/*
* 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 <png.h>
#include <zlib.h>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
#include "android-base/errors.h"
#include "android-base/logging.h"
#include "android-base/macros.h"
namespace aapt {
// Custom deleter that destroys libpng read and info structs.
class PngReadStructDeleter {
public:
PngReadStructDeleter(png_structp read_ptr, png_infop info_ptr)
: read_ptr_(read_ptr), info_ptr_(info_ptr) {}
~PngReadStructDeleter() {
png_destroy_read_struct(&read_ptr_, &info_ptr_, nullptr);
}
private:
png_structp read_ptr_;
png_infop info_ptr_;
DISALLOW_COPY_AND_ASSIGN(PngReadStructDeleter);
};
// Custom deleter that destroys libpng write and info structs.
class PngWriteStructDeleter {
public:
PngWriteStructDeleter(png_structp write_ptr, png_infop info_ptr)
: write_ptr_(write_ptr), info_ptr_(info_ptr) {}
~PngWriteStructDeleter() {
png_destroy_write_struct(&write_ptr_, &info_ptr_);
}
private:
png_structp write_ptr_;
png_infop info_ptr_;
DISALLOW_COPY_AND_ASSIGN(PngWriteStructDeleter);
};
// Custom warning logging method that uses IDiagnostics.
static void LogWarning(png_structp png_ptr, png_const_charp warning_msg) {
IDiagnostics* diag = (IDiagnostics*)png_get_error_ptr(png_ptr);
diag->Warn(DiagMessage() << warning_msg);
}
// Custom error logging method that uses IDiagnostics.
static void LogError(png_structp png_ptr, png_const_charp error_msg) {
IDiagnostics* diag = (IDiagnostics*)png_get_error_ptr(png_ptr);
diag->Error(DiagMessage() << error_msg);
// Causes libpng to longjmp to the spot where setjmp was set. This is how libpng does
// error handling. If this custom error handler method were to return, libpng would, by
// default, print the error message to stdout and call the same png_longjmp method.
png_longjmp(png_ptr, 1);
}
static void ReadDataFromStream(png_structp png_ptr, png_bytep buffer, png_size_t len) {
io::InputStream* in = (io::InputStream*)png_get_io_ptr(png_ptr);
const void* in_buffer;
size_t in_len;
if (!in->Next(&in_buffer, &in_len)) {
if (in->HadError()) {
std::stringstream error_msg_builder;
error_msg_builder << "failed reading from input";
if (!in->GetError().empty()) {
error_msg_builder << ": " << in->GetError();
}
std::string err = error_msg_builder.str();
png_error(png_ptr, err.c_str());
}
return;
}
const size_t bytes_read = std::min(in_len, len);
memcpy(buffer, in_buffer, bytes_read);
if (bytes_read != in_len) {
in->BackUp(in_len - bytes_read);
}
}
static void WriteDataToStream(png_structp png_ptr, png_bytep buffer, png_size_t len) {
io::OutputStream* out = (io::OutputStream*)png_get_io_ptr(png_ptr);
void* out_buffer;
size_t out_len;
while (len > 0) {
if (!out->Next(&out_buffer, &out_len)) {
if (out->HadError()) {
std::stringstream err_msg_builder;
err_msg_builder << "failed writing to output";
if (!out->GetError().empty()) {
err_msg_builder << ": " << out->GetError();
}
std::string err = out->GetError();
png_error(png_ptr, err.c_str());
}
return;
}
const size_t bytes_written = std::min(out_len, len);
memcpy(out_buffer, buffer, bytes_written);
// Advance the input buffer.
buffer += bytes_written;
len -= bytes_written;
// Advance the output buffer.
out_len -= bytes_written;
}
// If the entire output buffer wasn't used, backup.
if (out_len > 0) {
out->BackUp(out_len);
}
}
std::unique_ptr<Image> ReadPng(IAaptContext* context, const Source& source, 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;
size_t buffer_size;
if (!in->Next((const void**)&signature, &buffer_size)) {
context->GetDiagnostics()->Error(DiagMessage()
<< android::base::SystemErrorCodeToString(errno));
return {};
}
if (buffer_size < 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(buffer_size - 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 read_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
if (read_ptr == 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 info_ptr = png_create_info_struct(read_ptr);
if (info_ptr == nullptr) {
context->GetDiagnostics()->Error(DiagMessage() << "failed to create libpng read png_info");
png_destroy_read_struct(&read_ptr, nullptr, nullptr);
return {};
}
// Create a diagnostics that has the source information encoded.
