Merge pull request #737 from tc-wleite/WebP_FixAlphaSubsampling

WebP: Fix alpha decoding when source subsampling is used

imageio/imageio-webp/src/main/java/com/twelvemonkeys/imageio/plugins/webp/WebPImageReader.java

@@ -61,6 +61,8 @@ import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
 
+import static com.twelvemonkeys.imageio.plugins.webp.lossless.VP8LDecoder.copyIntoRasterWithParams;
+
 import static java.lang.Math.max;
 import static java.lang.Math.min;
 
@@ -536,21 +538,26 @@ final class WebPImageReader extends ImageReaderBase {
                 // Simulate header
                 imageInput.seek(imageInput.getStreamPosition() - 5);
 
-                WritableRaster tempRaster = Raster.createInterleavedRaster(DataBuffer.TYPE_BYTE, destination.getWidth(), destination.getHeight(), 4, null);
-                readVP8Lossless(tempRaster, param, width, height);
+                // Temp alpha raster must have same dimensions as the source, because of filtering.
+                WritableRaster tempRaster = Raster.createInterleavedRaster(DataBuffer.TYPE_BYTE, width, height, 4, null);
+                readVP8Lossless(tempRaster, null, width, height);
 
                 // Copy from green (band 1) in temp to alpha in destination
-                alphaRaster.setRect(tempRaster.createChild(0, 0, tempRaster.getWidth(), tempRaster.getHeight(), 0, 0, new int[] {1}));
+                WritableRaster alphaChannel = tempRaster.createWritableChild(0, 0, tempRaster.getWidth(), tempRaster.getHeight(), 0, 0, new int[]{1});
+                alphaFilter(alphaChannel, filtering);
+                copyIntoRasterWithParams(alphaChannel, alphaRaster, param);
                 break;
             default:
                 processWarningOccurred("Unknown WebP alpha compression: " + compression);
                 opaqueAlpha(alphaRaster);
                 break;
         }
+    }
 
+    private void alphaFilter(WritableRaster alphaRaster, int filtering) {
         if (filtering != AlphaFiltering.NONE) {
-            for (int y = 0; y < destination.getHeight(); y++) {
-                for (int x = 0; x < destination.getWidth(); x++) {
+            for (int y = 0; y < alphaRaster.getHeight(); y++) {
+                for (int x = 0; x < alphaRaster.getWidth(); x++) {
                     int predictorAlpha = getPredictorAlpha(alphaRaster, filtering, y, x);
                     alphaRaster.setSample(x, y, 0, alphaRaster.getSample(x, y, 0) + predictorAlpha % 256);
                 }

imageio/imageio-webp/src/main/java/com/twelvemonkeys/imageio/plugins/webp/lossless/VP8LDecoder.java

@@ -135,33 +135,36 @@ public final class VP8LDecoder {
         }
 
         if (fullSizeRaster != raster && param != null) {
-            // Copy into destination raster with settings applied
-            Rectangle sourceRegion = param.getSourceRegion();
-            int sourceXSubsampling = param.getSourceXSubsampling();
-            int sourceYSubsampling = param.getSourceYSubsampling();
-            int subsamplingXOffset = param.getSubsamplingXOffset();
-            int subsamplingYOffset = param.getSubsamplingYOffset();
-            Point destinationOffset = param.getDestinationOffset();
+            copyIntoRasterWithParams(fullSizeRaster, raster, param);
+        }
+    }
+    
+    /**
+     * Copy a source raster into a destination raster with settings applied.
+     */
+    public static void copyIntoRasterWithParams(final Raster srcRaster, final WritableRaster dstRaster,
+            final ImageReadParam param) {
+        Rectangle sourceRegion = param != null && param.getSourceRegion() != null ? param.getSourceRegion() : dstRaster.getBounds();
+        int sourceXSubsampling = param != null ? param.getSourceXSubsampling() : 1;
+        int sourceYSubsampling = param != null ? param.getSourceYSubsampling() : 1;
+        int subsamplingXOffset = param != null ? param.getSubsamplingXOffset() : 0;
+        int subsamplingYOffset = param != null ? param.getSubsamplingYOffset() : 0;
+        Point destinationOffset = param != null ? param.getDestinationOffset() : new Point(0, 0) ;
 
