[wip] Add canvas.toDataURL("image/png") compression control

Source/platform/image-encoders/skia/PNGImageEncoder.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
             *output++ = alpha;
         } else {
             *output++ = SkGetPackedR32(*input);
             *output++ = SkGetPackedG32(*input);
             *output++ = SkGetPackedB32(*input);
             *output++ = alpha;
         }
     }
 }
 
-static bool encodePixels(IntSize imageSize, unsigned char* inputPixels, bool premultiplied, Vector<unsigned char>* output)
+static bool encodePixels(IntSize imageSize, unsigned char* inputPixels, bool premultiplied, int quality, Vector<unsigned char>* output)
 {
     imageSize.clampNegativeToZero();
     Vector<unsigned char> row;
 
     png_struct* png = png_create_write_struct(PNG_LIBPNG_VER_STRING, 0, 0, 0);
     png_info* info = png_create_info_struct(png);
     if (!png || !info || setjmp(png_jmpbuf(png))) {
         png_destroy_write_struct(png ? &png : 0, info ? &info : 0);
         return false;
     }
 
-    // Optimize compression for speed.
-    // The parameters are the same as what libpng uses by default for RGB and RGBA images, except:
-    // - the zlib compression level is 3 instead of 6, to avoid the lazy Ziv-Lempel match searching;
-    // - the delta filter is 1 ("sub") instead of 5 ("all"), to reduce the filter computations.
-    // The zlib memory level (8) and strategy (Z_FILTERED) will be set inside libpng.
-    //
-    // Avoid the zlib strategies Z_HUFFMAN_ONLY or Z_RLE.
-    // Although they are the fastest for poorly-compressible images (e.g. photographs),
-    // they are very slow for highly-compressible images (e.g. text, drawings or business graphics).
-    png_set_compression_level(png, 3);
-    png_set_filter(png, PNG_FILTER_TYPE_BASE, PNG_FILTER_SUB);
+    int compression = static_cast<int>(quality / 10.0 + 0.5);
+    if (compression > Z_BEST_COMPRESSION)
+        compression = Z_BEST_COMPRESSION;
+    else if (compression < Z_BEST_SPEED)
+        compression = Z_BEST_SPEED;
+
+    if (quality <= 0) {
+        png_set_compression_level(png, Z_BEST_SPEED);
+        png_set_filter(png, PNG_FILTER_TYPE_BASE, PNG_FILTER_NONE);
+    } else if (compression <= 3) {
+        // Optimize compression for speed.
+        // The parameters are the libpng defaults for RGB and RGBA images, except that:
+        //   - the zlib compression level is lower than the libpng default 6,
+        //     to avoid the lazy Ziv-Lempel match searching.
+        //   - the delta filter is 1 ("sub"), instead of 5 ("all"), to reduce
+        //     filter computations.
+        // Note: the zlib memory level (8) and strategy (Z_FILTERED) will be set inside
+        // libpng.
+        // Note: avoid the zlib strategies Z_HUFFMAN_ONLY or Z_RLE. Although they are the
+        // fastest for poorly-compressible images (e.g. photographs) they are very slow
+        // for highly-compressible images (e.g. text, drawings, and business graphics).
+        png_set_compression_level(png, compression);
+        png_set_filter(png, PNG_FILTER_TYPE_BASE, PNG_FILTER_SUB);
+    } else if (compression <= 7) {
+        png_set_compression_level(png, compression);
+        png_set_filter(png, PNG_FILTER_TYPE_BASE, PNG_FILTER_PAETH);
+    } else {
+        png_set_compression_level(png, compression);
+    }
 
     png_set_write_fn(png, output, writeOutput, 0);
     png_set_IHDR(png, info, imageSize.width(), imageSize.height(),
                  8, PNG_COLOR_TYPE_RGB_ALPHA, 0, 0, 0);
     png_write_info(png, info);
 
     unsigned char* pixels = inputPixels;
     row.resize(imageSize.width() * sizeof(SkPMColor));
     const size_t pixelRowStride = imageSize.width() * 4;
     for (int y = 0; y < imageSize.height(); ++y) {
         if (premultiplied) {
             preMultipliedBGRAtoRGBA(pixels, imageSize.width(), row.data());
             png_write_row(png, row.data());
         } else
             png_write_row(png, pixels);
         pixels += pixelRowStride;
     }
 
     png_write_end(png, info);
     png_destroy_write_struct(&png, &info);
     return true;
 }
 
-bool PNGImageEncoder::encode(const SkBitmap& bitmap, Vector<unsigned char>* output)
+bool PNGImageEncoder::encode(const SkBitmap& bitmap, int quality, Vector<unsigned char>* output)
 {
     SkAutoLockPixels bitmapLock(bitmap);
 
     if (bitmap.colorType() != kPMColor_SkColorType || !bitmap.getPixels())
         return false; // Only support 32 bit/pixel skia bitmaps.
 
-    return encodePixels(IntSize(bitmap.width(), bitmap.height()), static_cast<unsigned char*>(bitmap.getPixels()), true, output);
+    return encodePixels(IntSize(bitmap.width(), bitmap.height()), static_cast<unsigned char*>(bitmap.getPixels()), true, quality, output);
 }
 
-bool PNGImageEncoder::encode(const ImageDataBuffer& imageData, Vector<unsigned char>* output)
+bool PNGImageEncoder::encode(const ImageDataBuffer& imageData, int quality, Vector<unsigned char>* output)
 {
-    return encodePixels(imageData.size(), imageData.data(), false, output);
+    return encodePixels(imageData.size(), imageData.data(), false, quality, output);
 }
 
 } // namespace WebCore