SourcePathDiagnostics source_diag(source, context->GetDiagnostics());
// Automatically release PNG resources at end of scope.
PngReadStructDeleter png_read_deleter(read_ptr, info_ptr);
// 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(read_ptr))) {
return {};
}
// Handle warnings ourselves via IDiagnostics.
png_set_error_fn(read_ptr, (png_voidp)&source_diag, LogError, LogWarning);
// Set up the read functions which read from our custom data sources.
png_set_read_fn(read_ptr, (png_voidp)in, ReadDataFromStream);
// Skip the signature that we already read.
png_set_sig_bytes(read_ptr, kPngSignatureSize);
// Read the chunk headers.
png_read_info(read_ptr, info_ptr);
// Extract image meta-data from the various chunk headers.
uint32_t width, height;
int bit_depth, color_type, interlace_method, compression_method, filter_method;
png_get_IHDR(read_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_method, &compression_method, &filter_method);
// When the image is read, expand it so that it is in RGBA 8888 format
// so that image handling is uniform.
if (color_type == PNG_COLOR_TYPE_PALETTE) {
png_set_palette_to_rgb(read_ptr);
}
if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) {
png_set_expand_gray_1_2_4_to_8(read_ptr);
}
if (png_get_valid(read_ptr, info_ptr, PNG_INFO_tRNS)) {
png_set_tRNS_to_alpha(read_ptr);
}
if (bit_depth == 16) {
png_set_strip_16(read_ptr);
}
if (!(color_type & PNG_COLOR_MASK_ALPHA)) {
png_set_add_alpha(read_ptr, 0xFF, PNG_FILLER_AFTER);
}
if (color_type == PNG_COLOR_TYPE_GRAY ||
color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
png_set_gray_to_rgb(read_ptr);
}
if (interlace_method != PNG_INTERLACE_NONE) {
png_set_interlace_handling(read_ptr);
}
// Once all the options for reading have been set, we need to flush
// them to libpng.
png_read_update_info(read_ptr, info_ptr);
// 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()) {
source_diag.Error(DiagMessage()
<< "PNG image dimensions are too large: " << width << "x" << height);
return {};
}
std::unique_ptr<Image> output_image = util::make_unique<Image>();
output_image->width = static_cast<int32_t>(width);
output_image->height = static_cast<int32_t>(height);
const size_t row_bytes = png_get_rowbytes(read_ptr, info_ptr);
CHECK(row_bytes == 4 * width); // RGBA
// Allocate one large block to hold the image.
output_image->data = std::unique_ptr<uint8_t[]>(new uint8_t[height * row_bytes]);
// Create an array of rows that index into the data block.
output_image->rows = std::unique_ptr<uint8_t* []>(new uint8_t*[height]);
for (uint32_t h = 0; h < height; h++) {
output_image->rows[h] = output_image->data.get() + (h * row_bytes);
}
// Actually read the image pixels.
png_read_image(read_ptr, output_image->rows.get());
// Finish reading. This will read any other chunks after the image data.
png_read_end(read_ptr, info_ptr);
return output_image;
}
// 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 convertible_to_grayscale, bool has_nine_patch,
size_t color_palette_size, size_t alpha_palette_size) {
const size_t palette_chunk_size = 16 + color_palette_size * 3;
const size_t alpha_chunk_size = 16 + alpha_palette_size;
const size_t color_alpha_data_chunk_size = 16 + 4 * width * height;
const size_t color_data_chunk_size = 16 + 3 * width * height;
const size_t grayscale_alpha_data_chunk_size = 16 + 2 * width * height;
const size_t palette_data_chunk_size = 16 + width * height;
if (grayscale) {
if (alpha_palette_size == 0) {
// This is the smallest the data can be.
return PNG_COLOR_TYPE_GRAY;
} else if (color_palette_size <= 256 && !has_nine_patch) {
// This grayscale has alpha and can fit within a palette.
