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

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

Index: Source/platform/image-encoders/skia/PNGImageEncoder.cpp
diff --git a/Source/platform/image-encoders/skia/PNGImageEncoder.cpp b/Source/platform/image-encoders/skia/PNGImageEncoder.cpp
index b5da17265c610f5d8c300eb19e4e6bcc10488cf8..a38c85c78a7f2f89da09ae6db5a2d2296061ca4d 100644
--- a/Source/platform/image-encoders/skia/PNGImageEncoder.cpp
+++ b/Source/platform/image-encoders/skia/PNGImageEncoder.cpp
@@ -68,7 +68,7 @@ static void preMultipliedBGRAtoRGBA(const void* pixels, int pixelCount, unsigned
     }
 }
 
-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;
@@ -80,17 +80,35 @@ static bool encodePixels(IntSize imageSize, unsigned char* inputPixels, bool pre
         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(),
@@ -114,19 +132,19 @@ static bool encodePixels(IntSize imageSize, unsigned char* inputPixels, bool pre
     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