// Encode PNGs without libpng.
std::string encodePNG(const PremultipliedImage& pre) {
// Make copy of the image so that we can unpremultiply it.
const auto src = util::unpremultiply(pre.clone());
// PNG magic bytes
const char preamble[8] = { char(0x89), 'P', 'N', 'G', '\r', '\n', 0x1a, '\n' };
// IHDR chunk for our RGBA image.
const char ihdr[13] = {
NETWORK_BYTE_UINT32(src.size.width), // width
NETWORK_BYTE_UINT32(src.size.height), // height
8, // bit depth == 8 bits
6, // color type == RGBA
0, // compression method == deflate
0, // filter method == default
0, // interlace method == none
};
// Prepare the (compressed) data chunk.
const auto stride = src.stride();
std::string idat;
for (uint32_t y = 0; y < src.size.height; y++) {
// Every scanline needs to be prefixed with one byte that indicates the filter type.
idat.append(1, 0); // filter type 0
idat.append((const char*)(src.data.get() + y * stride), stride);
}
idat = util::compress(idat);
// Assemble the PNG.
std::string png;
png.reserve((8 /* preamble */) + (12 + 13 /* IHDR */) +
(12 + idat.size() /* IDAT */) + (12 /* IEND */));
png.append(preamble, 8);
addChunk(png, "IHDR", ihdr, 13);
addChunk(png, "IDAT", idat.data(), static_cast<uint32_t>(idat.size()));
addChunk(png, "IEND");
return png;
}