-            if (sourceRegion == null) {
-                sourceRegion = raster.getBounds();
-            }
+        if (sourceXSubsampling == 1 && sourceYSubsampling == 1) {
+            // Only apply offset (and limit to requested region)
+            dstRaster.setRect(destinationOffset.x, destinationOffset.y, srcRaster);
+        }
+        else {
+            // Manual copy, more efficient way might exist
+            byte[] rgba = new byte[4];
+            int xEnd = dstRaster.getWidth() + dstRaster.getMinX();
+            int yEnd = dstRaster.getHeight() + dstRaster.getMinY();
 
-            if (sourceXSubsampling == 1 && sourceYSubsampling == 1) {
-                // Only apply offset (and limit to requested region)
-                raster.setRect(destinationOffset.x, destinationOffset.y, fullSizeRaster);
-            }
-            else {
-                // Manual copy, more efficient way might exist
-                byte[] rgba = new byte[4];
-                int xEnd = raster.getWidth() + raster.getMinX();
-                int yEnd = raster.getHeight() + raster.getMinY();
-
-                for (int xDst = destinationOffset.x, xSrc = sourceRegion.x + subsamplingXOffset; xDst < xEnd; xDst++, xSrc += sourceXSubsampling) {
-                    for (int yDst = destinationOffset.y, ySrc = sourceRegion.y + subsamplingYOffset; yDst < yEnd; yDst++, ySrc += sourceYSubsampling) {
-                        fullSizeRaster.getDataElements(xSrc, ySrc, rgba);
-                        raster.setDataElements(xDst, yDst, rgba);
-                    }
+            for (int xDst = destinationOffset.x, xSrc = sourceRegion.x + subsamplingXOffset; xDst < xEnd; xDst++, xSrc += sourceXSubsampling) {
+                for (int yDst = destinationOffset.y, ySrc = sourceRegion.y + subsamplingYOffset; yDst < yEnd; yDst++, ySrc += sourceYSubsampling) {
+                    srcRaster.getDataElements(xSrc, ySrc, rgba);
+                    dstRaster.setDataElements(xDst, yDst, rgba);
                 }
             }
         }

imageio/imageio-webp/src/test/java/com/twelvemonkeys/imageio/plugins/webp/WebPImageReaderTest.java

@@ -58,7 +58,9 @@ public class WebPImageReaderTest extends ImageReaderAbstractTest<WebPImageReader
                         new Dimension(400, 400), new Dimension(400, 400), new Dimension(400, 394),
                         new Dimension(371, 394), new Dimension(394, 382), new Dimension(400, 388),
                         new Dimension(394, 383), new Dimension(394, 394), new Dimension(372, 394),
-                        new Dimension(400, 400), new Dimension(320, 382))
+                        new Dimension(400, 400), new Dimension(320, 382)),
+                // Alpha transparency and Alpha filtering
+                new TestData(getClassLoaderResource("/webp/alpha_filter.webp"), new Dimension(1600, 1600))
         );
     }
 
@@ -162,4 +164,27 @@ public class WebPImageReaderTest extends ImageReaderAbstractTest<WebPImageReader
             reader.dispose();
         }
     }
+
+    @Test
+    public void testAlphaSubsampling() throws IOException {
+        WebPImageReader reader = createReader();
+
+        try (ImageInputStream stream = ImageIO.createImageInputStream(getClassLoaderResource("/webp/alpha_filter.webp"))) {
+            reader.setInput(stream);
+
+            // Read the image using a subsampling factor of 2 
+            ImageReadParam param = new ImageReadParam();
+            param.setSourceSubsampling(2, 2, 0, 0);
+
+            BufferedImage image = reader.read(0, param);
+
+            assertRGBEquals("Expected transparent at (100, 265)", 0x00000000, image.getRGB(100, 265) & 0xFF000000, 8);
+            assertRGBEquals("Expected transparent at (512, 320)", 0x00000000, image.getRGB(512, 320) & 0xFF000000, 8);
+            assertRGBEquals("Expected opaque at (666, 444)", 0xFF000000, image.getRGB(666, 444) & 0xFF000000, 8);
+            assertRGBEquals("Expected opaque corner (799, 799)", 0xFF000000, image.getRGB(699, 699) & 0xFF000000, 8);
+        }
+        finally {
+            reader.dispose();
+        }
+    }
 }