Smooth Scaling Rounding error fix for win32 #62

This commit contributes to fixing the implementation of Smooth scaling
of the ImageData to get rid of the rounding errors because of multiple
scale ups and downs with fractional scale factor. The commit moves
and modifies the DPIUtil::autoScaleImageData method implementation in
ImageData class to adapt the same.

contributes to
#62 and
#127

Author: amartya4256

bundles/org.eclipse.swt/Eclipse SWT/common/org/eclipse/swt/graphics/ImageData.java

@@ -1114,6 +1114,27 @@ int getByteOrder() {
 	return depth != 16 ? MSB_FIRST : LSB_FIRST;
 }
 
+/**
+ * Returns the scaled ImageData using smooth scaling.
+ *
+ * @since 3.130
+ */
+public ImageData scaleToUsingSmoothScaling(Device device, int width, int height) {
+	Image original = new Image (device, (ImageDataProvider) zoom -> this);
+	/* Create a 24 bit image data with alpha channel */
+	final ImageData resultData = new ImageData (width, height, 24, new PaletteData (0xFF, 0xFF00, 0xFF0000));
+	resultData.alphaData = new byte [width * height];
+	Image resultImage = new Image (device, (ImageDataProvider) zoom -> resultData);
+	GC gc = new GC (resultImage);
+	gc.setAntialias (SWT.ON);
+	gc.drawImage (original, 0, 0, this.width, this.height, 0, 0, width, height, false);
+	gc.dispose ();
+	original.dispose ();
+	ImageData result = resultImage.getImageData (DPIUtil.getDeviceZoom());
+	resultImage.dispose ();
+	return result;
+}
+
 /**
  * Returns a copy of the receiver which has been stretched or
  * shrunk to the specified size. If either the width or height

bundles/org.eclipse.swt/Eclipse SWT/common/org/eclipse/swt/internal/DPIUtil.java

@@ -294,26 +294,9 @@ private static ImageData autoScaleImageData (Device device, final ImageData imag
 	int scaledHeight = Math.round (height * scaleFactor);
 	boolean useSmoothScaling = autoScaleMethod == AutoScaleMethod.SMOOTH && imageData.getTransparencyType() != SWT.TRANSPARENCY_MASK;
 	if (useSmoothScaling) {
-		Image original = new Image (device, (ImageDataProvider) zoom -> imageData);
-		/* Create a 24 bit image data with alpha channel */
-		final ImageData resultData = new ImageData (scaledWidth, scaledHeight, 24, new PaletteData (0xFF, 0xFF00, 0xFF0000));
-		resultData.alphaData = new byte [scaledWidth * scaledHeight];
-		Image resultImage = new Image (device, (ImageDataProvider) zoom -> resultData);
-		GC gc = new GC (resultImage);
-		gc.setAntialias (SWT.ON);
-		gc.drawImage (original, 0, 0, autoScaleDown (width), autoScaleDown (height),
-				/* E.g. destWidth here is effectively DPIUtil.autoScaleDown (scaledWidth), but avoiding rounding errors.
-				 * Nevertheless, we still have some rounding errors due to the point-based API GC#drawImage(..).
-				 */
-				0, 0, Math.round (autoScaleDown (width * scaleFactor)), Math.round (autoScaleDown (height * scaleFactor)));
-		gc.dispose ();
-		original.dispose ();
-		ImageData result = resultImage.getImageData (getDeviceZoom ());
-		resultImage.dispose ();
-		return result;
-	} else {
-		return imageData.scaledTo (scaledWidth, scaledHeight);
+		return imageData.scaleToUsingSmoothScaling(device, scaledWidth, scaledHeight);
 	}
+	return imageData.scaledTo (scaledWidth, scaledHeight);
 }
 
 /**