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Added offset to gain computation in low-luminance regions #358

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Added offset to gain computation in low-luminance regions #358

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10 changes: 6 additions & 4 deletions lib/src/jpegr.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -896,14 +896,16 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
hdr_rgb = hdrGamutConversionFn(hdr_rgb);
hdr_rgb = clipNegatives(hdr_rgb);

constexpr float kComputeGainOffset = 1.0f; // Offset to prevent excessive gain in low luminance areas.

if (mUseMultiChannelGainMap) {
Color sdr_rgb_nits = sdr_rgb * kSdrWhiteNits;
Color hdr_rgb_nits = hdr_rgb * hdrSampleToNitsFactor;
size_t pixel_idx = (x + y * map_width) * 3;

gainmap_data[pixel_idx] = computeGain(sdr_rgb_nits.r, hdr_rgb_nits.r);
gainmap_data[pixel_idx + 1] = computeGain(sdr_rgb_nits.g, hdr_rgb_nits.g);
gainmap_data[pixel_idx + 2] = computeGain(sdr_rgb_nits.b, hdr_rgb_nits.b);
gainmap_data[pixel_idx] = computeGain(sdr_rgb_nits.r + kComputeGainOffset, hdr_rgb_nits.r + kComputeGainOffset);
gainmap_data[pixel_idx + 1] = computeGain(sdr_rgb_nits.g + kComputeGainOffset, hdr_rgb_nits.g + kComputeGainOffset);
gainmap_data[pixel_idx + 2] = computeGain(sdr_rgb_nits.b + kComputeGainOffset, hdr_rgb_nits.b + kComputeGainOffset);
for (int i = 0; i < 3; i++) {
gainmap_min_th[i] = (std::min)(gainmap_data[pixel_idx + i], gainmap_min_th[i]);
gainmap_max_th[i] = (std::max)(gainmap_data[pixel_idx + i], gainmap_max_th[i]);
Expand All @@ -921,7 +923,7 @@ uhdr_error_info_t JpegR::generateGainMap(uhdr_raw_image_t* sdr_intent, uhdr_raw_
}

size_t pixel_idx = x + y * map_width;
gainmap_data[pixel_idx] = computeGain(sdr_y_nits, hdr_y_nits);
gainmap_data[pixel_idx] = computeGain(sdr_y_nits + kComputeGainOffset, hdr_y_nits + kComputeGainOffset);
gainmap_min_th[0] = (std::min)(gainmap_data[pixel_idx], gainmap_min_th[0]);
gainmap_max_th[0] = (std::max)(gainmap_data[pixel_idx], gainmap_max_th[0]);
}
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