1 files changed, 77 insertions, 0 deletions
diff --git a/platform/default/src/mbgl/util/png_writer.cpp b/platform/default/src/mbgl/util/png_writer.cpp
new file mode 100644
index 0000000000..b89e253f85
--- /dev/null
+++ b/platform/default/src/mbgl/util/png_writer.cpp
@@ -0,0 +1,77 @@
+#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