Calculate Scaling for Font without Device#getDPI()

getDPI always returns points for primary monitor and can produce faulty behavior. Printing the code is broken because Printer DPI (400) is different from the Monitor DPI (100). Now we compute points from the zoom level instead of currentFontDPI.

Author: ShahzaibIbrahim

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/StyledText.java

@@ -187,6 +187,7 @@ public class StyledText extends Canvas {
 	AccessibleAdapter accAdapter;
 	MouseNavigator mouseNavigator;
 	boolean middleClickPressed;
+	private final static int BASE_DPI = 96;
 
 	//block selection
 	boolean blockSelection;
@@ -304,9 +305,11 @@ void cacheLineData(StyledText styledText) {
 				}
 			}
 		}
-		Point screenDPI = styledText.getDisplay().getDPI();
+
 		Point printerDPI = printer.getDPI();
 		resources = new HashMap<> ();
+		int scaleFactorX = printerDPI.x / BASE_DPI;
+		int scaleFactorY = printerDPI.y / BASE_DPI;
 		for (int i = 0; i < lineCount; i++) {
 			Color color = printerRenderer.getLineBackground(i, null);
 			if (color != null) {
@@ -323,7 +326,7 @@ void cacheLineData(StyledText styledText) {
 			}
 			int indent = printerRenderer.getLineIndent(i, 0);
 			if (indent != 0) {
-				printerRenderer.setLineIndent(i, 1, indent * printerDPI.x / screenDPI.x);
+				printerRenderer.setLineIndent(i, 1, indent * scaleFactorX);
 			}
 		}
 		StyleRange[] styles = printerRenderer.styles;
@@ -367,17 +370,17 @@ void cacheLineData(StyledText styledText) {
 			if (!printOptions.printTextFontStyle) {
 				style.fontStyle = SWT.NORMAL;
 			}
-			style.rise = style.rise * printerDPI.y / screenDPI.y;
+			style.rise = style.rise * scaleFactorY;
 			GlyphMetrics metrics = style.metrics;
 			if (metrics != null) {
-				metrics.ascent = metrics.ascent * printerDPI.y / screenDPI.y;
-				metrics.descent = metrics.descent * printerDPI.y / screenDPI.y;
-				metrics.width = metrics.width * printerDPI.x / screenDPI.x;
+				metrics.ascent = metrics.ascent * scaleFactorY;
+				metrics.descent = metrics.descent * scaleFactorY;
+				metrics.width = metrics.width * scaleFactorX;
 			}
 		}
-		lineSpacing = styledText.lineSpacing * printerDPI.y / screenDPI.y;
+		lineSpacing = styledText.lineSpacing * scaleFactorY;
 		if (printOptions.printLineNumbers) {
-			printMargin = 3 * printerDPI.x / screenDPI.x;
+			printMargin = 3 * scaleFactorX;
 		}
 	}
 	/**

bundles/org.eclipse.swt/Eclipse SWT Printing/win32/org/eclipse/swt/printing/Printer.java

@@ -268,6 +268,12 @@ protected void create(DeviceData deviceData) {
 	if (handle == 0) SWT.error(SWT.ERROR_NO_HANDLES);
 }
 
+@Override
+protected int getDeviceZoom() {
+	// Printer directly renders on pixel basis, so the zoom must be fixed to 100
+	return 100;
+}
+
 /**
  * Invokes platform specific functionality to allocate a new GC handle.
  * <p>
@@ -295,8 +301,9 @@ public long internal_new_GC(GCData data) {
 			data.style |= SWT.LEFT_TO_RIGHT;
 		}
 		data.device = this;
-		data.font = Font.win32_new(this, OS.GetCurrentObject(handle, OS.OBJ_FONT));
+		data.font = Font.win32_new(this, OS.GetCurrentObject(handle, OS.OBJ_FONT), getDeviceZoom());
 		isGCCreated = true;
+		data.nativeZoom = getDeviceZoom();
 	}
 	return handle;
 }

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

@@ -268,12 +268,20 @@ int computePixels(float height) {
 }
 
 float computePoints(LOGFONT logFont, long hFont) {
-	return computePoints(logFont, hFont, -1);
+	return computePoints(logFont, hFont, SWT.DEFAULT);
 }
 
