Deprecating Display#getDPI

Having the scale factor being based on the screen DPI leads to
unexpected result e.g. Image too big/small. Having a screen dpi
independent factor leads to consistent results

Author: ShahzaibIbrahim

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

@@ -624,7 +624,7 @@ public Point getDPI() {
 }
 
 Point getIndependentDPI() {
-	return super.getDPI();
+	return new Point(72, 72);
 }
 
 /**

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

@@ -391,7 +391,18 @@ public int getDepth () {
  * @exception SWTException <ul>
  *    <li>ERROR_DEVICE_DISPOSED - if the receiver has been disposed</li>
  * </ul>
+ *
+ * @deprecated <p>This method returns a single global DPI value
+ * that does not reflect per-monitor DPI settings on modern operating systems.
+ * In environments with different scaling factors across monitors, it may provide
+ * a misleading or meaningless result, as it does not correspond to the actual DPI
+ * of any specific monitor.</p>
+ *
+ * <p>Note: While deprecated for general {@code Device} instances like {@code Display},
+ * this method may still be validly used when called on a {@code Printer} instance,
+ * where a single global DPI value is meaningful and expected.</p>
  */
+@Deprecated
 public Point getDPI () {
 	checkDevice ();
 	return getScreenDPI();
@@ -611,8 +622,9 @@ protected void init () {
 	/* Initialize the system font slot */
 	boolean smallFonts = System.getProperty("org.eclipse.swt.internal.carbon.smallFonts") != null;
 	double systemFontSize = smallFonts ? NSFont.smallSystemFontSize() : NSFont.systemFontSize();
-	Point dpi = this.dpi = getDPI(), screenDPI = getScreenDPI();
-	NSFont font = NSFont.systemFontOfSize(systemFontSize * dpi.y / screenDPI.y);
+	final int DOTS_PER_INCH = 96;
+	Point dpi = this.dpi = getDPI();
+	NSFont font = NSFont.systemFontOfSize(systemFontSize * dpi.y / DOTS_PER_INCH);
 	font.retain();
 	systemFont = Font.cocoa_new(this, font);
 }

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

@@ -244,6 +244,7 @@ public boolean equals(Object object) {
  */
 public FontData[] getFontData() {
 	if (isDisposed()) SWT.error(SWT.ERROR_GRAPHIC_DISPOSED);
+	final int DOTS_PER_INCH = 96;
 	NSAutoreleasePool pool = null;
 	if (!NSThread.isMainThread()) pool = (NSAutoreleasePool) new NSAutoreleasePool().alloc().init();
 	try {
@@ -258,8 +259,8 @@ public FontData[] getFontData() {
 		if ((traits & OS.NSBoldFontMask) != 0) style |= SWT.BOLD;
 		if ((extraTraits & OS.NSItalicFontMask) != 0) style |= SWT.ITALIC;
 		if ((extraTraits & OS.NSBoldFontMask) != 0) style |= SWT.BOLD;
-		Point dpi = device.dpi, screenDPI = device.getScreenDPI();
-		FontData data = new FontData(name, (float)handle.pointSize() * screenDPI.y / dpi.y, style);
+		Point dpi = device.dpi;
+		FontData data = new FontData(name, (float)handle.pointSize() * DOTS_PER_INCH / dpi.y, style);
 		data.nsName = nsName;
 		return new FontData[]{data};
 	} finally {
@@ -312,8 +313,9 @@ public int hashCode() {
 void init(String name, float height, int style, String nsName) {
 	if (name == null) SWT.error(SWT.ERROR_NULL_ARGUMENT);
 	if (height < 0) SWT.error(SWT.ERROR_INVALID_ARGUMENT);
-	Point dpi = device.dpi, screenDPI = device.getScreenDPI();
-	float size = height * dpi.y / screenDPI.y;
+	final int DOTS_PER_INCH = 72;
+	Point dpi = device.dpi;
+	float size = height * dpi.y / DOTS_PER_INCH;
 	NSFont systemFont = NSFont.systemFontOfSize(size);
 	NSFont boldSystemFont = NSFont.boldSystemFontOfSize(size);
 	String systemFontName = systemFont.familyName().getString();

