[Win32] Poor text anti-alias when advanced mode is enabled

[ptziegler]

Describe the bug
Drawing a string when advanced mode is enabled looks very rough.

To Reproduce

package test;

import org.eclipse.swt.SWT;
import org.eclipse.swt.graphics.AutoscalingMode;
import org.eclipse.swt.graphics.Font;
import org.eclipse.swt.graphics.FontData;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Transform;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class TextAntiAlias {

	public static void main(String[] args) {
		Shell shell = new Shell();
		shell.setLayout(new FillLayout());
		shell.setSize(600, 200);
		
		FontData fd = shell.getFont().getFontData()[0];
		fd.height = 80;
		Font f = new Font(shell.getDisplay(),  fd);
		
		Composite c1 = new Composite(shell, SWT.NONE);
		c1.addPaintListener(event -> {
			GC gc = event.gc;
			gc.setAdvanced(true);
			gc.setFont(f);
			gc.drawString("Paper", 0, 0);
		});
		
		Composite c2 = new Composite(shell, SWT.NONE);
		c2.addPaintListener(event -> {
			GC gc = event.gc;
			gc.setFont(f);
			gc.drawString("Paper", 0, 0);
		});

		Display d = shell.getDisplay();
		shell.open();
		while(!shell.isDisposed()) {
			while(!d.readAndDispatch()) {
				d.sleep();
			}
		}
		
		f.dispose();
	}
}

Expected behavior

The GC should produce a crisp string, regardless of whether advanced mode is enabled. As an additional note: calling setTextAntialias(SWT.ON) causes both strings to look the like the left one. Meaning that explicitly enabling text-antialias leads to a worse result.

Screenshots

Environment:

1. Select the platform(s) on which the behavior is seen:

• • All OS
• • Windows
• • Linux
• • macOS

[ptziegler]

These default values seem to be the problem. When I explicitly enable text anti-alias, TextRenderingHintClearTypeGridFit is used. But when I hard-code the mode TextRenderingHintAntiAliasGridFit, everything looks the way I'd expect.

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/GC.java

Lines 5789 to 5796 in 9e86574

	int[] type = new int[1];
	OS.SystemParametersInfo(OS.SPI_GETFONTSMOOTHINGTYPE, 0, type, 0);
	if (type[0] == OS.FE_FONTSMOOTHINGCLEARTYPE) {
	textMode = Gdip.TextRenderingHintClearTypeGridFit;
	} else {
	textMode = Gdip.TextRenderingHintAntiAliasGridFit;
	}
	break;

It's been like that since 093c579, so sadly the history doesn't give any indication why this case distinction matters.

[ptziegler]

Perhaps as a hint:

TextRenderingHintClearTypeGridFit
Specifies that a character is drawn using its glyph ClearType bitmap and hinting. This type of text rendering cannot be used along with CompositingModeSourceCopy.

with:

CompositingModeSourceCopy
Specifies that when a color is rendered, it overwrites the background color. This mode cannot be used along with TextRenderingHintClearTypeGridFit.

I have to check this, but it sounds like this mode is used by the GC and is therefore incompatible with the rendering hint.

[HeikoKlare]

It's been like that since 093c579, so sadly the history doesn't give any indication why this case distinction matters.

The line of code was initially introduced with this commit: 7bbbc62
But that does not explain the distinction either.

[ptziegler]

The line of code was initially introduced with this commit: 7bbbc62

Thanks for the clarification. I'm always a little bit confused by this "restore HEAD" commit. 😅

I did some additional tests and can confirm that this problem is unrelated to the compositing mode. When calling GetCompositingMode() prior to setting the rendering hint, 0 (i.e. CompositingModeSourceOver) is returned.

The reason it shows up when calling setTextAntialias(...) is also a red herring, because internally, this will also enable advanced mode. I haven't really found anything meaningful regarding ClearType in combination with GDI+ which to me, indicates that something is missing on the SWT side. But I don't have a single clue what that is.

Perhaps the "fix" is to only use ClearType when advanced mode is disabled?

[ptziegler]

I haven't really found anything meaningful regarding ClearType in combination with GDI+ which to me, indicates that something is missing on the SWT side.

Or at the very least I had several AI suggestions that ClearType might produce worse results at large font sizes. But all sources it provided were just hearsay.

[HeikoKlare]

Perhaps the "fix" is to only use ClearType when advanced mode is disabled?

I have to admit that I currently also have now clue about a proper solution here. But I cannot imagine that ClearType is something that conflicts with GDI+ (advanced mode) in general. Quite the contrary, some Microsoft documentation explicitly states that ClearType-based anti-aliasing is something that is provided with GDI+, e.g. https://learn.microsoft.com/en-us/windows/win32/gdiplus/-gdiplus-antialiasing-with-text-use:

GDI+ provides traditional antialiasing and a new kind of antialiasing based on Microsoft ClearType display technology

[tmssngr]

When looking closer at the above screenshots with a magnifier, both sides show some weird white outline. I don't see them when launching the snippet (the right "Paper" looks better and antialiased, the left not):