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improve gainmap computation logic for dark pixels #333

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Merged
DichenZhang1 merged 1 commit into from
Dec 2, 2024
Merged

improve gainmap computation logic for dark pixels #333

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8 changes: 8 additions & 0 deletions lib/include/ultrahdr/gainmapmath.h
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Original file line number Diff line number Diff line change
Expand Up @@ -536,6 +536,14 @@ PutPixelFn putPixelFn(uhdr_img_fmt_t format);

////////////////////////////////////////////////////////////////////////////////
// common utils
static const float kHdrOffset = 1e-7f;
static const float kSdrOffset = 1e-7f;

static inline float clipNegatives(float value) { return (value < 0.0f) ? 0.0f : value; }

static inline Color clipNegatives(Color e) {
return {{{clipNegatives(e.r), clipNegatives(e.g), clipNegatives(e.b)}}};
}

// maximum limit of normalized pixel value in float representation
static const float kMaxPixelFloat = 1.0f;
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12 changes: 7 additions & 5 deletions lib/src/gainmapmath.cpp
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Expand Up @@ -782,12 +782,14 @@ uint8_t encodeGain(float y_sdr, float y_hdr, uhdr_gainmap_metadata_ext_t* metada
}

float computeGain(float sdr, float hdr) {
if (sdr == 0.0f) return 0.0f; // for sdr black return no gain
if (hdr == 0.0f) { // for hdr black, return a gain large enough to attenuate the sdr pel
float offset = (1.0f / 64);
return log2(offset / (offset + sdr));
float gain = log2((hdr + kHdrOffset) / (sdr + kSdrOffset));
if (sdr < 2.f / 255.0f) {
// If sdr is zero and hdr is non zero, it can result in very large gain values. In compression -
// decompression process, if the same sdr pixel increases to 1, the hdr recovered pixel will
// blow out. Dont allow dark pixels to signal large gains.
gain = (std::min)(gain, 2.3f);
}
return log2(hdr / sdr);
return gain;
}

uint8_t affineMapGain(float gainlog2, float mingainlog2, float maxgainlog2, float gamma) {
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23 changes: 9 additions & 14 deletions lib/src/jpegr.cpp
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Expand Up @@ -697,9 +697,6 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
const bool isSdrIntentRgb = isPixelFormatRgb(sdr_intent->fmt);
const float hdrSampleToNitsFactor =
hdr_intent->ct == UHDR_CT_LINEAR ? kSdrWhiteNits : hdr_white_nits;
ColorTransformFn clampPixel = hdr_intent->ct == UHDR_CT_LINEAR
? static_cast<ColorTransformFn>(clampPixelFloatLinear)
: static_cast<ColorTransformFn>(clampPixelFloat);
while (jobQueue.dequeueJob(rowStart, rowEnd)) {
for (size_t y = rowStart; y < rowEnd; ++y) {
for (size_t x = 0; x < dest->w; ++x) {
Expand Down Expand Up @@ -730,7 +727,7 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
Color hdr_rgb = hdrInvOetf(hdr_rgb_gamma);
hdr_rgb = hdrOotfFn(hdr_rgb, hdrLuminanceFn);
hdr_rgb = hdrGamutConversionFn(hdr_rgb);
hdr_rgb = clampPixel(hdr_rgb);
hdr_rgb = clipNegatives(hdr_rgb);

if (mUseMultiChannelGainMap) {
Color sdr_rgb_nits = sdr_rgb * kSdrWhiteNits;
Expand Down Expand Up @@ -807,9 +804,6 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
const bool isSdrIntentRgb = isPixelFormatRgb(sdr_intent->fmt);
const float hdrSampleToNitsFactor =
hdr_intent->ct == UHDR_CT_LINEAR ? kSdrWhiteNits : hdr_white_nits;
ColorTransformFn clampPixel = hdr_intent->ct == UHDR_CT_LINEAR
? static_cast<ColorTransformFn>(clampPixelFloatLinear)
: static_cast<ColorTransformFn>(clampPixelFloat);
float gainmap_min_th[3] = {127.0f, 127.0f, 127.0f};
float gainmap_max_th[3] = {-128.0f, -128.0f, -128.0f};

