[Analysis] Evaluate inconsistencies when Device of a Font and its GC differs

[akoch-yatta]

Based on #349 there is multiple questions to be evaluated and answered:

• If the passed font belongs to a different device than the device of the GC, can this lead to unexpected issues? If so, how to unify?
• System fonts are always fetched via the device of the GC -> isn't that inconsistent? You can create a custom font with the same details as the system font has and compare behavior.

If these questions must be solved with code changes, I would create new issues for that.

This Snippet can be extended to analyse the behavior. Note that a new font is fetched internally in GC if the gc has a different zoom than the font.

import org.eclipse.swt.graphics.Font;
import org.eclipse.swt.graphics.FontData;
import org.eclipse.swt.graphics.FontMetrics;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.printing.Printer;
import org.eclipse.swt.widgets.Display;

class PrintTest {

	public static void main(String[] args) {
		var printer = new Printer();
		var image = new Image(printer, 20, 20);

		FontData systemFontData = Display.getDefault().getSystemFont().getFontData()[0];
		var font = new Font(Display.getDefault(), systemFontData.getName(), systemFontData.getHeight(), 0);
		var fontPrinter = new Font(printer, systemFontData.getName(), systemFontData.getHeight(), 0);

		printMetric(systemFontData, "System Font");
		printMetric(font.getFontData()[0], "GC Font");
		printMetric(fontPrinter.getFontData()[0], "Printer Font");

		var gc = new GC(image);
		printMetric(gc.getFontMetrics(), "GC");

		gc.setFont(font);
		printMetric(gc.getFontMetrics(), "GC with Font");

		gc.setFont(fontPrinter);
		printMetric(gc.getFontMetrics(), "GC with Printer Font");
	}

	private static void printMetric(FontData fontData, String context) {
		System.out.println("### " + context);
		System.out.println("Name: " + fontData.getName());
		System.out.println("Height: " + fontData.height);
		System.out.println("Height in px: " + (-fontData.data.lfHeight));
	}

	private static void printMetric(FontMetrics textMetric, String context) {
		System.out.println("### " + context);
		System.out.println("Ascent: " + textMetric.getAscent());
		System.out.println("Descent: " + textMetric.getDescent());
		System.out.println("Height: " + textMetric.getHeight());
		System.out.println("Leading: " + textMetric.getLeading());
		System.out.println("Average Char Width: " + textMetric.getAverageCharWidth());
	}
}

[ShahzaibIbrahim]

If the passed font belongs to a different device than the device of the GC, can this lead to unexpected issues? If so, how to unify?

If you pass different device to the font from device of GC, I would expect it to have issues. Why would it be unexpected? I don't understand

[akoch-yatta]

If the passed font belongs to a different device than the device of the GC, can this lead to unexpected issues? If so, how to unify?

If you pass different device to the font from device of GC, I would expect it to have issues. Why would it be unexpected? I don't understand

Probably yes, but I would like to understand the scenarios. So, my assumption would have been, that always using the font device to get scaled fonts is wrong and always using the device is the GC is correct. The opposite seems to be the case, but I don't know why. Whats the contract here.

[fedejeanne]

Since there are no visual effects, I set this to priority low.

[HeikoKlare]

Since we had regressions by differing devices of GCs and their fonts, a further analysis as proposed here should maybe be of higher priority, let's say at least medium or even high to maybe do it soon as it would be more relevant for the upcoming release than other low (or even medium) issues that we have, which could wait for the next development cycle.
@akoch-yatta wdyt?