Description Andreas Loth 2019-01-21 07:43:59 EST

org.eclipse.swt.graphics.Device#getScreenDPI()

Point getScreenDPI () {
    int dpi = 96; //default value
    if (GTK.GTK_VERSION >= OS.VERSION(3, 22, 0)) {
        long /*int*/ display = GDK.gdk_display_get_default();
        long /*int*/ pMonitor = GDK.gdk_display_get_primary_monitor(display);
        if (pMonitor == 0) {
            pMonitor = GDK.gdk_display_get_monitor(display, 0);
        }
        int widthMM = GDK.gdk_monitor_get_width_mm(pMonitor); // returns 0
        int scaleFactor = GDK.gdk_monitor_get_scale_factor(pMonitor);
        GdkRectangle monitorGeometry = new GdkRectangle ();
        GDK.gdk_monitor_get_geometry(pMonitor, monitorGeometry);
        dpi = Compatibility.round (254 * monitorGeometry.width * scaleFactor, widthMM * 10); // = 2147483647
    }
    return new Point (dpi, dpi);
}


Monitors report that their size is 0 mm x 0 mm (some monitors do this, TigerVNC does not transmit the monitors physical size, resulting in 0 mm x 0 mm).

This causes GDK.gdk_monitor_get_width_mm(pMonitor) to return 0, which leads to widthMM == 0.
The method Compatibility.round requires the second parameter to be non-zero according to its Javadoc. 0 is passed in this situation.
This leads to a floating point division by zero. After casting this value to an int, Integer.MAX_VALUE is returned as dpi value.

Proposed fix:
Check widthMM for zero (or even negative values?) and return the default DPI in this case.

The TigerVNC project even said that using the physical size of a monitor is a problem itself and just the scale factor should be used:
https://github.com/TigerVNC/tigervnc/issues/789