Smooth Scaling Rounding error fix for win32 eclipse-platform#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 replicates
and modifies the DPIUtil::autoScaleImageData method implementation in
Image class to adapt the same.

contributes to
eclipse-platform#62 and
eclipse-platform#127

Author: amartya4256

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

@@ -292,7 +292,7 @@ private static ImageData autoScaleImageData (Device device, final ImageData imag
 	int height = imageData.height;
 	int scaledWidth = Math.round (width * scaleFactor);
 	int scaledHeight = Math.round (height * scaleFactor);
-	boolean useSmoothScaling = autoScaleMethod == AutoScaleMethod.SMOOTH && imageData.getTransparencyType() != SWT.TRANSPARENCY_MASK;
+	boolean useSmoothScaling = isSmoothScalingEnabled() && imageData.getTransparencyType() != SWT.TRANSPARENCY_MASK;
 	if (useSmoothScaling) {
 		Image original = new Image (device, (ImageDataProvider) zoom -> imageData);
 		/* Create a 24 bit image data with alpha channel */
@@ -316,6 +316,10 @@ private static ImageData autoScaleImageData (Device device, final ImageData imag
 	}
 }
 
+public static boolean isSmoothScalingEnabled() {
+	return autoScaleMethod == AutoScaleMethod.SMOOTH;
+}
+
 /**
  * Returns a new rectangle as per the scaleFactor.
  */
@@ -628,6 +632,10 @@ public static boolean useCairoAutoScale() {
 }
 
 public static int getZoomForAutoscaleProperty (int nativeDeviceZoom) {
+	return getZoomForAutoscaleProperty(nativeDeviceZoom, autoScaleValue);
+}
+
+private static int getZoomForAutoscaleProperty (int nativeDeviceZoom, String autoScaleValue) {
 	int zoom = 0;
 	if (autoScaleValue != null) {
 		if ("false".equalsIgnoreCase (autoScaleValue)) {

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

@@ -372,7 +372,7 @@ public Image(Device device, ImageData data) {
 	if (data == null) SWT.error(SWT.ERROR_NULL_ARGUMENT);
 	initialNativeZoom = DPIUtil.getNativeDeviceZoom();
 	int deviceZoom = getZoom();
-	data = DPIUtil.scaleImageData(device, new ElementAtZoom<>(data, 100), deviceZoom);
+	data = scaleImageData(data, deviceZoom, 100);
 	init(data, deviceZoom);
 	init();
 	this.device.registerResourceWithZoomSupport(this);
@@ -416,8 +416,8 @@ public Image(Device device, ImageData source, ImageData mask) {
 		SWT.error(SWT.ERROR_INVALID_ARGUMENT);
 	}
 	initialNativeZoom = DPIUtil.getNativeDeviceZoom();
-	source = DPIUtil.autoScaleUp(device, source);
-	mask = DPIUtil.autoScaleUp(device, mask);
+	source = scaleImageData(source, getZoom(), 100);
+	mask = scaleImageData(mask, getZoom(), 100);
 	mask = ImageData.convertMask(mask);
 	initIconHandle(this.device, source, mask, getZoom());
 	init();
@@ -481,7 +481,8 @@ public Image (Device device, InputStream stream) {
 	super(device);
 	initialNativeZoom = DPIUtil.getNativeDeviceZoom();
 	int deviceZoom = getZoom();
-	ImageData data = DPIUtil.scaleImageData(device, ImageDataLoader.load(stream, FileFormat.DEFAULT_ZOOM, deviceZoom), deviceZoom);
+	ElementAtZoom<ImageData> imageCandidate = ImageDataLoader.load(stream, FileFormat.DEFAULT_ZOOM, deviceZoom);
+	ImageData data = scaleImageData(imageCandidate.element(), deviceZoom, imageCandidate.zoom());
 	init(data, deviceZoom);
 	init();
 	this.device.registerResourceWithZoomSupport(this);
@@ -524,7 +525,8 @@ public Image (Device device, String filename) {
 	if (filename == null) SWT.error(SWT.ERROR_NULL_ARGUMENT);
 	initialNativeZoom = DPIUtil.getNativeDeviceZoom();
 	int deviceZoom = getZoom();
-	ImageData data = DPIUtil.scaleImageData(device, ImageDataLoader.load(filename, FileFormat.DEFAULT_ZOOM, deviceZoom), deviceZoom);
+	ElementAtZoom<ImageData> imageCandidate = ImageDataLoader.load(filename, FileFormat.DEFAULT_ZOOM, deviceZoom);
+	ImageData data = scaleImageData(imageCandidate.element(), deviceZoom, imageCandidate.zoom());
 	init(data, deviceZoom);
 	init();
 	this.device.registerResourceWithZoomSupport(this);
@@ -1264,7 +1266,7 @@ private ImageData getScaledImageData (int zoom) {
 	}
 	TreeSet<Integer> availableZooms = new TreeSet<>(zoomLevelToImageHandle.keySet());
 	int closestZoom = Optional.ofNullable(availableZooms.higher(zoom)).orElse(availableZooms.lower(zoom));
-	return DPIUtil.scaleImageData (device, getImageMetadata(closestZoom).getImageData(), zoom, closestZoom);
+	return scaleImageData(getImageMetadata(closestZoom).getImageData(), zoom, closestZoom);
 }
 
