Filter dialogs: the preview keeps square pixels (fixes #4640) (#4655)

The in-dialog preview clamped the texture UV to the image but still drew
it over the whole preview region. Whenever the region overhung the image
on one axis (a 100% view of a small document in the wide preview), that
axis was stretched to fill it. The preview now draws only the part of the
source rectangle that lies on the image, at its own place and scale in the
region, with the checkerboard under the image only.

Co-authored-by: Brandon Thomas <[email protected]>

crates/ui-egui/src/filter_preview_ui.rs

@@ -101,10 +101,7 @@ pub(crate) fn window(app: &PhotocraftApp, dialog: u64, ui: &mut egui::Ui, f: &mu
     let width = ui.available_width().max(140.0);
     let (rect, response) = ui.allocate_exact_size(vec2(width, PREVIEW_H), Sense::click_and_drag());
     let (x0, y0, sw, sh) = source_rect(cx, cy, zoom, rect.size(), doc);
-    let uv = Rect::from_min_max(
-        pos2((x0 / doc.x).clamp(0.0, 1.0), (y0 / doc.y).clamp(0.0, 1.0)),
-        pos2(((x0 + sw) / doc.x).clamp(0.0, 1.0), ((y0 + sh) / doc.y).clamp(0.0, 1.0)),
-    );
+    let (dest, uv) = placement(rect, (x0, y0, sw, sh), zoom, doc);
 
     // Press-and-hold shows the unfiltered image, as Photoshop's preview does.
     let holding = response.is_pointer_button_down_on();
@@ -112,8 +109,8 @@ pub(crate) fn window(app: &PhotocraftApp, dialog: u64, ui: &mut egui::Ui, f: &mu
     let painter = ui.painter_at(rect);
     painter.rect_filled(rect, t.radius_sm, t.canvas);
     if let Some((tex, _)) = shown {
-        crate::widgets::checker(&painter, rect, 8.0);
-        painter.image(tex.id(), rect, uv, Color32::WHITE);
+        crate::widgets::checker(&painter, dest, 8.0);
+        painter.image(tex.id(), dest, uv, Color32::WHITE);
     }
     painter.rect_stroke(rect, t.radius_sm, Stroke::new(1.0, t.field_border), egui::StrokeKind::Inside);
 
@@ -190,6 +187,21 @@ fn origin(centre: f32, size: f32, total: f32) -> f32 {
     if size >= total { (total - size) / 2.0 } else { (centre - size / 2.0).clamp(0.0, total - size) }
 }
 
+/// Where the image is drawn in `rect` and which part of the texture fills it, for the `source`
+/// rectangle `(x0, y0, width, height)` in document pixels shown at `zoom`. Only the part of the
+/// source that lies on the image is drawn, at its own place in the region: stretching the clamped
+/// texture over the whole region made the pixels non-square whenever the region overhung the image
+/// on one axis (#4640).
+fn placement(rect: Rect, source: (f32, f32, f32, f32), zoom: f32, doc: Vec2) -> (Rect, Rect) {
+    let zoom = if zoom.is_finite() && zoom > 0.0 { zoom } else { DEFAULT_ZOOM };
+    let (x0, y0, sw, sh) = source;
+    let (vx0, vy0) = (x0.max(0.0), y0.max(0.0));
+    let (vx1, vy1) = ((x0 + sw).min(doc.x).max(vx0), (y0 + sh).min(doc.y).max(vy0));
+    let at = |x: f32, y: f32| rect.min + vec2((x - x0) * zoom, (y - y0) * zoom);
+    let uv = Rect::from_min_max(pos2(vx0 / doc.x, vy0 / doc.y), pos2(vx1 / doc.x, vy1 / doc.y));
+    (Rect::from_min_max(at(vx0, vy0), at(vx1, vy1)), uv)
+}
+
 /// The document pixel under a pointer at `local` (relative to the region's top-left) in a view whose
 /// source rectangle starts at `(x0, y0)` and is shown at `zoom`.
 fn point_to_source(local: Vec2, x0: f32, y0: f32, zoom: f32, doc: Vec2) -> (f32, f32) {
@@ -425,6 +437,24 @@ mod tests {
         assert!(x0 / doc.x < 0.0 && (x0 + sw) / doc.x > 1.0);
     }
 
+    #[test]
+    fn the_image_keeps_square_pixels_when_the_region_overhangs_it() {
+        // #4640: a 370×240 document in a 650×150 region at 100% is wider than the image but shorter
+        // than it, and at 50% it fits inside on both axes. Either way one document pixel must cover
+        // the same number of points across and down, and the image is centred, not stretched.
+        let rect = Rect::from_min_size(pos2(10.0, 20.0), vec2(650.0, 150.0));
+        let doc = vec2(370.0, 240.0);
+        for zoom in [1.0, 0.5] {
+            let (dest, uv) = placement(rect, source_rect(185.0, 120.0, zoom, rect.size(), doc), zoom, doc);
+            let across = dest.width() / (uv.width() * doc.x);
+            let down = dest.height() / (uv.height() * doc.y);
+            assert!((across - zoom).abs() < 1e-4 && (down - zoom).abs() < 1e-4, "zoom {zoom}: {across} × {down} points per pixel");
+            assert!((dest.center().x - rect.center().x).abs() < 1e-3, "zoom {zoom}: centred across");
+            assert!(rect.contains_rect(dest), "zoom {zoom}: drawn inside the region");
+            assert!(Rect::from_min_max(pos2(0.0, 0.0), pos2(1.0, 1.0)).contains_rect(uv), "zoom {zoom}: {uv:?} stays on the texture");
+        }
+    }
+
     #[test]
     fn a_click_maps_to_the_document_point_under_it() {
         let doc = vec2(1000.0, 500.0);