// See if it is worth fitting into a palette.
const size_t palette_threshold = palette_chunk_size + alpha_chunk_size +
palette_data_chunk_size +
kPaletteOverheadConstant;
if (grayscale_alpha_data_chunk_size > palette_threshold) {
return PNG_COLOR_TYPE_PALETTE;
}
}
return PNG_COLOR_TYPE_GRAY_ALPHA;
}
if (color_palette_size <= 256 && !has_nine_patch) {
// This image can fit inside a palette. Let's see if it is worth it.
size_t total_size_with_palette =
palette_data_chunk_size + palette_chunk_size;
size_t total_size_without_palette = color_data_chunk_size;
if (alpha_palette_size > 0) {
total_size_with_palette += alpha_palette_size;
total_size_without_palette = color_alpha_data_chunk_size;
}
if (total_size_without_palette >
total_size_with_palette + kPaletteOverheadConstant) {
return PNG_COLOR_TYPE_PALETTE;
}
}
if (convertible_to_grayscale) {
if (alpha_palette_size == 0) {
return PNG_COLOR_TYPE_GRAY;
} else {
return PNG_COLOR_TYPE_GRAY_ALPHA;
}
}
if (alpha_palette_size == 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 write_ptr, png_infop write_info_ptr,
std::unordered_map<uint32_t, int>* color_palette,
std::unordered_set<uint32_t>* alpha_palette) {
CHECK(color_palette->size() <= 256);
CHECK(alpha_palette->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 : *alpha_palette) {
(*color_palette)[color] = index++;
}
// Assign the rest of the entries.
for (auto& entry : *color_palette) {
if (entry.second == -1) {
entry.second = index++;
}
}
// Create the PNG color palette struct.
auto color_palette_bytes = std::unique_ptr<png_color[]>(new png_color[color_palette->size()]);
std::unique_ptr<png_byte[]> alpha_palette_bytes;
if (!alpha_palette->empty()) {
alpha_palette_bytes = std::unique_ptr<png_byte[]>(new png_byte[alpha_palette->size()]);
}
for (const auto& entry : *color_palette) {
const uint32_t color = entry.first;
const int index = entry.second;
CHECK(index >= 0);
CHECK(static_cast<size_t>(index) < color_palette->size());
png_colorp slot = color_palette_bytes.get() + index;
slot->red = color >> 24;
slot->green = color >> 16;
slot->blue = color >> 8;
const png_byte alpha = color & 0x000000ff;
if (alpha != 0xff && alpha_palette_bytes) {
CHECK(static_cast<size_t>(index) < alpha_palette->size());
alpha_palette_bytes[index] = alpha;
}
}
// The bytes get copied here, so it is safe to release color_palette_bytes at
// the end of function
// scope.
png_set_PLTE(write_ptr, write_info_ptr, color_palette_bytes.get(), color_palette->size());
if (alpha_palette_bytes) {
png_set_tRNS(write_ptr, write_info_ptr, alpha_palette_bytes.get(), alpha_palette->size(),
nullptr);
}
}
// Write the 9-patch custom PNG chunks to write_info_ptr. This must be done
// before writing image data.
static void WriteNinePatch(png_structp write_ptr, png_infop write_info_ptr,
const NinePatch* nine_patch) {
// The order of the chunks is important.
