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Encode reversed PNG images as grayscale or indexed when possible (#1703)

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This commit is contained in:
Rangi
2025-06-19 09:48:27 -04:00
committed by GitHub
parent 37bf9fae01
commit b13d623ad4
1 changed files with 78 additions and 26 deletions
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84
src/gfx/reverse.cpp
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@@ -422,38 +422,73 @@ void reverse() {
}
png_set_write_fn(png, &pngFile, writePng, flushPng);
int pngColorType = options.palettes.empty() ? PNG_COLOR_TYPE_GRAY
: palettes.size() == 1 ? PNG_COLOR_TYPE_PALETTE
: PNG_COLOR_TYPE_RGB_ALPHA;
int pngDepth = options.palettes.empty() ? options.bitDepth : 8;
png_set_IHDR(
png,
pngInfo,
width * 8,
height * 8,
8,
PNG_COLOR_TYPE_RGB_ALPHA,
pngDepth,
pngColorType,
PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT
);
png_write_info(png, pngInfo);
if (pngColorType != PNG_COLOR_TYPE_GRAY) {
png_color_8 sbitChunk;
sbitChunk.red = 5;
sbitChunk.green = 5;
sbitChunk.blue = 5;
if (pngColorType == PNG_COLOR_TYPE_RGB_ALPHA) {
sbitChunk.alpha = 1;
}
png_set_sBIT(png, pngInfo, &sbitChunk);
}
constexpr uint8_t SIZEOF_TILE = 4 * 8; // 4 bytes/pixel (RGBA @ 8 bits/channel) * 8 pixels/tile
size_t const SIZEOF_ROW = width * SIZEOF_TILE;
std::vector<uint8_t> tileRow(8 * SIZEOF_ROW, 0xFF); // Data for 8 rows of pixels
if (pngColorType == PNG_COLOR_TYPE_PALETTE) {
assume(palettes.size() == 1);
png_color pngPalette[4] = {};
png_byte pngTrans[4] = {};
int nbPngColors = 0, nbPngTrans = 0;
for (auto const &color : palettes[0]) {
if (!color.has_value()) {
continue;
}
pngPalette[nbPngColors].red = color->red;
pngPalette[nbPngColors].green = color->green;
pngPalette[nbPngColors].blue = color->blue;
pngTrans[nbPngColors] = color->alpha;
++nbPngColors;
if (color->alpha < 255) {
nbPngTrans = nbPngColors;
}
}
png_set_PLTE(png, pngInfo, pngPalette, nbPngColors);
if (nbPngTrans > 0) {
png_set_tRNS(png, pngInfo, pngTrans, nbPngTrans, nullptr);
}
}
png_write_info(png, pngInfo);
// N bits/pixel * 8 pixels/tile row / 8 bits/byte = N bytes/tile row
uint8_t const bytesPerTileRow = pngColorType == PNG_COLOR_TYPE_RGB_ALPHA ? 32 : pngDepth;
size_t const bytesPerRow = width * bytesPerTileRow;
std::vector<uint8_t> tileRow(8 * bytesPerRow, 0xFF); // Data for 8 rows of pixels
uint8_t * const rowPtrs[8] = {
&tileRow.data()[0 * SIZEOF_ROW],
&tileRow.data()[1 * SIZEOF_ROW],
&tileRow.data()[2 * SIZEOF_ROW],
&tileRow.data()[3 * SIZEOF_ROW],
&tileRow.data()[4 * SIZEOF_ROW],
&tileRow.data()[5 * SIZEOF_ROW],
&tileRow.data()[6 * SIZEOF_ROW],
&tileRow.data()[7 * SIZEOF_ROW],
&tileRow.data()[0 * bytesPerRow],
&tileRow.data()[1 * bytesPerRow],
&tileRow.data()[2 * bytesPerRow],
&tileRow.data()[3 * bytesPerRow],
&tileRow.data()[4 * bytesPerRow],
&tileRow.data()[5 * bytesPerRow],
&tileRow.data()[6 * bytesPerRow],
&tileRow.data()[7 * bytesPerRow],
};
for (size_t ty = 0; ty < height; ++ty) {
@@ -486,19 +521,36 @@ void reverse() {
bitplane0 = flipTable[bitplane0];
bitplane1 = flipTable[bitplane1];
}
uint8_t *ptr = &rowPtrs[y][tx * SIZEOF_TILE];
uint8_t *ptr = &rowPtrs[y][tx * bytesPerTileRow];
uint16_t gray = 0;
for (uint8_t x = 0; x < 8; ++x) {
uint8_t bit0 = bitplane0 & 0x80, bit1 = bitplane1 & 0x80;
Rgba const &pixel = *palette[bit0 >> 7 | bit1 >> 6];
uint8_t colorID = bit0 >> 7 | bit1 >> 6;
Rgba const &pixel = *palette[colorID];
if (pngColorType == PNG_COLOR_TYPE_GRAY) {
gray = gray << pngDepth | (pixel.red & ((1 << pngDepth) - 1));
} else if (pngColorType == PNG_COLOR_TYPE_PALETTE) {
*ptr++ = palID * 4 + colorID;
} else {
*ptr++ = pixel.red;
*ptr++ = pixel.green;
*ptr++ = pixel.blue;
*ptr++ = pixel.alpha;
}
// Shift the pixel out
bitplane0 <<= 1;
bitplane1 <<= 1;
}
if (pngDepth == 1) {
*ptr = gray;
} else if (pngDepth == 2) {
*ptr++ = gray >> 8;
*ptr = gray & 0xff;
}
}
}
// We never modify the pointers, and neither should libpng, despite the overly lax function