Getting Physical DPI with Fractional Scaling

Platform
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1

I’m trying to obtain both the logical (scaled) DPI and the real (physical) DPI for a monitor when fractional scaling is enabled. I need the logical DPI to size my widgets correctly and the physical DPI to render textures at their true size so they stay sharp.

What I have so far (logical DPI)

// Get display geometry
let geometry = monitor.geometry();
let logical_width = geometry.width() as f64;
let logical_height = geometry.height() as f64;

// Physical size of the panel (mm) – obtained from monitor.physical_width() / height()
let width_mm = monitor.physical_width() as f64;
let height_mm = monitor.physical_height() as f64;

// Calculate the logical DPI
let layout_dpi_x = (logical_width / (width_mm / 25.4)).round();
let layout_dpi_y = (logical_height / (height_mm / 25.4)).round();
let layout_dpi = ((layout_dpi_x + layout_dpi_y) / 2.0).round() as u32;

This works fine on a 1× scale, but once I enable a fractional scale (e.g., 125 % or 150 %), monitor.geometry() returns the scaled size, so layout_dpi reflects the logical DPI rather than the true pixel density.

Is there a way to get the physical DPI or the physical display resolution directly, or failing that, a way to retrieve the scale factor that GNOME is applying (e.g., 1.25, 1.5, not necessarily the direct integer scale that is then downscaled by GTK) so I can convert between logical and physical DPI?

2

Using the DPI is not something that will help you, because hardware lies.

You can get the scaling factor communicated by the compositor to GTK using Gdk.Monitor.get_scale(), but it’s rare that you’d ever need this. You should ask the scaling factor of the surface that will draw a widget, by getting the root widget using Gtk.Widget.get_root(), then using Gtk.Native.get_surface(). Once you have the surface, you can use Gdk.Surface.get_scale() to get the scaling factor.