8356137: GifImageDecode can produce opaque image when disposal method changes

src/java.desktop/share/classes/sun/awt/image/GifImageDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1995, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -31,6 +31,7 @@
  */
 package sun.awt.image;
 
+import java.util.Arrays;
 import java.util.Hashtable;
 import java.io.InputStream;
 import java.io.IOException;
@@ -569,6 +570,9 @@ private boolean readImage(boolean first, int disposal_method, int delay)
                               0, 0);
                 }
             }
+            if (model.getTransparentPixel() > 0) {
+                Arrays.fill(saved_image, (byte) model.getTransparentPixel());
+            }
         }
 
         int hints = (interlace ? interlaceflags : normalflags);

test/jdk/sun/awt/image/gif/GifBuilder.java

@@ -0,0 +1,166 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import javax.imageio.IIOImage;
+import javax.imageio.ImageIO;
+import javax.imageio.ImageTypeSpecifier;
+import javax.imageio.ImageWriter;
+import javax.imageio.ImageWriteParam;
+import javax.imageio.metadata.IIOMetadata;
+import javax.imageio.metadata.IIOMetadataNode;
+import javax.imageio.stream.ImageOutputStream;
+import java.awt.AlphaComposite;
+import java.awt.Color;
+import java.awt.Graphics2D;
+import java.awt.image.BufferedImage;
+import java.awt.image.IndexColorModel;
+import java.io.File;
+import java.io.IOException;
+import java.net.URL;
+
+/**
+ * This constructs sample gif files used to test different combinations
+ * of gif frame disposal methods and transparent pixel indices.
+ */
+public class GifBuilder {
+
+    /**
+     * Different disposal methods for gif frames. These names exactly
+     * correspond to the String identifier ImageIO uses.
+     */
+    public enum Disposal {none, doNotDispose, restoreToBackgroundColor,
+        restoreToPrevious};
+
+
+    /**
+     * @param disposal the frame disposal method
+     * @param isFirstTableIndexTransparent if true then the transparent pixel
+     *                                     is set to 0. If false then the
+     *                                     transparent pixel is set to the
+     *                                     last index.
+     */
+    public record FrameDescription(Disposal disposal, boolean
+            isFirstTableIndexTransparent) {}
+
+    /**
+     * This creates a sample gif image based on a series of FrameDescriptions,
+     * and the calls {@link GifComparison#run(URL)}
+     */
+    public static void test(FrameDescription... frameDescriptions)
+            throws Throwable {
+        File file = createTestFile(frameDescriptions);
+        try {
+            GifComparison.run(file.toURI().toURL());
+        } finally {
+            file.delete();
+        }
+    }
+
+    private static File createTestFile(FrameDescription... frameDescriptions)
+            throws IOException {
+        Color[] colors = new Color[] {
+                Color.red,
+                Color.yellow,
+                Color.green,
+                Color.cyan
+        };
+        File file = File.createTempFile("GifBuilder", ".gif");
+        ImageOutputStream ios = ImageIO.createImageOutputStream(file);
+        ImageWriter gifWriter = ImageIO.getImageWritersByFormatName("GIF").
+                next();
+        gifWriter.setOutput(ios);
+        ImageWriteParam wparam = gifWriter.getDefaultWriteParam();
+        IIOMetadata streamMetadata = gifWriter.
+                getDefaultStreamMetadata(wparam);
+        gifWriter.prepareWriteSequence(streamMetadata);
+
+        IndexColorModel icm = createIndexColorModel(colors, colors.length - 1);
+
+        ImageTypeSpecifier s = ImageTypeSpecifier.createFromBufferedImageType(
+                BufferedImage.TYPE_BYTE_INDEXED);
+        IIOMetadata metadata = gifWriter.getDefaultImageMetadata(s, wparam);
+        String metaFormatName = metadata.getNativeMetadataFormatName();
+
+        for (FrameDescription frameDescription : frameDescriptions) {
+
+            // prepare the image:
+            int width = 100 + 50 * (icm.getMapSize() - 2);
+            BufferedImage bi = new BufferedImage(width, 100,
+                    BufferedImage.TYPE_BYTE_INDEXED, icm);
+            Graphics2D g = bi.createGraphics();
+            g.setComposite(AlphaComposite.Clear);
+            g.fillRect(0, 0, bi.getWidth(), bi.getHeight());
+            g.setComposite(AlphaComposite.SrcOver);
+            int x = 0;
+            for (int a = 0; a < icm.getMapSize() - 1; a++) {
+                if (a != icm.getTransparentPixel()) {
+                    Color color = new Color(icm.getRGB(a));
+                    g.setColor(color);
+                    g.fillOval(x, 0, 100, 100);
+                    x += 50;
+                }
+            }
+            g.dispose();
+
+            // wrap attributes for gifWriter:
+            int transparentPixel = frameDescription.isFirstTableIndexTransparent
+                    ? 0 : icm.getMapSize() - 1;
+            IIOMetadata frameMetadata = gifWriter.getDefaultImageMetadata(
+                    ImageTypeSpecifier.createFromRenderedImage(bi), wparam);
+            IIOMetadataNode root = new IIOMetadataNode(metaFormatName);
+            IIOMetadataNode gce = new IIOMetadataNode(
+                    "GraphicControlExtension");
+            gce.setAttribute("disposalMethod",
+                    frameDescription.disposal.name());
+            gce.setAttribute("userInputFlag", "FALSE");
+            gce.setAttribute("transparentColorFlag", "TRUE");
+            gce.setAttribute("delayTime", "0");
+            gce.setAttribute("transparentColorIndex",
+                    Integer.toString(transparentPixel));
+            root.appendChild(gce);
+            frameMetadata.mergeTree(metaFormatName, root);
+            IIOImage img = new IIOImage(bi,  null, frameMetadata);
+            gifWriter.writeToSequence(img, wparam);
+        }
+        gifWriter.endWriteSequence();
+        ios.flush();
+        ios.close();
+
+        return file;
+    }
+
+    private static IndexColorModel createIndexColorModel(Color[] colors,
+                                                 int transparentIndex) {
+        byte[] r = new byte[colors.length];
+        byte[] g = new byte[colors.length];
+        byte[] b = new byte[colors.length];
+        for (int a = 0; a < colors.length; a++) {
+            r[a] = (byte) colors[a].getRed();
+            g[a] = (byte) colors[a].getGreen();
+            b[a] = (byte) colors[a].getBlue();
+        }
+        int bits = (int)(Math.log(colors.length) / Math.log(2) + .5);
+        return new IndexColorModel(bits, colors.length, r, g, b,
+                transparentIndex);
+    }
+}