Implement ideal scale factor guessing

1 files changed, 104 insertions, 0 deletions

diff --git a/src/utils/scale.rs b/src/utils/scale.rs
new file mode 100644
index 00000000..339d0678
--- /dev/null
+++ b/src/utils/scale.rs
@@ -0,0 +1,104 @@
+//! Default monitor scale calculation.
+//!
+//! This module follows logic and tests from Mutter:
+//! https://gitlab.gnome.org/GNOME/mutter/-/blob/gnome-46/src/backends/meta-monitor.c
+
+use smithay::utils::{Physical, Raw, Size};
+
+const MIN_SCALE: i32 = 1;
+const MAX_SCALE: i32 = 4;
+const MIN_LOGICAL_AREA: i32 = 800 * 480;
+
+const MOBILE_TARGET_DPI: f64 = 135.;
+const LARGE_TARGET_DPI: f64 = 110.;
+const LARGE_MIN_SIZE_INCHES: f64 = 20.;
+
+/// Calculates the ideal scale for a monitor.
+pub fn guess_monitor_scale(size_mm: Size<i32, Raw>, resolution: Size<i32, Physical>) -> f64 {
+    if size_mm.w == 0 || size_mm.h == 0 {
+        return 1.;
+    }
+
+    let diag_inches = f64::from(size_mm.w * size_mm.w + size_mm.h * size_mm.h).sqrt() / 25.4;
+
+    let target_dpi = if diag_inches < LARGE_MIN_SIZE_INCHES {
+        MOBILE_TARGET_DPI
+    } else {
+        LARGE_TARGET_DPI
+    };
+
+    let physical_dpi =
+        f64::from(resolution.w * resolution.w + resolution.h * resolution.h).sqrt() / diag_inches;
+    let perfect_scale = physical_dpi / target_dpi;
+
+    // For integer scaling factors (we currently only do integer), bias the perfect scale down.
+    let perfect_scale = perfect_scale - 0.15;
+
+    supported_scales(resolution)
+        .map(|scale| (scale, (scale - perfect_scale).abs()))
+        .min_by(|a, b| a.1.partial_cmp(&b.1).unwrap())
+        .map_or(1., |(scale, _)| scale)
+}
+
+fn supported_scales(resolution: Size<i32, Physical>) -> impl Iterator<Item = f64> {
+    (MIN_SCALE..=MAX_SCALE)
+        .filter(move |scale| is_valid_for_resolution(resolution, *scale))
+        .map(f64::from)
+}
+
+fn is_valid_for_resolution(resolution: Size<i32, Physical>, scale: i32) -> bool {
+    let logical = resolution.to_logical(scale);
+    logical.w * logical.h >= MIN_LOGICAL_AREA
+}
+
+#[cfg(test)]
+mod tests {
+    use k9::snapshot;
+
+    use super::*;
+
+    fn check(size_mm: (i32, i32), resolution: (i32, i32)) -> f64 {
+        guess_monitor_scale(Size::from(size_mm), Size::from(resolution))
+    }
+
+    #[test]
+    fn test_guess_monitor_scale() {
+        // Librem 5; not enough logical area when scaled
+        snapshot!(check((65, 129), (720, 1440)), "1.0");
+        // OnePlus 6
+        snapshot!(check((68, 144), (1080, 2280)), "2.0");
+        // Google Pixel 6a
+        snapshot!(check((64, 142), (1080, 2400)), "2.0");
+        // 13" MacBook Retina
+        snapshot!(check((286, 179), (2560, 1600)), "2.0");
+        // Surface Laptop Studio
+        snapshot!(check((303, 202), (2400, 1600)), "1.0");
+        // Dell XPS 9320
+        snapshot!(check((290, 180), (3840, 2400)), "2.0");
+        // Lenovo ThinkPad X1 Yoga Gen 6
+        snapshot!(check((300, 190), (3840, 2400)), "2.0");
+        // Generic 23" 1080p
+        snapshot!(check((509, 286), (1920, 1080)), "1.0");
+        // Generic 23" 4K
+        snapshot!(check((509, 286), (3840, 2160)), "2.0");
+        // Generic 27" 4K
+        snapshot!(check((598, 336), (3840, 2160)), "1.0");
+        // Generic 32" 4K
+        snapshot!(check((708, 398), (3840, 2160)), "1.0");
+        // Generic 25" 4K; ideal scale is 1.60, should round to 1.5 and 1.0
+        snapshot!(check((554, 312), (3840, 2160)), "1.0");
+        // Generic 23.5" 4K; ideal scale is 1.70, should round to 1.75 and 2.0
+        snapshot!(check((522, 294), (3840, 2160)), "2.0");
+        // Lenovo Legion 7 Gen 7 AMD 16"
+        snapshot!(check((340, 210), (2560, 1600)), "1.0");
+        // Acer Nitro XV320QU LV 31.5"
+        snapshot!(check((700, 390), (2560, 1440)), "1.0");
+        // Surface Pro 6
+        snapshot!(check((260, 170), (2736, 1824)), "2.0");
+    }
+
+    #[test]
+    fn guess_monitor_scale_unknown_size() {
+        assert_eq!(check((0, 0), (1920, 1080)), 1.);
+    }
+}