Improve fractional rendering by finding perfectly scalable size

src/text_box.rs

@@ -318,28 +318,46 @@ where
 
         let mut editor = self.editor.lock().unwrap();
 
+        let cosmic_theme = theme.cosmic();
+        let scrollbar_w = cosmic_theme.spacing.space_xxs as i32;
+
         let view_w = cmp::min(viewport.width as i32, layout.bounds().width as i32)
-            - self.padding.horizontal() as i32;
+            - self.padding.horizontal() as i32
+            - scrollbar_w;
         let view_h = cmp::min(viewport.height as i32, layout.bounds().height as i32)
             - self.padding.vertical() as i32;
 
         let scale_factor = style.scale_factor as f32;
         let metrics = self.metrics.scale(scale_factor);
 
-        let image_w = (view_w as f32 * scale_factor) as i32;
-        let image_h = (view_h as f32 * scale_factor) as i32;
+        let calculate_image_scaled = |view: i32| -> (i32, f32) {
+            // Get smallest set of physical pixels that fit inside the logical pixels
+            let image = ((view as f32) * scale_factor).floor() as i32;
+            // Convert that back into logical pixels
+            let scaled = (image as f32) / scale_factor;
+            (image, scaled)
+        };
+        let calculate_ideal = |view_start: i32| -> (i32, f32) {
+            // Search for a perfect match within 16 pixels
+            for i in 0..16 {
+                let view = view_start - i;
+                let (image, scaled) = calculate_image_scaled(view);
+                if view == scaled as i32 {
+                    return (image, scaled);
+                }
+            }
+            let (image, scaled) = calculate_image_scaled(view_start);
+            (image, scaled)
+        };
 
-        let cosmic_theme = theme.cosmic();
-        let scrollbar_w = (cosmic_theme.spacing.space_xxs as f32 * scale_factor) as i32;
+        let (image_w, scaled_w) = calculate_ideal(view_w);
+        let (image_h, scaled_h) = calculate_ideal(view_h);
 
-        if image_w <= scrollbar_w || image_h <= 0 {
+        if image_w <= 0 || image_h <= 0 {
             // Zero sized image
             return;
         }
 
-        // Adjust image width by scrollbar width
-        let image_w = image_w - scrollbar_w;
-
         // Lock font system (used throughout)
         let mut font_system = font_system().write().unwrap();
 
@@ -587,7 +605,7 @@ where
                     let rect = Rectangle::new(
                         [image_w as f32 / scale_factor, start_y as f32 / scale_factor].into(),
                         Size::new(
-                            scrollbar_w as f32 / scale_factor,
+                            scrollbar_w as f32,
                             (end_y as f32 - start_y as f32) / scale_factor,
                         ),
                     );
@@ -609,17 +627,16 @@ where
         let image_position = layout.position() + [self.padding.left, self.padding.top].into();
         if let Some(ref handle) = *handle_opt {
             let image_size = image::Renderer::dimensions(renderer, handle);
+            let scaled_size = Size::new(scaled_w as f32, scaled_h as f32);
+            log::debug!(
+                "text_box image {:?} scaled {:?} position {:?}",
+                image_size, scaled_size, image_position
+            );
             image::Renderer::draw(
                 renderer,
                 handle.clone(),
                 image::FilterMethod::Nearest,
-                Rectangle::new(
-                    image_position,
-                    Size::new(
-                        image_size.width as f32 / scale_factor,
-                        image_size.height as f32 / scale_factor,
-                    ),
-                ),
+                Rectangle::new(image_position, scaled_size),
                 [0.0; 4],
             );
         }