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# Objective - Fixes #4019 - Fix lighting of double-sided materials when using a negative scale - The FlightHelmet.gltf model's hose uses a double-sided material. Loading the model with a uniform scale of -1.0, and comparing against Blender, it was identified that negating the world-space tangent, bitangent, and interpolated normal produces incorrect lighting. Discussion with Morten Mikkelsen clarified that this is both incorrect and unnecessary. ## Solution - Remove the code that negates the T, B, and N vectors (the interpolated world-space tangent, calculated world-space bitangent, and interpolated world-space normal) when seeing the back face of a double-sided material with negative scale. - Negate the world normal for a double-sided back face only when not using normal mapping ### Before, on `main`, flipping T, B, and N <img width="932" alt="Screenshot 2022-08-22 at 15 11 53" src="https://user-images.githubusercontent.com/302146/185965366-f776ff2c-cfa1-46d1-9c84-fdcb399c273c.png"> ### After, on this PR <img width="932" alt="Screenshot 2022-08-22 at 15 12 11" src="https://user-images.githubusercontent.com/302146/185965420-8be493e2-3b1a-4188-bd13-fd6b17a76fe7.png"> ### Double-sided material without normal maps https://user-images.githubusercontent.com/302146/185988113-44a384e7-0b55-4946-9b99-20f8c803ab7e.mp4 --- ## Changelog - Fixed: Lighting of normal-mapped, double-sided materials applied to models with negative scale - Fixed: Lighting and shadowing of back faces with no normal-mapping and a double-sided material ## Migration Guide `prepare_normal` from the `bevy_pbr::pbr_functions` shader import has been reworked. Before: ```rust pbr_input.world_normal = in.world_normal; pbr_input.N = prepare_normal( pbr_input.material.flags, in.world_normal, #ifdef VERTEX_TANGENTS #ifdef STANDARDMATERIAL_NORMAL_MAP in.world_tangent, #endif #endif in.uv, in.is_front, ); ``` After: ```rust pbr_input.world_normal = prepare_world_normal( in.world_normal, (material.flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u, in.is_front, ); pbr_input.N = apply_normal_mapping( pbr_input.material.flags, pbr_input.world_normal, #ifdef VERTEX_TANGENTS #ifdef STANDARDMATERIAL_NORMAL_MAP in.world_tangent, #endif #endif in.uv, ); ``` |
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