-float computePoints(LOGFONT logFont, long hFont, int currentFontDPI) {
+float computePoints(LOGFONT logFont, long hFont, int zoom) {
 	long hDC = internal_new_GC (null);
-	int logPixelsY = OS.GetDeviceCaps(hDC, OS.LOGPIXELSY);
+
+	float conversionFactor = 72f;
+	if (isAutoScalable() && zoom != SWT.DEFAULT) {
+		// For auto scalable devices we need to use a dynamic
+		// DPI value that is extracted from the zoom
+		conversionFactor /= DPIUtil.mapZoomToDPI(zoom);
+	} else {
+		conversionFactor /= OS.GetDeviceCaps(hDC, OS.LOGPIXELSY);
+	}
 	int pixels = 0;
 	if (logFont.lfHeight > 0) {
 		/*
@@ -292,14 +300,7 @@ float computePoints(LOGFONT logFont, long hFont, int currentFontDPI) {
 		pixels = -logFont.lfHeight;
 	}
 	internal_dispose_GC (hDC, null);
-	float adjustedZoomFactor = 1.0f;
-	if (currentFontDPI > 0) {
-		// as Device::computePoints will always return point on the basis of the
-		// primary monitor zoom, a custom zoomFactor must be calculated if the font
-		// is used for a different zoom level
-		adjustedZoomFactor *= (float) logPixelsY / (float) currentFontDPI;
-	}
-	return adjustedZoomFactor * pixels * 72f / logPixelsY;
+	return pixels * conversionFactor;
 }
 
 /**

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

@@ -213,7 +213,7 @@ public FontData[] getFontData() {
 	if (isDisposed()) SWT.error(SWT.ERROR_GRAPHIC_DISPOSED);
 	LOGFONT logFont = new LOGFONT ();
 	OS.GetObject(handle, LOGFONT.sizeof, logFont);
-	float heightInPoints = device.computePoints(logFont, handle, DPIUtil.mapZoomToDPI(zoom));
+	float heightInPoints = device.computePoints(logFont, handle, zoom);
 	return new FontData[] {FontData.win32_new(logFont, heightInPoints)};
 }
 
@@ -280,7 +280,7 @@ private static int extractZoom(Device device) {
 	if (device == null) {
 		return DPIUtil.getNativeDeviceZoom();
 	}
-	return DPIUtil.mapDPIToZoom(device._getDPIx());
+	return device.getDeviceZoom();
 }
 
 /**
@@ -331,8 +331,15 @@ public static Font win32_new(Device device, long handle) {
  * @since 3.126
  */
 public static Font win32_new(Device device, long handle, int zoom) {
-	Font font = win32_new(device, handle);
+	Font font = new Font(device);
 	font.zoom = zoom;
+	font.handle = handle;
+	/*
+	 * When created this way, Font doesn't own its .handle, and
+	 * for this reason it can't be disposed. Tell leak detector
+	 * to just ignore it.
+	 */
+	font.ignoreNonDisposed();
 	return font;
 }
 

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/internal/ScalingSWTFontRegistry.java

@@ -53,7 +53,8 @@ private Font getScaledFont(int targetZoom) {
 
 		private Font scaleFont(int zoom) {
 			FontData fontData = baseFont.getFontData()[0];
-			fontData.data.lfHeight = computePixels(zoom, fontData);
+			int baseZoom = computeZoom(fontData);
+			fontData.data.lfHeight = fontData.data.lfHeight * Math.round(1.0f*zoom/baseZoom);
 			Font scaledFont = Font.win32_new(device, fontData, zoom);
 			addScaledFont(zoom, scaledFont);
 			return scaledFont;
@@ -168,24 +169,16 @@ private Font getOrCreateFont(ScaledFontContainer container, int zoom) {
 	}
 
 	private int computeZoom(FontData fontData) {
-		int dpi = device.getDPI().x;
-		int pixelsAtPrimaryMonitorZoom = computePixels(fontData.height);
-		int value = DPIUtil.mapDPIToZoom(dpi) * fontData.data.lfHeight / pixelsAtPrimaryMonitorZoom;
-		return value;
-	}
-
-	private int computePixels(int zoom, FontData fontData) {
-		int dpi = device.getDPI().x;
-		int adjustedLogFontHeight = computePixels(fontData.height);
-		int primaryZoom = DPIUtil.mapDPIToZoom(dpi);
-		if (zoom != primaryZoom) {
-			adjustedLogFontHeight *= (1f * zoom / primaryZoom);
+		int pixelHeight = fontData.data.lfHeight;
+		float pointHeight = fontData.height;
+		if (pixelHeight == 0 || pointHeight == 0.0) {
+			return 100;
 		}
-		return adjustedLogFontHeight;
+		float computedPoints = computePointsFromPixels(pixelHeight);
+		return Math.round(100.0f * computedPoints / pointHeight);
 	}
 
-	private int computePixels(float height) {
-		int dpi = device.getDPI().x;
-		return -(int)(0.5f + (height * dpi / 72f));
+	private float computePointsFromPixels(int pixelHeight) {
+		return -(pixelHeight / 96f * 72f);
 	}
 }