// 9-patch code in older platforms expects the 9-patch chunk to be last.
png_unknown_chunk unknown_chunks[3];
memset(unknown_chunks, 0, sizeof(unknown_chunks));
size_t index = 0;
size_t chunk_len = 0;
std::unique_ptr<uint8_t[]> serialized_outline =
nine_patch->SerializeRoundedRectOutline(&chunk_len);
strcpy((char*)unknown_chunks[index].name, "npOl");
unknown_chunks[index].size = chunk_len;
unknown_chunks[index].data = (png_bytep)serialized_outline.get();
unknown_chunks[index].location = PNG_HAVE_PLTE;
index++;
std::unique_ptr<uint8_t[]> serialized_layout_bounds;
if (nine_patch->layout_bounds.nonZero()) {
serialized_layout_bounds = nine_patch->SerializeLayoutBounds(&chunk_len);
strcpy((char*)unknown_chunks[index].name, "npLb");
unknown_chunks[index].size = chunk_len;
unknown_chunks[index].data = (png_bytep)serialized_layout_bounds.get();
unknown_chunks[index].location = PNG_HAVE_PLTE;
index++;
}
std::unique_ptr<uint8_t[]> serialized_nine_patch = nine_patch->SerializeBase(&chunk_len);
strcpy((char*)unknown_chunks[index].name, "npTc");
unknown_chunks[index].size = chunk_len;
unknown_chunks[index].data = (png_bytep)serialized_nine_patch.get();
unknown_chunks[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(write_ptr, 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(write_ptr, write_info_ptr, unknown_chunks, index);
}
bool WritePng(IAaptContext* context, const Image* image,
const NinePatch* nine_patch, 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 write_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
if (write_ptr == nullptr) {
context->GetDiagnostics()->Error(DiagMessage() << "failed to create libpng write png_struct");
return false;
}
// Allocate memory to store image header data.
png_infop write_info_ptr = png_create_info_struct(write_ptr);
if (write_info_ptr == nullptr) {
context->GetDiagnostics()->Error(DiagMessage() << "failed to create libpng write png_info");
png_destroy_write_struct(&write_ptr, nullptr);
return false;
}
// Automatically release PNG resources at end of scope.
PngWriteStructDeleter png_write_deleter(write_ptr, write_info_ptr);
// 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(write_ptr))) {
return false;
}
// Handle warnings with our IDiagnostics.
png_set_error_fn(write_ptr, (png_voidp)context->GetDiagnostics(), LogError, LogWarning);
// Set up the write functions which write to our custom data sources.
png_set_write_fn(write_ptr, (png_voidp)out, WriteDataToStream, nullptr);
// We want small files and can take the performance hit to achieve this goal.
png_set_compression_level(write_ptr, 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> color_palette;
std::unordered_set<uint32_t> alpha_palette;
bool needs_to_zero_rgb_channels_of_transparent_pixels = false;
bool grayscale = true;
int max_gray_deviation = 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.
needs_to_zero_rgb_channels_of_transparent_pixels =
needs_to_zero_rgb_channels_of_transparent_pixels ||
(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;
color_palette[color] = -1;
// If the pixel has non-opaque alpha, insert it into the
// alpha palette.
if (alpha != 0xff) {
alpha_palette.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.
max_gray_deviation = std::max(std::abs(red - green), max_gray_deviation);
max_gray_deviation = std::max(std::abs(green - blue), max_gray_deviation);
max_gray_deviation = std::max(std::abs(blue - red), max_gray_deviation);
}
}
if (context->IsVerbose()) {
DiagMessage msg;
msg << " paletteSize=" << color_palette.size()
<< " alphaPaletteSize=" << alpha_palette.size()
<< " maxGrayDeviation=" << max_gray_deviation
<< " grayScale=" << (grayscale ? "true" : "false");
context->GetDiagnostics()->Note(msg);
}
const bool convertible_to_grayscale = max_gray_deviation <= options.grayscale_tolerance;
const int new_color_type = PickColorType(
image->width, image->height, grayscale, convertible_to_grayscale,
nine_patch != nullptr, color_palette.size(), alpha_palette.size());
if (context->IsVerbose()) {
DiagMessage msg;
msg << "encoding PNG ";
if (nine_patch) {
msg << "(with 9-patch) as ";
}
switch (new_color_type) {
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 " << new_color_type;
break;
}
context->GetDiagnostics()->Note(msg);
}
png_set_IHDR(write_ptr, write_info_ptr, image->width, image->height, 8,
new_color_type, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
if (new_color_type & PNG_COLOR_MASK_PALETTE) {
// Assigns indices to the palette, and writes the encoded palette to the
// libpng writePtr.
