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* Copyright 2023 Google Inc.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
#ifndef SkGainmapInfo_DEFINED
#define SkGainmapInfo_DEFINED
#include "include/core/SkColor.h"
* Gainmap rendering parameters. Suppose our display has HDR to SDR ratio of H and we wish to
* display an image with gainmap on this display. Let B be the pixel value from the base image
* in a color space that has the primaries of the base image and a linear transfer function. Let
* G be the pixel value from the gainmap. Let D be the output pixel in the same color space as B.
* The value of D is computed as follows:
* First, let W be a weight parameter determing how much the gainmap will be applied.
* W = clamp((log(H) - log(fDisplayRatioSdr)) /
* (log(fDisplayRatioHdr) - log(fDisplayRatioSdr), 0, 1)
* Next, let L be the gainmap value in log space. We compute this from the value G that was
* sampled from the texture as follows:
* L = mix(log(fGainmapRatioMin), log(fGainmapRatioMax), pow(G, fGainmapGamma))
* Finally, apply the gainmap to compute D, the displayed pixel. If the base image is SDR then
* compute:
* D = (B + fEpsilonSdr) * exp(L * W) - fEpsilonHdr
* If the base image is HDR then compute:
* D = (B + fEpsilonHdr) * exp(L * (W - 1)) - fEpsilonSdr
* In the above math, log() is a natural logarithm and exp() is natural exponentiation. Note,
* however, that the base used for the log() and exp() functions does not affect the results of
* the computation (it cancels out, as long as the same base is used throughout).
struct SkGainmapInfo {
* Parameters for converting the gainmap from its image encoding to log space. These are
* specified per color channel. The alpha value is unused.
SkColor4f fGainmapRatioMin = {1.f, 1.f, 1.f, 1.0};
SkColor4f fGainmapRatioMax = {2.f, 2.f, 2.f, 1.0};
SkColor4f fGainmapGamma = {1.f, 1.f, 1.f, 1.f};
* Parameters sometimes used in gainmap computation to avoid numerical instability.
SkColor4f fEpsilonSdr = {0.f, 0.f, 0.f, 1.0};
SkColor4f fEpsilonHdr = {0.f, 0.f, 0.f, 1.0};
* If the output display's HDR to SDR ratio is less or equal than fDisplayRatioSdr then the SDR
* rendition is displayed. If the output display's HDR to SDR ratio is greater or equal than
* fDisplayRatioHdr then the HDR rendition is displayed. If the output display's HDR to SDR
* ratio is between these values then an interpolation between the two is displayed using the
* math above.
float fDisplayRatioSdr = 1.f;
float fDisplayRatioHdr = 2.f;
* Whether the base image is the SDR image or the HDR image.
enum class BaseImageType {
BaseImageType fBaseImageType = BaseImageType::kSDR;
// TODO(ccameron): Remove these parameters after the new parameters roll into Android.
SkColor4f fLogRatioMin = {0.f, 0.f, 0.f, 1.0};
SkColor4f fLogRatioMax = {1.f, 1.f, 1.f, 1.0};
float fHdrRatioMin = 1.f;
float fHdrRatioMax = 50.f;
* The type of file that created this gainmap.
enum class Type {
Type fType = Type::kUnknown;