 
@@ -1854,6 +1856,40 @@ public void setBackground(Color color) {
 	zoomLevelToImageHandle.values().forEach(imageHandle -> imageHandle.setBackground(backgroundColor));
 }
 
+private ImageData scaleImageData(final ImageData imageData, int targetZoom, int currentZoom) {
+	if (imageData == null || targetZoom == currentZoom || (device != null && !device.isAutoScalable())) return imageData;
+	float scaleFactor = (float) targetZoom / (float) currentZoom;
+	int width = imageData.width;
+	int height = imageData.height;
+	int scaledWidth = Math.round (width * scaleFactor);
+	int scaledHeight = Math.round (height * scaleFactor);
+	boolean useSmoothScaling = DPIUtil.isSmoothScalingEnabled() && imageData.getTransparencyType() != SWT.TRANSPARENCY_MASK;
+	if (useSmoothScaling) {
+		return scaleToUsingSmoothScaling(scaledWidth, scaledHeight, imageData);
+	}
+	return imageData.scaledTo (scaledWidth, scaledHeight);
+}
+
+private ImageData scaleToUsingSmoothScaling(int width, int height, ImageData imageData) {
+	Image original = new Image (device, (ImageDataProvider) zoom -> imageData);
+	/* 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, imageData.width, imageData.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, width, height, false);
+	gc.dispose ();
+	original.dispose ();
+	ImageData result = resultImage.getImageData (resultImage.getZoom());
+	resultImage.dispose ();
+	return result;
+}
+
 private int getZoom() {
 	return DPIUtil.getZoomForAutoscaleProperty(initialNativeZoom);
 }
@@ -2073,7 +2109,7 @@ ImageData getImageData(int zoom) {
 
 	private ImageData scaleIfNecessary(ElementAtZoom<ImageData> imageDataAtZoom, int zoom) {
 		if (imageDataAtZoom.zoom() != zoom) {
-			return DPIUtil.scaleImageData(device, imageDataAtZoom, zoom);
+			return scaleImageData(imageDataAtZoom.element(), zoom, imageDataAtZoom.zoom());
 		} else {
 			return imageDataAtZoom.element();
 		}
@@ -2298,13 +2334,13 @@ private class ImageDataProviderWrapper extends BaseImageProviderWrapper<ImageDat
 	@Override
 	ImageData getImageData(int zoom) {
 		ElementAtZoom<ImageData> data = DPIUtil.validateAndGetImageDataAtZoom (provider, zoom);
-		return DPIUtil.scaleImageData (device, data.element(), zoom, data.zoom());
+		return scaleImageData(data.element(), zoom, data.zoom());
 	}
 
 	@Override
 	ImageHandle getImageMetadata(int zoom) {
 		ElementAtZoom<ImageData> imageCandidate = DPIUtil.validateAndGetImageDataAtZoom (provider, zoom);
-		ImageData resizedData = DPIUtil.scaleImageData (device, imageCandidate.element(), zoom, imageCandidate.zoom());
+		ImageData resizedData = scaleImageData(imageCandidate.element(), zoom, imageCandidate.zoom());
 		ImageData newData = adaptImageDataIfDisabledOrGray(resizedData);
 		init(newData, zoom);
 		return zoomLevelToImageHandle.get(zoom);