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#include <mbgl/util/compression.hpp>
#include <mbgl/util/image.hpp>
#include <mbgl/util/premultiply.hpp>
#include <boost/crc.hpp>
#include <cassert>
#include <cstring>
#define NETWORK_BYTE_UINT32(value) \
char(value >> 24), char(value >> 16), char(value >> 8), char(value >> 0)
namespace {
void addChunk(std::string& png, const char* type, const char* data = "", const uint32_t size = 0) {
assert(strlen(type) == 4);
// Checksum encompasses type + data
boost::crc_32_type checksum;
checksum.process_bytes(type, 4);
checksum.process_bytes(data, size);
const char length[4] = { NETWORK_BYTE_UINT32(size) };
const char crc[4] = { NETWORK_BYTE_UINT32(checksum.checksum()) };
png.reserve(png.size() + 4 /* length */ + 4 /* type */ + size + 4 /* CRC */);
png.append(length, 4);
png.append(type, 4);
png.append(data, size);
png.append(crc, 4);
}
} // namespace
namespace mbgl {
// 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;
}
} // namespace mbgl
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