Expand Down Expand Up @@ -843,7 +837,7 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
Color hdr_rgb = hdrInvOetf(hdr_rgb_gamma);
hdr_rgb = hdrOotfFn(hdr_rgb, hdrLuminanceFn);
hdr_rgb = hdrGamutConversionFn(hdr_rgb);
hdr_rgb = clampPixel(hdr_rgb);
hdr_rgb = clipNegatives(hdr_rgb);

if (mUseMultiChannelGainMap) {
Color sdr_rgb_nits = sdr_rgb * kSdrWhiteNits;
Expand Down Expand Up @@ -907,10 +901,11 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
min_content_boost_log2 = (std::min)(gainmap_min[index], min_content_boost_log2);
max_content_boost_log2 = (std::max)(gainmap_max[index], max_content_boost_log2);
}
// -13.0 emphirically is a small enough gain factor that is capable of representing hdr
// black from any sdr luminance. Allowing further excursion might not offer any benefit and on
// the downside can cause bigger error during affine map and inverse map.
min_content_boost_log2 = (std::max)(-13.0f, min_content_boost_log2);
// gain coefficient range [-14.3, 15.6] is capable of representing hdr pels from sdr pels.
// Allowing further excursion might not offer any benefit and on the downside can cause bigger
// error during affine map and inverse affine map.
min_content_boost_log2 = (std::clamp)(min_content_boost_log2, -14.3f, 15.6f);
max_content_boost_log2 = (std::clamp)(max_content_boost_log2, -14.3f, 15.6f);
if (this->mMaxContentBoost != FLT_MAX) {
float suggestion = log2(this->mMaxContentBoost);
max_content_boost_log2 = (std::min)(max_content_boost_log2, suggestion);
Expand Down Expand Up @@ -969,8 +964,8 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
gainmap_metadata->max_content_boost = exp2(max_content_boost_log2);
gainmap_metadata->min_content_boost = exp2(min_content_boost_log2);
gainmap_metadata->gamma = this->mGamma;
gainmap_metadata->offset_sdr = 0.0f;
gainmap_metadata->offset_hdr = 0.0f;
gainmap_metadata->offset_sdr = kSdrOffset;
gainmap_metadata->offset_hdr = kHdrOffset;
gainmap_metadata->hdr_capacity_min = 1.0f;
if (this->mTargetDispPeakBrightness != -1.0f) {
gainmap_metadata->hdr_capacity_max = this->mTargetDispPeakBrightness / kSdrWhiteNits;
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4 changes: 2 additions & 2 deletions tests/gainmapmath_test.cpp
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Expand Up @@ -1276,7 +1276,7 @@ TEST_F(GainMapMathTest, EncodeGain) {
float max_boost = log2(4.0f);
float gamma = 1.0f;

EXPECT_EQ(affineMapGain(computeGain(0.0f, 1.0f), min_boost, max_boost, 1.0f), 128);
EXPECT_EQ(affineMapGain(computeGain(0.0f, 1.0f), min_boost, max_boost, 1.0f), 255);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 0.0f), min_boost, max_boost, 1.0f), 0);
EXPECT_EQ(affineMapGain(computeGain(0.5f, 0.0f), min_boost, max_boost, 1.0f), 0);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 1.0), min_boost, max_boost, 1.0f), 128);
Expand Down Expand Up @@ -1322,7 +1322,7 @@ TEST_F(GainMapMathTest, EncodeGain) {
EXPECT_EQ(affineMapGain(computeGain(1.0f, 1.0f), min_boost, max_boost, 1.0f), 64);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 8.0f), min_boost, max_boost, 1.0f), 255);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 4.0f), min_boost, max_boost, 1.0f), 191);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 2.0f), min_boost, max_boost, 1.0f), 128);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 2.0f), min_boost, max_boost, 1.0f), 127);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 0.7071f), min_boost, max_boost, 1.0f), 32);
EXPECT_EQ(affineMapGain(computeGain(1.0f, 0.5f), min_boost, max_boost, 1.0f), 0);
}
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