only needed for rare edge cases involving morph targets, prefer -vn to increase normal quality)

Thanks to @ruby0x1, @sciecode and @oisyn for contributions to this release!