WritePalette(write_ptr, write_info_ptr, &color_palette, &alpha_palette);
png_set_filter(write_ptr, 0, PNG_NO_FILTERS);
} else {
png_set_filter(write_ptr, 0, PNG_ALL_FILTERS);
}
if (nine_patch) {
WriteNinePatch(write_ptr, write_info_ptr, nine_patch);
}
// Flush our updates to the header.
png_write_info(write_ptr, write_info_ptr);
// Write out each row of image data according to its encoding.
if (new_color_type == PNG_COLOR_TYPE_PALETTE) {
// 1 byte/pixel.
auto out_row = std::unique_ptr<png_byte[]>(new png_byte[image->width]);
for (int32_t y = 0; y < image->height; y++) {
png_const_bytep in_row = image->rows[y];
for (int32_t x = 0; x < image->width; x++) {
int rr = *in_row++;
int gg = *in_row++;
int bb = *in_row++;
int aa = *in_row++;
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 = color_palette[color];
CHECK(idx != -1);
out_row[x] = static_cast<png_byte>(idx);
}
png_write_row(write_ptr, out_row.get());
}
} else if (new_color_type == PNG_COLOR_TYPE_GRAY ||
new_color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
const size_t bpp = new_color_type == PNG_COLOR_TYPE_GRAY ? 1 : 2;
auto out_row =
std::unique_ptr<png_byte[]>(new png_byte[image->width * bpp]);
for (int32_t y = 0; y < image->height; y++) {
png_const_bytep in_row = image->rows[y];
for (int32_t x = 0; x < image->width; x++) {
int rr = in_row[x * 4];
int gg = in_row[x * 4 + 1];
int bb = in_row[x * 4 + 2];
int aa = in_row[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.
out_row[x * bpp] = in_row[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
out_row[x * bpp] =
(png_byte)(rr * 0.2126f + gg * 0.7152f + bb * 0.0722f);
}
if (bpp == 2) {
// Write out alpha if we have it.
out_row[x * bpp + 1] = aa;
}
}
png_write_row(write_ptr, out_row.get());
}
} else if (new_color_type == PNG_COLOR_TYPE_RGB || new_color_type == PNG_COLOR_TYPE_RGBA) {
const size_t bpp = new_color_type == PNG_COLOR_TYPE_RGB ? 3 : 4;
if (needs_to_zero_rgb_channels_of_transparent_pixels) {
// The source RGBA data can't be used as-is, because we need to zero out
// the RGB values of transparent pixels.
auto out_row = std::unique_ptr<png_byte[]>(new png_byte[image->width * bpp]);
for (int32_t y = 0; y < image->height; y++) {
png_const_bytep in_row = image->rows[y];
for (int32_t x = 0; x < image->width; x++) {
int rr = *in_row++;
int gg = *in_row++;
int bb = *in_row++;
int aa = *in_row++;
if (aa == 0) {
// Zero out the RGB channels when transparent.
rr = gg = bb = 0;
}
out_row[x * bpp] = rr;
out_row[x * bpp + 1] = gg;
out_row[x * bpp + 2] = bb;
if (bpp == 4) {
out_row[x * bpp + 3] = aa;
}
}
png_write_row(write_ptr, out_row.get());
}
} else {
// The source image can be used as-is, just tell libpng whether or not to
// ignore the alpha channel.
if (new_color_type == PNG_COLOR_TYPE_RGB) {
// Delete the extraneous alpha values that we appended to our buffer
// when reading the original values.
png_set_filler(write_ptr, 0, PNG_FILLER_AFTER);
}
png_write_image(write_ptr, image->rows.get());
}
} else {
LOG(FATAL) << "unreachable";
}
png_write_end(write_ptr, write_info_ptr);
return true;
}
} // namespace aapt