#392 refactor code for html2image conversion

the test is taken from PR #393 comments.

CHANGELOG.md

@@ -1,7 +1,7 @@
 # Changelog
 
 ## 9.9.5 (under construction) - see https://github.com/flyingsaucerproject/flyingsaucer/milestone/21
-*
+* #392 Fix transparent background of resized base64 encoded images - thanks to @Openhelios (#393)
 
 ## 9.9.4 (20.09.2024) - see https://github.com/flyingsaucerproject/flyingsaucer/milestone/20?closed=1
 * Set "Page only" as default initial view in PDF viewer (#390)

flying-saucer-core/src/main/java/org/xhtmlrenderer/swing/Java2DRenderer.java

@@ -34,11 +34,18 @@
 import org.xhtmlrenderer.simple.extend.XhtmlNamespaceHandler;
 import org.xhtmlrenderer.util.Configuration;
 import org.xhtmlrenderer.util.ImageUtil;
+import org.xml.sax.SAXException;
 
+import javax.xml.parsers.DocumentBuilderFactory;
+import javax.xml.parsers.ParserConfigurationException;
 import java.awt.*;
 import java.awt.image.BufferedImage;
+import java.io.ByteArrayInputStream;
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
 
 /**
  * <p>Renders an XML files, formatted with CSS, as an image. Input is a document in the form of file or URL,
@@ -352,4 +359,13 @@ public boolean isFocus(Element e) {
             return false;
         }
     }
+
+    public static BufferedImage htmlAsImage(String html, int widthInPixels) throws SAXException {
+        try (InputStream in = new ByteArrayInputStream(html.getBytes(UTF_8))) {
+            Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(in);
+            return new Java2DRenderer(document, widthInPixels).getImage();
+        } catch (IOException | ParserConfigurationException e) {
+            throw new RuntimeException(e);
+        }
+    }
 }

flying-saucer-core/src/main/java/org/xhtmlrenderer/util/ImageUtil.java

@@ -32,6 +32,13 @@
 import java.util.Map;
 import java.util.logging.Level;
 
+import static java.awt.Transparency.OPAQUE;
+import static java.awt.Transparency.TRANSLUCENT;
+import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR;
+import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR_PRE;
+import static java.awt.image.BufferedImage.TYPE_INT_ARGB;
+import static java.awt.image.BufferedImage.TYPE_INT_ARGB_PRE;
+
 /**
  * Static utility methods for working with images. Meant to suggest "best practices" for the most straightforward
  * cases of working with images.
@@ -98,23 +105,24 @@ public static BufferedImage makeCompatible(BufferedImage image) {
     @Nonnull
     @CheckReturnValue
     public static BufferedImage createCompatibleBufferedImage(int width, int height, int biType) {
-        final BufferedImage image;
-
         GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
         if (ge.isHeadlessInstance()) {
-            image = new BufferedImage(width, height, biType);
+            return new BufferedImage(width, height, biType);
         } else {
             GraphicsConfiguration gc = getGraphicsConfiguration();
-
-            // TODO: check type using image type - can be sniffed; see Filthy Rich Clients
-            int type = (biType == BufferedImage.TYPE_INT_ARGB || biType == BufferedImage.TYPE_INT_ARGB_PRE 
-                || biType == BufferedImage.TYPE_4BYTE_ABGR || biType == BufferedImage.TYPE_4BYTE_ABGR_PRE ?
-                    Transparency.TRANSLUCENT : Transparency.OPAQUE);
-
-            image = gc.createCompatibleImage(width, height, type);
+            return gc.createCompatibleImage(width, height, detectTransparency(biType));
         }
+    }
 
-        return image;
+    static int detectTransparency(int biType) {
+        // TODO: check type using image type - can be sniffed; see Filthy Rich Clients
+        return switch (biType) {
+            case TYPE_INT_ARGB,
+                 TYPE_INT_ARGB_PRE,
+                 TYPE_4BYTE_ABGR,
+                 TYPE_4BYTE_ABGR_PRE -> TRANSLUCENT;
+            default -> OPAQUE;
+        };
     }
 
     @Nonnull
@@ -204,7 +212,7 @@ public static BufferedImage getScaledInstance(BufferedImage orgImage, int target
         Object hint = Configuration.valueFromClassConstant("xr.image.render-quality",
                 RenderingHints.VALUE_INTERPOLATION_BICUBIC);
 
-        ScalingOptions opt = new ScalingOptions(targetWidth, targetHeight, BufferedImage.TYPE_INT_ARGB, quality, hint);
+        ScalingOptions opt = new ScalingOptions(targetWidth, targetHeight, TYPE_INT_ARGB, quality, hint);
 
         return getScaledInstance(opt, orgImage);
     }
@@ -236,7 +244,7 @@ public static BufferedImage convertToBufferedImage(Image awtImg, int type) {
     @Nonnull
     @CheckReturnValue
     public static BufferedImage createTransparentImage(int width, int height) {
-        BufferedImage bi = createCompatibleBufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
+        BufferedImage bi = createCompatibleBufferedImage(width, height, TYPE_INT_ARGB);
         Graphics2D g2d = bi.createGraphics();
 