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

@@ -455,7 +455,18 @@ public int getDepth () {
  * @exception SWTException <ul>
  *    <li>ERROR_DEVICE_DISPOSED - if the receiver has been disposed</li>
  * </ul>
+ *
+ * @deprecated <p>This method returns a single global DPI value
+ * that does not reflect per-monitor DPI settings on modern operating systems.
+ * In environments with different scaling factors across monitors, it may provide
+ * a misleading or meaningless result, as it does not correspond to the actual DPI
+ * of any specific monitor.</p>
+ *
+ * <p>Note: While deprecated for general {@code Device} instances like {@code Display},
+ * this method may still be validly used when called on a {@code Printer} instance,
+ * where a single global DPI value is meaningful and expected.</p>
  */
+@Deprecated
 public Point getDPI () {
 	checkDevice ();
 	return getScreenDPI();
@@ -641,6 +652,7 @@ public boolean getWarnings () {
  * @see #create
  */
 protected void init () {
+	final int DOTS_PER_INCH = 96;
 	if (debug) {
 		if (xDisplay != 0) {
 			/* Create the warning and error callbacks */
@@ -723,10 +735,9 @@ protected void init () {
 		}
 	}
 	defaultFont = OS.pango_font_description_copy (defaultFont);
-	Point dpi = getDPI(), screenDPI = getScreenDPI();
-	if (dpi.y != screenDPI.y) {
+	if (this.dpi.y != DOTS_PER_INCH) {
 		int size = OS.pango_font_description_get_size(defaultFont);
-		OS.pango_font_description_set_size(defaultFont, size * dpi.y / screenDPI.y);
+		OS.pango_font_description_set_size(defaultFont, size * this.dpi.y / DOTS_PER_INCH);
 	}
 	systemFont = Font.gtk_new (this, defaultFont);
 

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

@@ -184,15 +184,15 @@ public boolean equals(Object object) {
  */
 public FontData[] getFontData() {
 	if (isDisposed()) SWT.error(SWT.ERROR_GRAPHIC_DISPOSED);
-
+	final int DOTS_PER_INCH = 96;
 	long family = OS.pango_font_description_get_family(handle);
 	int length = C.strlen(family);
 	byte[] buffer = new byte[length];
 	C.memmove(buffer, family, length);
 	String name = new String(Converter.mbcsToWcs(buffer));
 	float height = (float)OS.pango_font_description_get_size(handle) / OS.PANGO_SCALE;
-	Point dpi = device.dpi, screenDPI = device.getScreenDPI();
-	float size = height * screenDPI.y / dpi.y;
+	Point dpi = device.dpi;
+	float size = height * DOTS_PER_INCH / dpi.y;
 	int pangoStyle = OS.pango_font_description_get_style(handle);
 	int pangoWeight = OS.pango_font_description_get_weight(handle);
 	int style = SWT.NORMAL;
@@ -254,8 +254,9 @@ public int hashCode() {
 void init(String name, float height, int style, byte[] fontString) {
 	if (name == null) SWT.error(SWT.ERROR_NULL_ARGUMENT);
 	if (height < 0) SWT.error(SWT.ERROR_INVALID_ARGUMENT);
-	Point dpi = device.dpi, screenDPI = device.getScreenDPI();
-	float size = height * dpi.y / screenDPI.y;
+	final int DOTS_PER_INCH = 96;
+	Point dpi = device.dpi;
+	float size = height * dpi.y / DOTS_PER_INCH;
 	if (fontString != null) {
 		handle = OS.pango_font_description_from_string (fontString);
 		if (handle == 0) SWT.error(SWT.ERROR_NO_HANDLES);

