Fix inverted normal mapping on WebGPU (derivative TBN)#9099
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The derivative-based tangent frame (TBN.js) is scaled by the tbnBasis uniform, which only accounted for the projection Y flip applied when rendering with flipY. On WebGPU, screen-space dpdy has the opposite sign to WebGL's dFdy (framebuffer space is Y-down), which negates the computed tangent and bitangent and rotates the tangent-space normal 180 degrees, producing inverted normal-mapped lighting for meshes without vertex tangents. Fold the backend into the sign so it stays consistent. Fixes #5735
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Jul 22, 2026
The derivative-based tangent frame (TBN.js) is scaled by the tbnBasis uniform, which only accounted for the projection Y flip applied when rendering with flipY. On WebGPU, screen-space dpdy has the opposite sign to WebGL's dFdy (framebuffer space is Y-down), which negates the computed tangent and bitangent and rotates the tangent-space normal 180 degrees, producing inverted normal-mapped lighting for meshes without vertex tangents. Fold the backend into the sign so it stays consistent. Fixes #5735 Co-authored-by: Martin Valigursky <mvaligursky@snapchat.com>
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Fixes #5735, fixes #6984, fixes #8905 — inverted normal-mapped lighting on WebGPU for meshes that have no vertex tangents (the tangent frame is derived from screen-space derivatives). Confirmed against the Khronos NormalTangentTest — WebGPU now matches WebGL2 and the reference. (#6984 and #8905 are duplicate reports of the same issue.)
Cause:
TBN.jsis scaled by thetbnBasisuniform, which was set purely from the render target'sflipY(originally added to correct offscreen render-to-texture on WebGL).dpdyhas the opposite sign to WebGL'sdFdy. This negates the computed tangent and bitangent, rotating the tangent-space normal 180° and producing inverted lighting.Changes:
Renderer.setCameraUniforms: fold the backend into thetbnBasissign —(device.isWebGPU !== !!flipY) ? -1 : 1. WebGL behavior is unchanged; WebGPU picks up the extra inherent flip.Notes: