AAPT2: Refactor PngCrunching
PngCrunching now has a slightly better heuristic of choosing to encode
an image as a palette or RGB. For small images, RGB compresses much better
than a palette.
The original PNG is used as-is (minus some optional chunks being stripped)
if the resulting crunched PNG is larger than the original.
9-patch handling is abstracted away from PNGs, paving the way
for other 9-patches, like WebP.
TODO: handle PNGs with 9-patch chunks already present, which
should just be passed through. This will allow for 3rd party
tools to generate 9-patches.
TODO: implement cheap transparency: when one color is used to represent
transparent, and all other colors are opaque.
Bug:30053276
Change-Id: I5167f53b91d1efa462d9f03d6b9108d9b541c0c1
diff --git a/tools/aapt2/compile/PngCrunch.cpp b/tools/aapt2/compile/PngCrunch.cpp
new file mode 100644
index 0000000..a2e3f4f
--- /dev/null
+++ b/tools/aapt2/compile/PngCrunch.cpp
@@ -0,0 +1,724 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "compile/Png.h"
+
+#include <algorithm>
+#include <android-base/errors.h>
+#include <android-base/macros.h>
+#include <png.h>
+#include <unordered_map>
+#include <unordered_set>
+#include <zlib.h>
+
+namespace aapt {
+
+// Size in bytes of the PNG signature.
+constexpr size_t kPngSignatureSize = 8u;
+
+/**
+ * Custom deleter that destroys libpng read and info structs.
+ */
+class PngReadStructDeleter {
+public:
+ explicit PngReadStructDeleter(png_structp readPtr, png_infop infoPtr) :
+ mReadPtr(readPtr), mInfoPtr(infoPtr) {
+ }
+
+ ~PngReadStructDeleter() {
+ png_destroy_read_struct(&mReadPtr, &mInfoPtr, nullptr);
+ }
+
+private:
+ png_structp mReadPtr;
+ png_infop mInfoPtr;
+
+ DISALLOW_COPY_AND_ASSIGN(PngReadStructDeleter);
+};
+
+/**
+ * Custom deleter that destroys libpng write and info structs.
+ */
+class PngWriteStructDeleter {
+public:
+ explicit PngWriteStructDeleter(png_structp writePtr, png_infop infoPtr) :
+ mWritePtr(writePtr), mInfoPtr(infoPtr) {
+ }
+
+ ~PngWriteStructDeleter() {
+ png_destroy_write_struct(&mWritePtr, &mInfoPtr);
+ }
+
+private:
+ png_structp mWritePtr;
+ png_infop mInfoPtr;
+
+ DISALLOW_COPY_AND_ASSIGN(PngWriteStructDeleter);
+};
+
+// Custom warning logging method that uses IDiagnostics.
+static void logWarning(png_structp pngPtr, png_const_charp warningMsg) {
+ IDiagnostics* diag = (IDiagnostics*) png_get_error_ptr(pngPtr);
+ diag->warn(DiagMessage() << warningMsg);
+}
+
+// Custom error logging method that uses IDiagnostics.
+static void logError(png_structp pngPtr, png_const_charp errorMsg) {
+ IDiagnostics* diag = (IDiagnostics*) png_get_error_ptr(pngPtr);
+ diag->error(DiagMessage() << errorMsg);
+}
+
+static void readDataFromStream(png_structp pngPtr, png_bytep buffer, png_size_t len) {
+ io::InputStream* in = (io::InputStream*) png_get_io_ptr(pngPtr);
+
+ const void* inBuffer;
+ int inLen;
+ if (!in->Next(&inBuffer, &inLen)) {
+ if (in->HadError()) {
+ std::string err = in->GetError();
+ png_error(pngPtr, err.c_str());
+ }
+ return;
+ }
+
+ const size_t bytesRead = std::min(static_cast<size_t>(inLen), len);
+ memcpy(buffer, inBuffer, bytesRead);
+ if (bytesRead != static_cast<size_t>(inLen)) {
+ in->BackUp(inLen - static_cast<int>(bytesRead));
+ }
+}
+
+static void writeDataToStream(png_structp pngPtr, png_bytep buffer, png_size_t len) {
+ io::OutputStream* out = (io::OutputStream*) png_get_io_ptr(pngPtr);
+
+ void* outBuffer;
+ int outLen;
+ while (len > 0) {
+ if (!out->Next(&outBuffer, &outLen)) {
+ if (out->HadError()) {
+ std::string err = out->GetError();
+ png_error(pngPtr, err.c_str());
+ }
+ return;
+ }
+
+ const size_t bytesWritten = std::min(static_cast<size_t>(outLen), len);
+ memcpy(outBuffer, buffer, bytesWritten);
+
+ // Advance the input buffer.
+ buffer += bytesWritten;
+ len -= bytesWritten;
+
+ // Advance the output buffer.
+ outLen -= static_cast<int>(bytesWritten);
+ }
+
+ // If the entire output buffer wasn't used, backup.
+ if (outLen > 0) {
+ out->BackUp(outLen);
+ }
+}
+
+std::unique_ptr<Image> readPng(IAaptContext* context, io::InputStream* in) {
+ // Read the first 8 bytes of the file looking for the PNG signature.
+ // Bail early if it does not match.
+ const png_byte* signature;
+ int bufferSize;
+ if (!in->Next((const void**) &signature, &bufferSize)) {
+ context->getDiagnostics()->error(DiagMessage()
+ << android::base::SystemErrorCodeToString(errno));
+ return {};
+ }
+
+ if (static_cast<size_t>(bufferSize) < kPngSignatureSize
+ || png_sig_cmp(signature, 0, kPngSignatureSize) != 0) {
+ context->getDiagnostics()->error(DiagMessage()
+ << "file signature does not match PNG signature");
+ return {};
+ }
+
+ // Start at the beginning of the first chunk.
+ in->BackUp(bufferSize - static_cast<int>(kPngSignatureSize));
+
+ // Create and initialize the png_struct with the default error and warning handlers.
+ // The header version is also passed in to ensure that this was built against the same
+ // version of libpng.
+ png_structp readPtr = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
+ if (readPtr == nullptr) {
+ context->getDiagnostics()->error(DiagMessage()
+ << "failed to create libpng read png_struct");
+ return {};
+ }
+
+ // Create and initialize the memory for image header and data.
+ png_infop infoPtr = png_create_info_struct(readPtr);
+ if (infoPtr == nullptr) {
+ context->getDiagnostics()->error(DiagMessage() << "failed to create libpng read png_info");
+ png_destroy_read_struct(&readPtr, nullptr, nullptr);
+ return {};
+ }
+
+ // Automatically release PNG resources at end of scope.
+ PngReadStructDeleter pngReadDeleter(readPtr, infoPtr);
+
+ // libpng uses longjmp to jump to an error handling routine.
+ // setjmp will only return true if it was jumped to, aka there was
+ // an error.
+ if (setjmp(png_jmpbuf(readPtr))) {
+ return {};
+ }
+
+ // Handle warnings ourselves via IDiagnostics.
+ png_set_error_fn(readPtr, (png_voidp) context->getDiagnostics(), logError, logWarning);
+
+ // Set up the read functions which read from our custom data sources.
+ png_set_read_fn(readPtr, (png_voidp) in, readDataFromStream);
+
+ // Skip the signature that we already read.
+ png_set_sig_bytes(readPtr, kPngSignatureSize);
+
+ // Read the chunk headers.
+ png_read_info(readPtr, infoPtr);
+
+ // Extract image meta-data from the various chunk headers.
+ uint32_t width, height;
+ int bitDepth, colorType, interlaceMethod, compressionMethod, filterMethod;
+ png_get_IHDR(readPtr, infoPtr, &width, &height, &bitDepth, &colorType, &interlaceMethod,
+ &compressionMethod, &filterMethod);
+
+ // When the image is read, expand it so that it is in RGBA 8888 format
+ // so that image handling is uniform.
+
+ if (colorType == PNG_COLOR_TYPE_PALETTE) {
+ png_set_palette_to_rgb(readPtr);
+ }
+
+ if (colorType == PNG_COLOR_TYPE_GRAY && bitDepth < 8) {
+ png_set_expand_gray_1_2_4_to_8(readPtr);
+ }
+
+ if (png_get_valid(readPtr, infoPtr, PNG_INFO_tRNS)) {
+ png_set_tRNS_to_alpha(readPtr);
+ }
+
+ if (bitDepth == 16) {
+ png_set_strip_16(readPtr);
+ }
+
+ if (!(colorType & PNG_COLOR_MASK_ALPHA)) {
+ png_set_add_alpha(readPtr, 0xFF, PNG_FILLER_AFTER);
+ }
+
+ if (colorType == PNG_COLOR_TYPE_GRAY || colorType == PNG_COLOR_TYPE_GRAY_ALPHA) {
+ png_set_gray_to_rgb(readPtr);
+ }
+
+ if (interlaceMethod != PNG_INTERLACE_NONE) {
+ png_set_interlace_handling(readPtr);
+ }
+
+ // Once all the options for reading have been set, we need to flush
+ // them to libpng.
+ png_read_update_info(readPtr, infoPtr);
+
+ // 9-patch uses int32_t to index images, so we cap the image dimensions to something
+ // that can always be represented by 9-patch.
+ if (width > std::numeric_limits<int32_t>::max() ||
+ height > std::numeric_limits<int32_t>::max()) {
+ context->getDiagnostics()->error(DiagMessage() << "PNG image dimensions are too large: "
+ << width << "x" << height);
+ return {};
+ }
+
+ std::unique_ptr<Image> outputImage = util::make_unique<Image>();
+ outputImage->width = static_cast<int32_t>(width);
+ outputImage->height = static_cast<int32_t>(height);
+
+ const size_t rowBytes = png_get_rowbytes(readPtr, infoPtr);
+ assert(rowBytes == 4 * width); // RGBA
+
+ // Allocate one large block to hold the image.
+ outputImage->data = std::unique_ptr<uint8_t[]>(new uint8_t[height * rowBytes]);
+
+ // Create an array of rows that index into the data block.
+ outputImage->rows = std::unique_ptr<uint8_t*[]>(new uint8_t*[height]);
+ for (uint32_t h = 0; h < height; h++) {
+ outputImage->rows[h] = outputImage->data.get() + (h * rowBytes);
+ }
+
+ // Actually read the image pixels.
+ png_read_image(readPtr, outputImage->rows.get());
+
+ // Finish reading. This will read any other chunks after the image data.
+ png_read_end(readPtr, infoPtr);
+
+ return outputImage;
+}
+
+/**
+ * Experimentally chosen constant to be added to the overhead of using color type
+ * PNG_COLOR_TYPE_PALETTE to account for the uncompressability of the palette chunk.
+ * Without this, many small PNGs encoded with palettes are larger after compression than
+ * the same PNGs encoded as RGBA.
+ */
+constexpr static const size_t kPaletteOverheadConstant = 1024u * 10u;
+
+// Pick a color type by which to encode the image, based on which color type will take
+// the least amount of disk space.
+//
+// 9-patch images traditionally have not been encoded with palettes.
+// The original rationale was to avoid dithering until after scaling,
+// but I don't think this would be an issue with palettes. Either way,
+// our naive size estimation tends to be wrong for small images like 9-patches
+// and using palettes balloons the size of the resulting 9-patch.
+// In order to not regress in size, restrict 9-patch to not use palettes.
+
+// The options are:
+//
+// - RGB
+// - RGBA
+// - RGB + cheap alpha
+// - Color palette
+// - Color palette + cheap alpha
+// - Color palette + alpha palette
+// - Grayscale
+// - Grayscale + cheap alpha
+// - Grayscale + alpha
+//
+static int pickColorType(int32_t width, int32_t height,
+ bool grayScale, bool convertibleToGrayScale, bool hasNinePatch,
+ size_t colorPaletteSize, size_t alphaPaletteSize) {
+ const size_t paletteChunkSize = 16 + colorPaletteSize * 3;
+ const size_t alphaChunkSize = 16 + alphaPaletteSize;
+ const size_t colorAlphaDataChunkSize = 16 + 4 * width * height;
+ const size_t colorDataChunkSize = 16 + 3 * width * height;
+ const size_t grayScaleAlphaDataChunkSize = 16 + 2 * width * height;
+ const size_t paletteDataChunkSize = 16 + width * height;
+
+ if (grayScale) {
+ if (alphaPaletteSize == 0) {
+ // This is the smallest the data can be.
+ return PNG_COLOR_TYPE_GRAY;
+ } else if (colorPaletteSize <= 256 && !hasNinePatch) {
+ // This grayscale has alpha and can fit within a palette.
+ // See if it is worth fitting into a palette.
+ const size_t paletteThreshold = paletteChunkSize + alphaChunkSize +
+ paletteDataChunkSize + kPaletteOverheadConstant;
+ if (grayScaleAlphaDataChunkSize > paletteThreshold) {
+ return PNG_COLOR_TYPE_PALETTE;
+ }
+ }
+ return PNG_COLOR_TYPE_GRAY_ALPHA;
+ }
+
+
+ if (colorPaletteSize <= 256 && !hasNinePatch) {
+ // This image can fit inside a palette. Let's see if it is worth it.
+ size_t totalSizeWithPalette = paletteDataChunkSize + paletteChunkSize;
+ size_t totalSizeWithoutPalette = colorDataChunkSize;
+ if (alphaPaletteSize > 0) {
+ totalSizeWithPalette += alphaPaletteSize;
+ totalSizeWithoutPalette = colorAlphaDataChunkSize;
+ }
+
+ if (totalSizeWithoutPalette > totalSizeWithPalette + kPaletteOverheadConstant) {
+ return PNG_COLOR_TYPE_PALETTE;
+ }
+ }
+
+ if (convertibleToGrayScale) {
+ if (alphaPaletteSize == 0) {
+ return PNG_COLOR_TYPE_GRAY;
+ } else {
+ return PNG_COLOR_TYPE_GRAY_ALPHA;
+ }
+ }
+
+ if (alphaPaletteSize == 0) {
+ return PNG_COLOR_TYPE_RGB;
+ }
+ return PNG_COLOR_TYPE_RGBA;
+}
+
+// Assigns indices to the color and alpha palettes, encodes them, and then invokes
+// png_set_PLTE/png_set_tRNS.
+// This must be done before writing image data.
+// Image data must be transformed to use the indices assigned within the palette.
+static void writePalette(png_structp writePtr, png_infop writeInfoPtr,
+ std::unordered_map<uint32_t, int>* colorPalette,
+ std::unordered_set<uint32_t>* alphaPalette) {
+ assert(colorPalette->size() <= 256);
+ assert(alphaPalette->size() <= 256);
+
+ // Populate the PNG palette struct and assign indices to the color
+ // palette.
+
+ // Colors in the alpha palette should have smaller indices.
+ // This will ensure that we can truncate the alpha palette if it is
+ // smaller than the color palette.
+ int index = 0;
+ for (uint32_t color : *alphaPalette) {
+ (*colorPalette)[color] = index++;
+ }
+
+ // Assign the rest of the entries.
+ for (auto& entry : *colorPalette) {
+ if (entry.second == -1) {
+ entry.second = index++;
+ }
+ }
+
+ // Create the PNG color palette struct.
+ auto colorPaletteBytes = std::unique_ptr<png_color[]>(new png_color[colorPalette->size()]);
+
+ std::unique_ptr<png_byte[]> alphaPaletteBytes;
+ if (!alphaPalette->empty()) {
+ alphaPaletteBytes = std::unique_ptr<png_byte[]>(new png_byte[alphaPalette->size()]);
+ }
+
+ for (const auto& entry : *colorPalette) {
+ const uint32_t color = entry.first;
+ const int index = entry.second;
+ assert(index >= 0);
+ assert(static_cast<size_t>(index) < colorPalette->size());
+
+ png_colorp slot = colorPaletteBytes.get() + index;
+ slot->red = color >> 24;
+ slot->green = color >> 16;
+ slot->blue = color >> 8;
+
+ const png_byte alpha = color & 0x000000ff;
+ if (alpha != 0xff && alphaPaletteBytes) {
+ assert(static_cast<size_t>(index) < alphaPalette->size());
+ alphaPaletteBytes[index] = alpha;
+ }
+ }
+
+ // The bytes get copied here, so it is safe to release colorPaletteBytes at the end of function
+ // scope.
+ png_set_PLTE(writePtr, writeInfoPtr, colorPaletteBytes.get(), colorPalette->size());
+
+ if (alphaPaletteBytes) {
+ png_set_tRNS(writePtr, writeInfoPtr, alphaPaletteBytes.get(), alphaPalette->size(),
+ nullptr);
+ }
+}
+
+// Write the 9-patch custom PNG chunks to writeInfoPtr. This must be done before
+// writing image data.
+static void writeNinePatch(png_structp writePtr, png_infop writeInfoPtr,
+ const NinePatch* ninePatch) {
+ // The order of the chunks is important.
+ // 9-patch code in older platforms expects the 9-patch chunk to
+ // be last.
+
+ png_unknown_chunk unknownChunks[3];
+ memset(unknownChunks, 0, sizeof(unknownChunks));
+
+ size_t index = 0;
+ size_t chunkLen = 0;
+
+ std::unique_ptr<uint8_t[]> serializedOutline =
+ ninePatch->serializeRoundedRectOutline(&chunkLen);
+ strcpy((char*) unknownChunks[index].name, "npOl");
+ unknownChunks[index].size = chunkLen;
+ unknownChunks[index].data = (png_bytep) serializedOutline.get();
+ unknownChunks[index].location = PNG_HAVE_PLTE;
+ index++;
+
+ std::unique_ptr<uint8_t[]> serializedLayoutBounds;
+ if (ninePatch->layoutBounds.nonZero()) {
+ serializedLayoutBounds = ninePatch->serializeLayoutBounds(&chunkLen);
+ strcpy((char*) unknownChunks[index].name, "npLb");
+ unknownChunks[index].size = chunkLen;
+ unknownChunks[index].data = (png_bytep) serializedLayoutBounds.get();
+ unknownChunks[index].location = PNG_HAVE_PLTE;
+ index++;
+ }
+
+ std::unique_ptr<uint8_t[]> serializedNinePatch = ninePatch->serializeBase(&chunkLen);
+ strcpy((char*) unknownChunks[index].name, "npTc");
+ unknownChunks[index].size = chunkLen;
+ unknownChunks[index].data = (png_bytep) serializedNinePatch.get();
+ unknownChunks[index].location = PNG_HAVE_PLTE;
+ index++;
+
+ // Handle all unknown chunks. We are manually setting the chunks here,
+ // so we will only ever handle our custom chunks.
+ png_set_keep_unknown_chunks(writePtr, PNG_HANDLE_CHUNK_ALWAYS, nullptr, 0);
+
+ // Set the actual chunks here. The data gets copied, so our buffers can
+ // safely go out of scope.
+ png_set_unknown_chunks(writePtr, writeInfoPtr, unknownChunks, index);
+}
+
+bool writePng(IAaptContext* context, const Image* image, const NinePatch* ninePatch,
+ io::OutputStream* out, const PngOptions& options) {
+ // Create and initialize the write png_struct with the default error and warning handlers.
+ // The header version is also passed in to ensure that this was built against the same
+ // version of libpng.
+ png_structp writePtr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
+ nullptr, nullptr, nullptr);
+ if (writePtr == nullptr) {
+ context->getDiagnostics()->error(DiagMessage()
+ << "failed to create libpng write png_struct");
+ return false;
+ }
+
+ // Allocate memory to store image header data.
+ png_infop writeInfoPtr = png_create_info_struct(writePtr);
+ if (writeInfoPtr == nullptr) {
+ context->getDiagnostics()->error(DiagMessage() << "failed to create libpng write png_info");
+ png_destroy_write_struct(&writePtr, nullptr);
+ return false;
+ }
+
+ // Automatically release PNG resources at end of scope.
+ PngWriteStructDeleter pngWriteDeleter(writePtr, writeInfoPtr);
+
+ // libpng uses longjmp to jump to error handling routines.
+ // setjmp will return true only if it was jumped to, aka, there was an error.
+ if (setjmp(png_jmpbuf(writePtr))) {
+ return false;
+ }
+
+ // Handle warnings with our IDiagnostics.
+ png_set_error_fn(writePtr, (png_voidp) context->getDiagnostics(), logError, logWarning);
+
+ // Set up the write functions which write to our custom data sources.
+ png_set_write_fn(writePtr, (png_voidp) out, writeDataToStream, nullptr);
+
+ // We want small files and can take the performance hit to achieve this goal.
+ png_set_compression_level(writePtr, Z_BEST_COMPRESSION);
+
+ // Begin analysis of the image data.
+ // Scan the entire image and determine if:
+ // 1. Every pixel has R == G == B (grayscale)
+ // 2. Every pixel has A == 255 (opaque)
+ // 3. There are no more than 256 distinct RGBA colors (palette).
+ std::unordered_map<uint32_t, int> colorPalette;
+ std::unordered_set<uint32_t> alphaPalette;
+ bool needsToZeroRGBChannelsOfTransparentPixels = false;
+ bool grayScale = true;
+ int maxGrayDeviation = 0;
+
+ for (int32_t y = 0; y < image->height; y++) {
+ const uint8_t* row = image->rows[y];
+ for (int32_t x = 0; x < image->width; x++) {
+ int red = *row++;
+ int green = *row++;
+ int blue = *row++;
+ int alpha = *row++;
+
+ if (alpha == 0) {
+ // The color is completely transparent.
+ // For purposes of palettes and grayscale optimization,
+ // treat all channels as 0x00.
+ needsToZeroRGBChannelsOfTransparentPixels =
+ needsToZeroRGBChannelsOfTransparentPixels ||
+ (red != 0 || green != 0 || blue != 0);
+ red = green = blue = 0;
+ }
+
+ // Insert the color into the color palette.
+ const uint32_t color = red << 24 | green << 16 | blue << 8 | alpha;
+ colorPalette[color] = -1;
+
+ // If the pixel has non-opaque alpha, insert it into the
+ // alpha palette.
+ if (alpha != 0xff) {
+ alphaPalette.insert(color);
+ }
+
+ // Check if the image is indeed grayscale.
+ if (grayScale) {
+ if (red != green || red != blue) {
+ grayScale = false;
+ }
+ }
+
+ // Calculate the gray scale deviation so that it can be compared
+ // with the threshold.
+ maxGrayDeviation = std::max(std::abs(red - green), maxGrayDeviation);
+ maxGrayDeviation = std::max(std::abs(green - blue), maxGrayDeviation);
+ maxGrayDeviation = std::max(std::abs(blue - red), maxGrayDeviation);
+ }
+ }
+
+ if (context->verbose()) {
+ DiagMessage msg;
+ msg << " paletteSize=" << colorPalette.size()
+ << " alphaPaletteSize=" << alphaPalette.size()
+ << " maxGrayDeviation=" << maxGrayDeviation
+ << " grayScale=" << (grayScale ? "true" : "false");
+ context->getDiagnostics()->note(msg);
+ }
+
+ const bool convertibleToGrayScale = maxGrayDeviation <= options.grayScaleTolerance;
+
+ const int newColorType = pickColorType(image->width, image->height, grayScale,
+ convertibleToGrayScale, ninePatch != nullptr,
+ colorPalette.size(), alphaPalette.size());
+
+ if (context->verbose()) {
+ DiagMessage msg;
+ msg << "encoding PNG ";
+ if (ninePatch) {
+ msg << "(with 9-patch) as ";
+ }
+ switch (newColorType) {
+ case PNG_COLOR_TYPE_GRAY:
+ msg << "GRAY";
+ break;
+ case PNG_COLOR_TYPE_GRAY_ALPHA:
+ msg << "GRAY + ALPHA";
+ break;
+ case PNG_COLOR_TYPE_RGB:
+ msg << "RGB";
+ break;
+ case PNG_COLOR_TYPE_RGB_ALPHA:
+ msg << "RGBA";
+ break;
+ case PNG_COLOR_TYPE_PALETTE:
+ msg << "PALETTE";
+ break;
+ default:
+ msg << "unknown type " << newColorType;
+ break;
+ }
+ context->getDiagnostics()->note(msg);
+ }
+
+ png_set_IHDR(writePtr, writeInfoPtr, image->width, image->height, 8, newColorType,
+ PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+
+ if (newColorType & PNG_COLOR_MASK_PALETTE) {
+ // Assigns indices to the palette, and writes the encoded palette to the libpng writePtr.
+ writePalette(writePtr, writeInfoPtr, &colorPalette, &alphaPalette);
+ png_set_filter(writePtr, 0, PNG_NO_FILTERS);
+ } else {
+ png_set_filter(writePtr, 0, PNG_ALL_FILTERS);
+ }
+
+ if (ninePatch) {
+ writeNinePatch(writePtr, writeInfoPtr, ninePatch);
+ }
+
+ // Flush our updates to the header.
+ png_write_info(writePtr, writeInfoPtr);
+
+ // Write out each row of image data according to its encoding.
+ if (newColorType == PNG_COLOR_TYPE_PALETTE) {
+ // 1 byte/pixel.
+ auto outRow = std::unique_ptr<png_byte[]>(new png_byte[image->width]);
+
+ for (int32_t y = 0; y < image->height; y++) {
+ png_const_bytep inRow = image->rows[y];
+ for (int32_t x = 0; x < image->width; x++) {
+ int rr = *inRow++;
+ int gg = *inRow++;
+ int bb = *inRow++;
+ int aa = *inRow++;
+ if (aa == 0) {
+ // Zero out color channels when transparent.
+ rr = gg = bb = 0;
+ }
+
+ const uint32_t color = rr << 24 | gg << 16 | bb << 8 | aa;
+ const int idx = colorPalette[color];
+ assert(idx != -1);
+ outRow[x] = static_cast<png_byte>(idx);
+ }
+ png_write_row(writePtr, outRow.get());
+ }
+ } else if (newColorType == PNG_COLOR_TYPE_GRAY || newColorType == PNG_COLOR_TYPE_GRAY_ALPHA) {
+ const size_t bpp = newColorType == PNG_COLOR_TYPE_GRAY ? 1 : 2;
+ auto outRow = std::unique_ptr<png_byte[]>(new png_byte[image->width * bpp]);
+
+ for (int32_t y = 0; y < image->height; y++) {
+ png_const_bytep inRow = image->rows[y];
+ for (int32_t x = 0; x < image->width; x++) {
+ int rr = inRow[x * 4];
+ int gg = inRow[x * 4 + 1];
+ int bb = inRow[x * 4 + 2];
+ int aa = inRow[x * 4 + 3];
+ if (aa == 0) {
+ // Zero out the gray channel when transparent.
+ rr = gg = bb = 0;
+ }
+
+ if (grayScale) {
+ // The image was already grayscale, red == green == blue.
+ outRow[x * bpp] = inRow[x * 4];
+ } else {
+ // The image is convertible to grayscale, use linear-luminance of
+ // sRGB colorspace: https://en.wikipedia.org/wiki/Grayscale#Colorimetric_.28luminance-preserving.29_conversion_to_grayscale
+ outRow[x * bpp] = (png_byte) (rr * 0.2126f + gg * 0.7152f + bb * 0.0722f);
+ }
+
+ if (bpp == 2) {
+ // Write out alpha if we have it.
+ outRow[x * bpp + 1] = aa;
+ }
+ }
+ png_write_row(writePtr, outRow.get());
+ }
+ } else if (newColorType == PNG_COLOR_TYPE_RGB || newColorType == PNG_COLOR_TYPE_RGBA) {
+ const size_t bpp = newColorType == PNG_COLOR_TYPE_RGB ? 3 : 4;
+ if (needsToZeroRGBChannelsOfTransparentPixels) {
+ // The source RGBA data can't be used as-is, because we need to zero out the RGB
+ // values of transparent pixels.
+ auto outRow = std::unique_ptr<png_byte[]>(new png_byte[image->width * bpp]);
+
+ for (int32_t y = 0; y < image->height; y++) {
+ png_const_bytep inRow = image->rows[y];
+ for (int32_t x = 0; x < image->width; x++) {
+ int rr = *inRow++;
+ int gg = *inRow++;
+ int bb = *inRow++;
+ int aa = *inRow++;
+ if (aa == 0) {
+ // Zero out the RGB channels when transparent.
+ rr = gg = bb = 0;
+ }
+ outRow[x * bpp] = rr;
+ outRow[x * bpp + 1] = gg;
+ outRow[x * bpp + 2] = bb;
+ if (bpp == 4) {
+ outRow[x * bpp + 3] = aa;
+ }
+ }
+ png_write_row(writePtr, outRow.get());
+ }
+ } else {
+ // The source image can be used as-is, just tell libpng whether or not to ignore
+ // the alpha channel.
+ if (newColorType == PNG_COLOR_TYPE_RGB) {
+ // Delete the extraneous alpha values that we appended to our buffer
+ // when reading the original values.
+ png_set_filler(writePtr, 0, PNG_FILLER_AFTER);
+ }
+ png_write_image(writePtr, image->rows.get());
+ }
+ } else {
+ assert(false && "unreachable");
+ }
+
+ png_write_end(writePtr, writeInfoPtr);
+ return true;
+}
+
+} // namespace aapt