SWTGraphic scaling inconsistent across platforms

[ptziegler]

This was brought up here and here. When a plain SWTGraphic is used, scaling is done internally using an SWT Transform. This class seems to behave differently on different platforms.

For example the following snippet:

package test;

import org.eclipse.draw2d.SWTGraphics;
import org.eclipse.draw2d.ScaledGraphics;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Pattern;
import org.eclipse.swt.graphics.Transform;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class Test {
	public static void main(String[] args) {
		// System.setProperty("swt.autoScale.updateOnRuntime", "true");
		Shell shell = new Shell();
		shell.setSize(400, 400);

		Display d = shell.getDisplay();

		Color white = new Color(255, 255, 255);
		Color gray = new Color(128, 128, 128);

		shell.addPaintListener(event -> {
			GC gc = event.gc;

			for (int y = 0 ; y < shell.getSize().y; y += 50) {
				gc.setBackground(y % 20 == 0 ? white : gray);
				gc.fillRectangle(0, y, shell.getSize().x, 50);
			}
			
			SWTGraphics g1 = new SWTGraphics(gc);
			g1.translate(50, 100);
			g1.scale(2.5f);
			g1.setBackgroundColor(white);
			g1.drawText("Hello World", 0, 0);
			g1.dispose();

			gc.setTransform(null);
			

			ScaledGraphics g2 = new ScaledGraphics(new SWTGraphics(gc));
			g2.translate(50, 100);
			g2.scale(2.5f);
			g2.setBackgroundColor(white);
			g2.drawText("Hello World", 0, 0);
			g2.dispose();
		});

		shell.open();
		
		while(!shell.isDisposed()) {
			while(!d.readAndDispatch()) {
				d.sleep();
			}
		}
	}
}

[Phillipus]

Looks the same on Mac. What should it look like?

[ptziegler]

Looks the same on Mac. What should it look like?

@Phillipus you are correct. I've updated the snippet to better show the problem. This time using a side-by-side comparision of the SWTGraphics and the ScaledGraphics. Given that I perform the same operation with both Graphics instances, I would expect the string to be drawn at exactly the same position. But you clearly see rounding errors that only get worse with the scaling factor.

[ptziegler]

For reference on Linux:

[ptziegler]

Problem is this method here:

gef-classic/org.eclipse.draw2d/src/org/eclipse/draw2d/ScaledGraphics.java

Lines 1162 to 1177 in 574b59b

	private Point zoomTextPoint(int x, int y) {
	if (localCache.font != localFont) {
	// Font is different, re-calculate its height
	FontMetrics metric = FigureUtilities.getFontMetrics(localFont);
	localCache.height = metric.getHeight() - metric.getDescent();
	localCache.font = localFont;
	}
	if (targetCache.font != graphics.getFont()) {
	FontMetrics metric = graphics.getFontMetrics();
	targetCache.font = graphics.getFont();
	targetCache.height = metric.getHeight() - metric.getDescent();
	}
	return new Point(((int) (Math.floor((x * zoom) + fractionalX))),
	(int) (Math.floor((y + localCache.height - 1) * zoom - targetCache.height + 1 + fractionalY)));
	}
	}

When at 125% zoom, and with localFont == graphics.getFont(). The call to FigureUtilities.getFontMetrics(localFont).getHeight() return 15pt, but the call to graphics.getFontMetrics().getHeight() returns 20pt. I.e. the height in points is scaled by the monitor zoom.

[ptziegler]

Hm... the problem is that the internal Shell in FigureUtilities is initialized with FigureUtilities.configureForAutoscalingMode(), while the Shell in this example is not.

But that's not the case for the logic editor.