GC with Image in unsupported use cases

Logging a warning for GC initialized with unsupported use cases.
1. all dynamic images (retrieved via any of the existing providers).
2. all images for which handles in other zoom values have already been
created.

Author: ShahzaibIbrahim

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/Image.java

@@ -80,6 +80,8 @@
  */
 public final class Image extends Resource implements Drawable {
 
+	static boolean strictChecks = System.getProperty("org.eclipse.swt.internal.enableStrictChecks") != null;
+
 	/**
 	 * specifies whether the receiver is a bitmap or an icon
 	 * (one of <code>SWT.BITMAP</code>, <code>SWT.ICON</code>)
@@ -1690,6 +1692,12 @@ private ImageHandle init(ImageData i, int zoom) {
 	}
 }
 
+private void assertCondition(boolean condition, String message) {
+    if (condition && strictChecks) {
+        System.err.print(message + "\nSee https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet384.java for supported variants.");
+    }
+}
+
 /**
  * Invokes platform specific functionality to allocate a new GC handle.
  * <p>
@@ -1716,6 +1724,9 @@ public long internal_new_GC (GCData data) {
 		SWT.error(SWT.ERROR_INVALID_ARGUMENT);
 	}
 
+	assertCondition(imageProvider != null && (imageProvider instanceof ImageDataProviderWrapper || imageProvider instanceof ImageFileNameProviderWrapper), "Image initialized with ImageDataProvider or ImageFileNameProvider is not supposed to be modified.");
+	assertCondition(!zoomLevelToImageHandle.isEmpty() && (zoomLevelToImageHandle.size() != 1 || !zoomLevelToImageHandle.containsKey(getZoom())), "Images with handles created for a different zoom level should not be modified.");
+
 	/* Create a compatible HDC for the device */
 	long hDC = device.internal_new_GC(null);
 	long imageDC = OS.CreateCompatibleDC(hDC);

examples/org.eclipse.swt.snippets/Snippets.md

@@ -214,6 +214,7 @@ To contribute a new snippet, [create a snippet contribution as a pull request](h
 - [draw an image scaled to half size and double size](https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet355.java) – [(preview)](https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/examples/org.eclipse.swt.snippets/previews/Snippet355.png "Preview for Snippet 355")
 - [draw an image at various zoom/dpi levels](https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet367.java) – [(preview)](https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/examples/org.eclipse.swt.snippets/previews/Snippet367.png "Preview for Snippet 367")
 - [draw a disabled/grayed image at various zoom levels](https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet382.java) – [(preview)](https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/examples/org.eclipse.swt.snippets/previews/Snippet382.png "Preview for Snippet 382")
+- [draw an image with ImageGcProvider to draw a watermark over it](https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet384.java) – [(preview)](https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/examples/org.eclipse.swt.snippets/previews/Snippet384.png "Preview for Snippet 384")
 
 ### **ImageData**
 - [display an animated GIF](https://github.com/eclipse-platform/eclipse.platform.swt/tree/master/examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet141.java)

examples/org.eclipse.swt.snippets/previews/Snippet384.png

@@ -0,0 +1,84 @@
+package org.eclipse.swt.snippets;
+/*******************************************************************************
+ * Copyright (c) 2025 Yatta Solutions
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Yatta Solutions - initial API and implementation
+ *******************************************************************************/
+
+import org.eclipse.swt.*;
+import org.eclipse.swt.graphics.*;
+import org.eclipse.swt.widgets.*;
+
+/**
+ * Snippet to show how to draw on a image loaded from the file system which has multiple variant for different zoom levels.
+ * Mainly motivated by custom drawing behavior in the win32 implementation
+ * <p>
+ * For a list of all SWT example snippets see
+ * http://www.eclipse.org/swt/snippets/
+ * </p>
+ */
+public class Snippet384 {
+
+    public static void main(String[] args) {
+		System.setProperty("swt.autoScale.updateOnRuntime", "true");
+        Display display = new Display();
+        Shell shell = new Shell(display);
+        shell.setText("Watermark over Image using GC");
+        shell.setSize(1200 ,900);
+
+        final ImageFileNameProvider filenameProvider = zoom -> {
+    		String path = null;
+    		switch (zoom) {
+    		case 150:
+    			path = "bin/org/eclipse/swt/snippets/red.jpeg";
+    			break;
+    		case 100:
+    			path = "bin/org/eclipse/swt/snippets/black.jpg";
+    			break;
+    		default:
+    			path = "bin/org/eclipse/swt/snippets/black.jpg";
+    		}
+    		return path;
+    	};
+        Image loadedImage = new Image(display, filenameProvider);
+
+        int width = loadedImage.getBounds().width;
+        int height = loadedImage.getBounds().height;
+
+        Image composedImage = new Image(display, (gc, iX, iY) -> {
+            gc.drawImage(loadedImage, 0, 0);
+
+            gc.setForeground(display.getSystemColor(SWT.COLOR_WHITE));
+            Font font = new Font(display, "Arial", 24, SWT.BOLD);
+            gc.setFont(font);
+            gc.setAlpha(128); // semi-transparent
+            String watermark = "WATERMARK";
+            Point extent = gc.textExtent(watermark);
+            int x = (width - extent.x) / 2;
+            int y = (height - extent.y) / 2;
+            gc.drawText(watermark, x, y, SWT.DRAW_TRANSPARENT);
+            font.dispose();
+        }, width, height);
+
+		shell.addPaintListener(e -> {
+            e.gc.drawImage(composedImage, 0, 0);
+        });
+
+        shell.open();
+        while (!shell.isDisposed()) {
+            if (!display.readAndDispatch()) display.sleep();
+        }
+
+        loadedImage.dispose();
+        composedImage.dispose();
+        display.dispose();
+    }
+}