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Fix inverse tone mapping issues (google#4437)
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Bring color grading back into the Rec.709 color space to match
previous behaviors. This change also implements an exact inverse
tone map function for the "Filmic" operator.
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@romainguy
romainguy authored Aug 3, 2021
1 parent af6f19e commit 91b5cc4
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Showing 7 changed files with 29 additions and 23 deletions.
4 changes: 2 additions & 2 deletions docs/Materials.html
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Expand Up @@ -2639,8 +2639,8 @@
<tr><td style="text-align:left"> <strong class="asterisk">getUV0()</strong> </td><td style="text-align:center"> float2 </td><td style="text-align:left"> First interpolated set of UV coordinates, only available if the uv0 attribute is required </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">getUV1()</strong> </td><td style="text-align:center"> float2 </td><td style="text-align:left"> First interpolated set of UV coordinates, only available if the uv1 attribute is required </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">getMaskThreshold()</strong> </td><td style="text-align:center"> float </td><td style="text-align:left"> Returns the mask threshold, only available when <code>blending</code> is set to <code>masked</code> </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">inverseTonemap(float3)</strong> </td><td style="text-align:center"> float3 </td><td style="text-align:left"> Applies the inverse tone mapping operator to the specified linear sRGB color and returns a linear sRGB color. This operation may be an approximation </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">inverseTonemapSRGB(float3)</strong> </td><td style="text-align:center"> float3 </td><td style="text-align:left"> Applies the inverse tone mapping operator to the specified non-linear sRGB color and returns a linear sRGB color. This operation may be an approximation </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">inverseTonemap(float3)</strong> </td><td style="text-align:center"> float3 </td><td style="text-align:left"> Applies the inverse tone mapping operator to the specified linear sRGB color and returns a linear sRGB color. This operation may be an approximation and works best with the “Filmic” tone mapping operator </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">inverseTonemapSRGB(float3)</strong> </td><td style="text-align:center"> float3 </td><td style="text-align:left"> Applies the inverse tone mapping operator to the specified non-linear sRGB color and returns a linear sRGB color. This operation may be an approximation and works best with the “Filmic” tone mapping operator </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">luminance(float3)</strong> </td><td style="text-align:center"> float </td><td style="text-align:left"> Computes the luminance of the specified linear sRGB color </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">ycbcrToRgb(float, float2)</strong> </td><td style="text-align:center"> float3 </td><td style="text-align:left"> Converts a luminance and CbCr pair to a sRGB color </td></tr>
<tr><td style="text-align:left"> <strong class="asterisk">uvToRenderTargetUV(float2)</strong> </td><td style="text-align:center"> float2 </td><td style="text-align:left"> Transforms a UV coordinate to allow sampling from a <code>RenderTarget</code> attachment </td></tr>
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4 changes: 2 additions & 2 deletions docs/Materials.md.html
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Expand Up @@ -2177,8 +2177,8 @@
**getUV0()** | float2 | First interpolated set of UV coordinates, only available if the uv0 attribute is required
**getUV1()** | float2 | First interpolated set of UV coordinates, only available if the uv1 attribute is required
**getMaskThreshold()** | float | Returns the mask threshold, only available when `blending` is set to `masked`
**inverseTonemap(float3)** | float3 | Applies the inverse tone mapping operator to the specified linear sRGB color and returns a linear sRGB color. This operation may be an approximation
**inverseTonemapSRGB(float3)** | float3 | Applies the inverse tone mapping operator to the specified non-linear sRGB color and returns a linear sRGB color. This operation may be an approximation
**inverseTonemap(float3)** | float3 | Applies the inverse tone mapping operator to the specified linear sRGB color and returns a linear sRGB color. This operation may be an approximation and works best with the "Filmic" tone mapping operator
**inverseTonemapSRGB(float3)** | float3 | Applies the inverse tone mapping operator to the specified non-linear sRGB color and returns a linear sRGB color. This operation may be an approximation and works best with the "Filmic" tone mapping operator
**luminance(float3)** | float | Computes the luminance of the specified linear sRGB color
**ycbcrToRgb(float, float2)** | float3 | Converts a luminance and CbCr pair to a sRGB color
**uvToRenderTargetUV(float2)** | float2 | Transforms a UV coordinate to allow sampling from a `RenderTarget` attachment
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15 changes: 7 additions & 8 deletions filament/src/ColorGrading.cpp
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Expand Up @@ -517,8 +517,7 @@ FColorGrading::FColorGrading(FEngine& engine, const Builder& builder) {
v = chromaticAdaptation(v, config.adaptationTransform);
}

// Convert to color grading color space
v = sRGB_to_REC2020 * v;
// TODO: We should convert to a color grading color space here

if (builder->hasAdjustments) {
// Kill negative values before the next transforms
Expand All @@ -528,7 +527,7 @@ FColorGrading::FColorGrading(FEngine& engine, const Builder& builder) {
v = channelMixer(v, builder->outRed, builder->outGreen, builder->outBlue);

