[GTK][X11] Drawing on an image is shifted with DPI scaling

[jarkubik]

Describe the bug
Drawing on an Image object using a GC is shifted by a significant amount when run on X11 with DPI scaling.

To Reproduce
The following program executed on Linux with either full X11 session or in wayland with Xwayland.
To reproduce:

1. Set the GDK_BACKEND=x11 and GDK_SCALE=2 environment variable. The latter was not required on my system, the OS-level scaling was automatically detected and applied with the same result as setting the environment variable.
2. Start the program.

package test;

import org.eclipse.swt.SWT;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.graphics.ImageData;
import org.eclipse.swt.graphics.PaletteData;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Label;
import org.eclipse.swt.widgets.Shell;

public class Test
{
    private static Image repaint( Image source )
    {
        Rectangle bounds = source.getBounds();
        Image newImage = new Image( source.getDevice(), bounds.width, bounds.height );
        GC gc = new GC( newImage );
        gc.setAntialias( SWT.ON );
        gc.setInterpolation( SWT.HIGH );

        int offset = 0;
        // offset = bounds.height * ( DPIUtil.getDeviceZoom() - 100 ) / 200;
        gc.drawImage( source, 0, 0, bounds.width, bounds.height, 0, offset, bounds.width, bounds.height );

        gc.setForeground( new Color( source.getDevice(), 0xff, 00, 00 ) );
        gc.drawRectangle( 0, offset, bounds.width - 1, bounds.height - 1 );

        gc.dispose();
        return newImage;
    }

    public static void main( String[] args )
    {
        Display dpy = new Display();

        ImageData imageData = new ImageData( 256, 256, 24, new PaletteData( 0xff0000, 0x00ff00, 0x0000ff ) );
        for ( int i = 0; i < 256; ++i ) {
            for ( int j = i % 16; j < 256 - i % 16; ++j ) {
                imageData.setPixel( i, j, ( i << 8 ) + j );
            }
        }

        Image image = new Image( dpy, imageData );
        Shell shell = new Shell( dpy );
        shell.setText( "SWT " + SWT.getVersion() );
        Label label1 = new Label( shell, SWT.NONE );

        Label label2 = new Label( shell, SWT.NONE );

        label1.setSize( image.getBounds().width, image.getBounds().height );
        label1.setImage( image );
        image = repaint( image );
        label2.setBounds( label1.getBounds().width, 0, image.getBounds().width, image.getBounds().height );
        label2.setImage( image );
        shell.setSize( label1.getBounds().width + label2.getBounds().width,
                Math.max( label1.getBounds().height, label2.getBounds().height ) );
        shell.open();

        while ( !shell.isDisposed() ) {
            if ( !dpy.readAndDispatch() ) {
                dpy.sleep();
            }
        }

        dpy.dispose();
    }
}

Expected behavior
The re-painted image on the right side should look the same as the one on the left (except for the added red rectangle).

Screenshots

Environment:

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

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

2. Additional OS info (e.g. OS version, Linux Desktop, etc)
   Observed on Kubuntu 24.04 and 26.04 on both Xwayland and native X11 server

3. JRE/JDK version
   java-25-openjdk-amd64

Version since
The last version which shows the expected behavior is 4.7.3a:

In SWT 4.8 - 4.16 the image is smaller and shifted vertically:

In SWT 4.17 and newer it is not smaller but it is shifted by the amount relative to the DPI scaling:

Workaround (or) Additional context
I was able to workaround the issue in SWT 4.17 and later by shifting the drawing coordinates by amount proportional to the UI scaling (visible in the code snippet as a commented-out line). The workaround works for different scaling values between 1 and 5. SWT 4.8 - 4.16 would likely require a different shift value(s).

[Phillipus]

FWIW I can confirm the behaviour both on X11 (Linux Mint) and Wayland (Ubuntu 26.04). This is on latest I-build SWT and monitor display at 200% scaling.

(The version numbers you reference are for the Eclipse SDK, not SWT).

[jarkubik]

(The version numbers you reference are for the Eclipse SDK, not SWT).

I am sorry, the versioning of SWT is quite confusing. The standalone SWT sdk-s which I used for my tests are using these version numbers so I assumed this is the proper numbering. I have noticed that the OSGI metadata contain different version numbers, I'm not sure which ones to use when talking about SWT.

[akurtakov]

Tested with latest master on Fedora 44/ Gtk 3.24.52 and Gtk 4.22.4:

• XWayland/Gtk3 - broken with the reported 4.17+ behavior
• XWayland/Gtk4 - works
• Wayland/Gtk3 - works
• Wayland/Gtk4 - works

[Phillipus]

There seems to be some difference between Fedora and Ubuntu 26.04 (Wayland). I see reported behaviour on Ubuntu.

GTK3 - 3.24.52-0ubuntu1
GTK4 - 4.22.2+ds-1ubuntu1

[akurtakov]

Potential difference is the Wayland compositor in the XWayland case - for me it's Gnome (Mutter).

[Phillipus]

Maybe difference between setting GDK_SCALE=2 and not setting that when using 200% display?

[akurtakov]

I run on 200% display (thus setting GDK_SCALE makes no difference and it's anyway ignored on Wayland)

.

[Phillipus]

I had this enabled on Ubuntu 26.04. Disabling it makes the Snippet work.

[Phillipus]

I set the option "Built-in Scaling for Legacy Apps" off but now the problem is that whereas Eclipse itself displays crisply, a child launched Eclipse or RCP app is blurry. If I turn it on the launched instance is crisp but the default cursor is tiny.

Edit - strangely GDK_SCALE=2 fixed the blurriness.