[emath] RectTransform: transforms Pos2 from one Rect to another

Very useful for transforming coordinate systems, e.g. for painting

egui_demo_lib/src/apps/demo/dancing_strings.rs

@@ -34,6 +34,9 @@ impl super::View for DancingStrings {
             let desired_size = ui.available_width() * vec2(1.0, 0.35);
             let (_id, rect) = ui.allocate_space(desired_size);
 
+            let to_screen =
+                math::RectTransform::from_to(Rect::from_x_y_ranges(0.0..=1.0, -1.0..=1.0), rect);
+
             let mut shapes = vec![];
 
             for &mode in &[2, 3, 5] {
@@ -46,11 +49,7 @@ impl super::View for DancingStrings {
                         let t = i as f32 / (n as f32);
                         let amp = (time as f32 * speed * mode).sin() / mode;
                         let y = amp * (t * std::f32::consts::TAU / 2.0 * mode).sin();
-
-                        pos2(
-                            lerp(rect.x_range(), t),
-                            remap(y, -1.0..=1.0, rect.y_range()),
-                        )
+                        to_screen * pos2(t, y)
                     })
                     .collect();
 

egui_demo_lib/src/apps/demo/painting.rs

@@ -3,7 +3,8 @@ use egui::*;
 #[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "persistence", serde(default))]
 pub struct Painting {
-    lines: Vec<Vec<Vec2>>,
+    /// in 0-1 normalized coordinates
+    lines: Vec<Vec<Pos2>>,
     stroke: Stroke,
 }
 
@@ -32,6 +33,12 @@ impl Painting {
             ui.allocate_painter(ui.available_size_before_wrap_finite(), Sense::drag());
         let rect = response.rect;
 
+        let to_screen = emath::RectTransform::from_to(
+            Rect::from_min_size(Pos2::ZERO, rect.square_proportions()),
+            rect,
+        );
+        let from_screen = to_screen.inverse();
+
         if self.lines.is_empty() {
             self.lines.push(vec![]);
         }
@@ -39,7 +46,7 @@ impl Painting {
         let current_line = self.lines.last_mut().unwrap();
 
         if let Some(pointer_pos) = response.interact_pointer_pos() {
-            let canvas_pos = pointer_pos - rect.min;
+            let canvas_pos = from_screen * pointer_pos;
             if current_line.last() != Some(&canvas_pos) {
                 current_line.push(canvas_pos);
             }
@@ -49,7 +56,7 @@ impl Painting {
 
         for line in &self.lines {
             if line.len() >= 2 {
-                let points: Vec<Pos2> = line.iter().map(|p| rect.min + *p).collect();
+                let points: Vec<Pos2> = line.iter().map(|p| to_screen * *p).collect();
                 painter.add(Shape::line(points, self.stroke));
             }
         }

egui_demo_lib/src/apps/fractal_clock.rs

@@ -100,8 +100,6 @@ impl FractalClock {
     }
 
     fn paint(&mut self, painter: &Painter) {
-        let rect = painter.clip_rect();
-
         struct Hand {
             length: f32,
             angle: f32,
@@ -130,14 +128,18 @@ impl FractalClock {
             Hand::from_length_angle(0.5, angle_from_period(12.0 * 60.0 * 60.0)),
         ];
 
-        let scale = self.zoom * rect.width().min(rect.height());
         let mut shapes: Vec<Shape> = Vec::new();
-        let mut paint_line = |points: [Pos2; 2], color: Color32, width: f32| {
-            let line = [
-                rect.center() + scale * points[0].to_vec2(),
-                rect.center() + scale * points[1].to_vec2(),
-            ];
 
+        let rect = painter.clip_rect();
+        let to_screen = emath::RectTransform::from_to(
+            Rect::from_center_size(Pos2::ZERO, rect.square_proportions() / self.zoom),
+            rect,
+        );
+
+        let mut paint_line = |points: [Pos2; 2], color: Color32, width: f32| {
+            let line = [to_screen * points[0], to_screen * points[1]];
+
+            // culling
             if rect.intersects(Rect::from_two_pos(line[0], line[1])) {
                 shapes.push(Shape::line_segment(line, (width, color)));
             }
@@ -186,6 +188,9 @@ impl FractalClock {
             width *= self.width_factor;
 
             let luminance_u8 = (255.0 * luminance).round() as u8;
+            if luminance_u8 == 0 {
+                break;
+            }
 
             for &rotor in &hand_rotors {
                 for a in &nodes {