         // Make all filled pixels transparent

flying-saucer-core/src/test/java/org/xhtmlrenderer/swing/Java2DRendererTest.java

@@ -0,0 +1,57 @@
+package org.xhtmlrenderer.swing;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.imageio.ImageIO;
+import java.awt.image.BufferedImage;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.util.Base64;
+
+import static java.awt.Color.RED;
+import static java.awt.Color.WHITE;
+import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR;
+import static java.awt.image.BufferedImage.TYPE_4BYTE_ABGR_PRE;
+import static java.awt.image.BufferedImage.TYPE_INT_ARGB;
+import static java.awt.image.BufferedImage.TYPE_INT_ARGB_PRE;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class Java2DRendererTest {
+  private static final Logger log = LoggerFactory.getLogger(Java2DRendererTest.class);
+
+  @ParameterizedTest
+  @ValueSource(ints = {TYPE_INT_ARGB, TYPE_INT_ARGB_PRE, TYPE_4BYTE_ABGR, TYPE_4BYTE_ABGR_PRE})
+  void convertHtmlToImage_withResizedEmbeddedABGRImage(int imageType) throws Exception {
+    BufferedImage smallImg = create2PixelImage(imageType);
+      String html = """
+            <html><body>
+            <img width="100" src="data:image/png;base64,%s"/>
+            <h1>Hello</h1></body></html>""".formatted(imageBase64(smallImg));
+    BufferedImage htmlAsImage = Java2DRenderer.htmlAsImage(html, 100);
+
+    File result = new File("target/%s.%s.png".formatted(getClass().getSimpleName(), imageType));
+    ImageIO.write(htmlAsImage, "png", result);
+    log.info("Generated image from html: {}", result.getAbsolutePath());
+
+    assertEquals(RED.getRGB(), htmlAsImage.getRGB(75, 25));
+    assertEquals(WHITE.getRGB(), htmlAsImage.getRGB(25, 25));
+  }
+
+  private static BufferedImage create2PixelImage(int imageType) {
+    BufferedImage img = new BufferedImage(2, 1, imageType);
+    img.setRGB(0, 0, 0);
+    img.setRGB(1, 0, 0xFFFF0000);
+    return img;
+  }
+
+  private static String imageBase64(BufferedImage img) throws IOException {
+    try (ByteArrayOutputStream bytes = new ByteArrayOutputStream(96)) {
+      ImageIO.write(img, "png", bytes);
+      return Base64.getEncoder().encodeToString(bytes.toByteArray());
+    }
+  }
+}

flying-saucer-core/src/test/java/org/xhtmlrenderer/util/ImageUtilTest.java

@@ -4,7 +4,10 @@
 
 import java.awt.image.BufferedImage;
 
+import static java.awt.Transparency.OPAQUE;
+import static java.awt.Transparency.TRANSLUCENT;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.xhtmlrenderer.util.ImageUtil.detectTransparency;
 import static org.xhtmlrenderer.util.ImageUtil.isEmbeddedBase64Image;
 import static org.xhtmlrenderer.util.ImageUtil.loadEmbeddedBase64Image;
 
@@ -30,4 +33,16 @@ void embeddedBase64Image() {
         assertThat(image.getWidth()).isEqualTo(352);
         assertThat(image.getHeight()).isEqualTo(186);
     }
+
+    @Test
+    void detectsImageType() {
+        assertThat(detectTransparency(BufferedImage.TYPE_INT_ARGB)).isEqualTo(TRANSLUCENT);
+        assertThat(detectTransparency(BufferedImage.TYPE_INT_ARGB_PRE)).isEqualTo(TRANSLUCENT);
+        assertThat(detectTransparency(BufferedImage.TYPE_4BYTE_ABGR)).isEqualTo(TRANSLUCENT);
+        assertThat(detectTransparency(BufferedImage.TYPE_4BYTE_ABGR_PRE)).isEqualTo(TRANSLUCENT);
+
+        assertThat(detectTransparency(BufferedImage.TYPE_3BYTE_BGR)).isEqualTo(OPAQUE);
+        assertThat(detectTransparency(BufferedImage.TYPE_CUSTOM)).isEqualTo(OPAQUE);
+        assertThat(detectTransparency(BufferedImage.TYPE_INT_RGB)).isEqualTo(OPAQUE);
+    }
 }

pom.xml

@@ -63,6 +63,11 @@
       <artifactId>junit-jupiter-api</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-params</artifactId>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.assertj</groupId>
       <artifactId>assertj-core</artifactId>