Revert^2 "Move some image/9patch code to androidfw"

This reverts commit 917043bc2586743afda5a21386893fa8c787800b.

Reason for revert: Roll forward with fix

Test: Automatic
Bug: 296324826
Change-Id: I42a0b48c02fd497b2174c0c65f300265202f7ab1

1 files changed, 730 insertions, 0 deletions

diff --git a/libs/androidfw/PngCrunch.cpp b/libs/androidfw/PngCrunch.cpp
new file mode 100644
index 000000000000..cf3c0eeff402
--- /dev/null
+++ b/libs/androidfw/PngCrunch.cpp
@@ -0,0 +1,730 @@
+/*
+ * 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 <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"
+#include "androidfw/Png.h"
+
+namespace android {
+
+// 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) {
+  android::IDiagnostics* diag = (android::IDiagnostics*)png_get_error_ptr(png_ptr);
+  diag->Warn(android::DiagMessage() << warning_msg);
+}
+
+// Custom error logging method that uses IDiagnostics.
+static void LogError(png_structp png_ptr, png_const_charp error_msg) {
+  android::IDiagnostics* diag = (android::IDiagnostics*)png_get_error_ptr(png_ptr);
+  diag->Error(android::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) {
+  InputStream* in = (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) {
+  OutputStream* out = (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(InputStream* in, IDiagnostics* diag) {
+  // 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)) {
+    if (in->HadError()) {
+      diag->Error(android::DiagMessage() << "failed to read PNG signature: " << in->GetError());
+    } else {
+      diag->Error(android::DiagMessage() << "not enough data for PNG signature");
+    }
+    return {};
+  }
+
+  if (buffer_size < kPngSignatureSize || png_sig_cmp(signature, 0, kPngSignatureSize) != 0) {
+    diag->Error(android::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) {
+    diag->Error(android::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) {
+    diag->Error(android::DiagMessage() << "failed to create libpng read png_info");
+    png_destroy_read_struct(&read_ptr, nullptr, nullptr);
+    return {};
+  }
+
+  // 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)&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()) {
+    diag->Error(android::DiagMessage()
+                << "PNG image dimensions are too large: " << width << "x" << height);
+    return {};
+  }
+
+  std::unique_ptr<Image> output_image = std::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(const Image* image, const NinePatch* nine_patch, OutputStream* out,
+              const PngOptions& options, IDiagnostics* diag, bool verbose) {
+  // 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) {
+    diag->Error(android::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) {
+    diag->Error(android::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)&diag, 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 (verbose) {
+    android::DiagMessage msg;
+    msg << " paletteSize=" << color_palette.size() << " alphaPaletteSize=" << alpha_palette.size()
+        << " maxGrayDeviation=" << max_gray_deviation
+        << " grayScale=" << (grayscale ? "true" : "false");
+    diag->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 (verbose) {
+    android::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;
+    }
+    diag->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 android