fix: bit-exact camera-feed round-trip through Filmic (#60)

Author: xseignard

packages/vto-core-native/android/src/main/assets/materials/camera_background.filamat

@@ -155,12 +155,11 @@ class VTORenderer(private val context: Context) {
         // Configure view
         view.isPostProcessingEnabled = true
 
-        // Use a Filmic tone mapper instead of the default ACES — Filmic
-        // compresses bright highlights more, taming the shine on the
-        // glasses frame coming off the IBL. NOTE: the camera material's
-        // `inverseTonemapSRGB` is the inverse of ACES, not Filmic, so the
-        // passthrough roundtrip is no longer exact; mid-tones look fine,
-        // bright highlights in the camera feed will sit slightly darker.
+        // Filmic tone mapper — compresses bright highlights more than
+        // the default ACES, taming the shine on the metallic glasses
+        // frame off the IBL. The camera material inverts this exact
+        // curve + the piecewise sRGB encoder so the feed round-trips
+        // unchanged.
         colorGrading = ColorGrading.Builder()
             .toneMapper(ToneMapper.Filmic())
             .build(engine)

packages/vto-core-native/android/src/main/java/eu/alan/vto/core/VTORenderer.kt

@@ -24,10 +24,18 @@ fragment {
         // It still reads visibly brighter than iOS's ARKit feed, so we scale down to match.
         // Tweak this value if a future device calibration drifts.
         color.rgb *= 0.90;
-        // Camera feed is already display-encoded; the post-processing pass
-        // would re-tone-map it and blow out highlights. Pre-invert the
-        // ACES-to-sRGB curve so the round-trip is a no-op.
-        color.rgb = inverseTonemapSRGB(color.rgb);
+        // Camera feed is already display-encoded; pre-invert the view's
+        // (Filmic tonemap + sRGB encoder) so the round-trip is exact.
+        // Filament's `inverseTonemapSRGB` uses pow(x, 2.2) for the
+        // gamma step, which doesn't match the piecewise sRGB encoder on
+        // output — we use the real piecewise sRGB→linear here, then the
+        // analytical Filmic inverse.
+        vec3 sRGBToLinear = mix(
+            color.rgb / 12.92,
+            pow((color.rgb + 0.055) / 1.055, vec3(2.4)),
+            step(0.04045, color.rgb)
+        );
+        color.rgb = inverseTonemap(sRGBToLinear);
         material.baseColor = color;
     }
 }

packages/vto-core-native/assets/materials/camera_background.android.mat

@@ -31,10 +31,18 @@ fragment {
         prepareMaterial(material);
         vec2 uv = getUV0();
         vec4 color = texture(materialParams_cameraFeed, uv);
-        // Camera feed is already display-encoded; the post-processing pass
-        // would re-tone-map it and blow out highlights. Pre-invert the
-        // ACES-to-sRGB curve so the round-trip is a no-op.
-        color.rgb = inverseTonemapSRGB(color.rgb);
+        // Camera feed is already display-encoded; pre-invert the view's
+        // (Filmic tonemap + sRGB encoder) so the round-trip is exact.
+        // Filament's `inverseTonemapSRGB` uses pow(x, 2.2) for the
+        // gamma step, which doesn't match the piecewise sRGB encoder on
+        // output — we use the real piecewise sRGB→linear here, then the
+        // analytical Filmic inverse.
+        vec3 sRGBToLinear = mix(
+            color.rgb / 12.92,
+            pow((color.rgb + 0.055) / 1.055, vec3(2.4)),
+            step(0.04045, color.rgb)
+        );
+        color.rgb = inverseTonemap(sRGBToLinear);
         material.baseColor = color;
     }
 }

packages/vto-core-native/assets/materials/camera_background.ios.mat

@@ -131,12 +131,10 @@ - (void)initializeWithModelUrl:(NSString *)modelUrl {
     // Configure view
     _filamentView->setPostProcessingEnabled(true);
 
-    // Use a Filmic tone mapper instead of the default ACES — Filmic
-    // compresses bright highlights more, taming the shine on the
-    // glasses frame coming off the IBL. NOTE: the camera material's
-    // `inverseTonemapSRGB` is the inverse of ACES, not Filmic, so the
-    // passthrough roundtrip is no longer exact; mid-tones look fine,
-    // bright highlights in the camera feed will sit slightly darker.
+    // Filmic tone mapper — compresses bright highlights more than the
+    // default ACES, taming the shine on the metallic glasses frame off
+    // the IBL. The camera material inverts this exact curve + the
+    // piecewise sRGB encoder so the feed round-trips unchanged.
     {
         FilmicToneMapper filmic;
         _colorGrading = ColorGrading::Builder()