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PNG decoder: handle gAMA chunk #23

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Jan 5, 2018
Merged

PNG decoder: handle gAMA chunk #23

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36 changes: 33 additions & 3 deletions examples/utils.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@
#include <string.h>
#include <assert.h>
#include <stdio.h>
#include <math.h>

#ifdef SJPEG_HAVE_PNG
#include <png.h>
Expand Down Expand Up @@ -533,7 +534,8 @@ static const struct {
static bool ExtractMetadataFromPNG(png_structp png,
png_infop const head_info,
png_infop const end_info,
SjpegEncodeParam* const param) {
SjpegEncodeParam* const param,
double* const gamma) {
if (param == NULL) return true;
param->ResetMetadata();
for (int p = 0; p < 2; ++p) {
Expand Down Expand Up @@ -581,7 +583,7 @@ static bool ExtractMetadataFromPNG(png_structp png,
#else // >= libpng 1.5.0
png_bytep profile;
#endif
png_uint_32 len;
png_uint_32 len = 0;

if (png_get_iCCP(png, info,
&name, &comp_type, &profile, &len) == PNG_INFO_iCCP) {
Expand All @@ -591,6 +593,11 @@ static bool ExtractMetadataFromPNG(png_structp png,
}
fprintf(stderr, "[%s : %d bytes]\n", "ICCP", static_cast<int>(len));
}
if (len == 0) { // if there's no iCCP, we look for gamma values.
if (p == 0 && png_get_gAMA(png, info, gamma) == PNG_INFO_gAMA) {
fprintf(stderr, "[GAMMA: %lf]\n", *gamma);
}
}
}
}

Expand All @@ -613,6 +620,23 @@ static void ReadFunc(png_structp png_ptr, png_bytep data, png_size_t length) {
ctx->offset += length;
}

static void ApplyGamma(uint8_t* rgb, int64_t len, double g) {
if (g == 1.) return;
if (len < 512) {
// for small array, no need to build a LUT.
for (int64_t i = 0; i < len; ++i) {
if (rgb[i] > 0 && rgb[i] < 255) {
rgb[i] = (uint8_t)(255. * pow(rgb[i] / 255., g));
}
}
} else {
uint8_t lut[256];
for (int i = 0; i < 256; ++i) lut[i] = i;
ApplyGamma(lut, 256, g);
for (int64_t i = 0; i < len; ++i) rgb[i] = lut[rgb[i]];
}
}

vector<uint8_t> ReadPNG(const std::string& input,
int* const width_ptr, int* const height_ptr,
SjpegEncodeParam* const param) {
Expand All @@ -627,6 +651,7 @@ vector<uint8_t> ReadPNG(const std::string& input,
png_uint_32 width, height, y;
int64_t stride;
vector<uint8_t> rgb;
double gamma = 0.;

assert(input.size() > 0);

Expand Down Expand Up @@ -699,11 +724,16 @@ vector<uint8_t> ReadPNG(const std::string& input,
}
png_read_end(png, end_info);

if (!ExtractMetadataFromPNG(png, info, end_info, param)) {
if (!ExtractMetadataFromPNG(png, info, end_info, param, &gamma)) {
fprintf(stderr, "Error extracting PNG metadata!\n");
goto Error;
}

if (gamma != 0) {
gamma = 1. / (gamma * 2.2); // sRGB approximation
ApplyGamma(rgb.data(), stride * height, gamma);
}

if (width_ptr != NULL) *width_ptr = static_cast<int>(width);
if (height_ptr != NULL) *height_ptr = static_cast<int>(height);

Expand Down