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Display.java

@@ -5043,7 +5043,8 @@ String debugInfoForIndex(long index) {
 }
 
 void dpiChanged(int newScaleFactor) {
-	DPIUtil.setDeviceZoom (DPIUtil.mapDPIToZoom(getDPI().x * newScaleFactor));
+	final int DOTS_PER_INCH = 96;
+	DPIUtil.setDeviceZoom (DPIUtil.mapDPIToZoom(DOTS_PER_INCH * newScaleFactor));
 	Shell[] shells = getShells();
 	for (int i = 0; i < shells.length; i++) {
 		shells[i].layout(true, true);

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

@@ -519,7 +519,18 @@ public int getDepth () {
  * @exception SWTException <ul>
  *    <li>ERROR_DEVICE_DISPOSED - if the receiver has been disposed</li>
  * </ul>
+ *
+ * @deprecated <p>This method returns a single global DPI value
+ * that does not reflect per-monitor DPI settings on modern operating systems.
+ * In environments with different scaling factors across monitors, it may provide
+ * a misleading or meaningless result, as it does not correspond to the actual DPI
+ * of any specific monitor.</p>
+ *
+ * <p>Note: While deprecated for general {@code Device} instances like {@code Display},
+ * this method may still be validly used when called on a {@code Printer} instance,
+ * where a single global DPI value is meaningful and expected.</p>
  */
+@Deprecated
 public Point getDPI () {
 	checkDevice ();
 	long hDC = internal_new_GC (null);

examples/org.eclipse.swt.examples/src/org/eclipse/swt/examples/imageanalyzer/ImageAnalyzer.java

@@ -1165,16 +1165,16 @@ void menuPrint() {
 		if (image == null) return;
 
 		try {
+			final int DOTS_PER_INCH = 96;
 			// Ask the user to specify the printer.
 			PrintDialog dialog = new PrintDialog(shell, SWT.NONE);
 			if (printerData != null) dialog.setPrinterData(printerData);
 			printerData = dialog.open();
 			if (printerData == null) return;
 
 			Printer printer = new Printer(printerData);
-			Point screenDPI = display.getDPI();
 			Point printerDPI = printer.getDPI();
-			int scaleFactor = printerDPI.x / screenDPI.x;
+			int scaleFactor = printerDPI.x / DOTS_PER_INCH;
 			Rectangle trim = printer.computeTrim(0, 0, 0, 0);
 			if (printer.startJob(currentName)) {
 				if (printer.startPage()) {

examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet361.java

@@ -146,6 +146,7 @@ public void getName(AccessibleEvent e) {
 	}
 
 	private static void performPrintAction(final Display display, final Shell shell) {
+		final int DOTS_PER_INCH = 96;
 		Rectangle r = composite.getBounds();
 		Point p = shell.toDisplay(r.x, r.y);
 		org.eclipse.swt.graphics.Image snapshotImage
@@ -159,9 +160,8 @@ private static void performPrintAction(final Display display, final Shell shell)
 		data = dialog.open();
 		if (data != null) {
 			Printer printer = new Printer(data);
-			Point screenDPI = display.getDPI();
 			Point printerDPI = printer.getDPI();
-			int scaleFactor = printerDPI.x / screenDPI.x;
+			int scaleFactor = printerDPI.x / DOTS_PER_INCH;
 			Rectangle trim = printer.computeTrim(0, 0, 0, 0);
 			if (printer.startJob("Print Image")) {
 				ImageData imageData = snapshotImage.getImageData();

examples/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet367.java

@@ -185,7 +185,7 @@ public int getGcStyle() {
 
 		createSeparator(shell);
 
-		new Label (shell, SWT.NONE).setText ("5. 50x50 box\n(Display#getDPI(): " + display.getDPI().x + ")");
+		new Label (shell, SWT.NONE).setText ("5. 50x50 box");
 		Label box= new Label (shell, SWT.NONE);
 		box.setBackground(display.getSystemColor(SWT.COLOR_WIDGET_DARK_SHADOW));
 		box.setLayoutData (new GridData (50, 50));