// Shadows/mid-tones/highlights
v = tonalRanges(v, LUMA_REC2020,
v = tonalRanges(v, LUMA_REC709,
builder->shadows, builder->midtones, builder->highlights,
builder->tonalRanges);

Expand All @@ -546,10 +545,10 @@ FColorGrading::FColorGrading(FEngine& engine, const Builder& builder) {
v = LogC_to_linear(v);

// Vibrance in linear space
v = vibrance(v, LUMA_REC2020, builder->vibrance);
v = vibrance(v, LUMA_REC709, builder->vibrance);

// Saturation in linear space
v = saturation(v, LUMA_REC2020, builder->saturation);
v = saturation(v, LUMA_REC709, builder->saturation);

// Kill negative values before tone mapping
v = max(v, 0.0f);
Expand All @@ -566,9 +565,9 @@ FColorGrading::FColorGrading(FEngine& engine, const Builder& builder) {
v = (*builder->toneMapper)(v);
}

// Convert to output color space
// TODO: allow to customize the output color space,
v = REC2020_to_sRGB * v;
// TODO: We should convert to the output color space if we use a working
// color space that's not sRGB
// TODO: Allow the user to customize the output color space

v = saturate(v);

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12 changes: 12 additions & 0 deletions filament/src/ColorSpace.h
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Expand Up @@ -112,6 +112,18 @@ constexpr mat3f sRGB_to_AP1{
0.04737f, 0.01345f, 0.86961f
};

constexpr mat3f AP0_to_sRGB{
2.52169f, -0.27648f, -0.01538f,
-1.13413f, 1.37272f, -0.15298f,
-0.38756f, -0.09624f, 1.16835f
};

constexpr mat3f sRGB_to_AP0{
0.4397010f, 0.0897923f, 0.0175440f,
0.3829780f, 0.8134230f, 0.1115440f,
0.1773350f, 0.0967616f, 0.8707040f
};

constexpr mat3f sRGB_to_REC2020 = XYZ_to_REC2020 * sRGB_to_XYZ;

constexpr mat3f REC2020_to_sRGB = XYZ_to_sRGB * REC2020_to_XYZ;
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4 changes: 2 additions & 2 deletions filament/src/ToneMapper.cpp
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Expand Up @@ -135,7 +135,7 @@ float3 ACES(float3 color, float brightness) noexcept {
constexpr float RRT_SAT_FACTOR = 0.96f;
constexpr float ODT_SAT_FACTOR = 0.93f;

float3 ap0 = REC2020_to_AP0 * color;
float3 ap0 = sRGB_to_AP0 * color;

// Glow module
float saturation = rgb_2_saturation(ap0);
Expand Down Expand Up @@ -177,7 +177,7 @@ float3 ACES(float3 color, float brightness) noexcept {
// Apply desaturation to compensate for luminance difference
linearCV = mix(float3(dot(linearCV, LUMA_AP1)), linearCV, ODT_SAT_FACTOR);

return AP1_to_REC2020 * linearCV;
return AP1_to_sRGB * linearCV;
}

} // namespace aces
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4 changes: 0 additions & 4 deletions samples/gltf_viewer.cpp
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Expand Up @@ -343,10 +343,6 @@ static void createGroundPlane(Engine* engine, Scene* scene, App& app) {
app.scene.groundMaterial = shadowMaterial;
}

static LinearColor inverseTonemapSRGB(sRGBColor x) {
return (x * -0.155) / (x - 1.019);
}

static float sGlobalScale = 1.0f;
static float sGlobalScaleAnamorphism = 0.0f;

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9 changes: 4 additions & 5 deletions shaders/src/common_graphics.fs
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Expand Up @@ -47,8 +47,8 @@ vec3 ycbcrToRgb(float luminance, vec2 cbcr) {
/*
* The input must be in the [0, 1] range.
*/
vec3 Inverse_Tonemap_Unreal(const vec3 x) {
return (x * -0.155) / (x - 1.019);
vec3 Inverse_Tonemap_Filmic(const vec3 x) {
return (0.03 - 0.59 * x - sqrt(0.0009 + 1.3702 * x - 1.0127 * x * x)) / (-5.02 + 4.86 * x);
}

/**
Expand All @@ -61,7 +61,7 @@ vec3 Inverse_Tonemap_Unreal(const vec3 x) {
vec3 inverseTonemapSRGB(vec3 color) {
// sRGB input
color = clamp(color, 0.0, 1.0);
return Inverse_Tonemap_Unreal(color);
return Inverse_Tonemap_Filmic(pow(color, vec3(2.2)));
}

/**
Expand All @@ -73,8 +73,7 @@ vec3 inverseTonemapSRGB(vec3 color) {
*/
vec3 inverseTonemap(vec3 linear) {
// Linear input
linear = clamp(linear, 0.0, 1.0);
return Inverse_Tonemap_Unreal(pow(linear, vec3(1.0 / 2.2)));
return Inverse_Tonemap_Filmic(clamp(linear, 0.0, 1.0));
}

//------------------------------------------------------------------------------
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