Add RectShape::blur_width to implement shadows (emilk#4267)

This is mostly a refactor, but has some performance benefits:
* We (re)use the same tessellator as for everything else, leading to
less allocations
* We cull shapes before rendering them

Adding `RectShape::blur_width` means it can also be used for other
effects, such as glow.

crates/egui/src/containers/frame.rs

@@ -291,9 +291,8 @@ impl Frame {
         if shadow == Default::default() {
             frame_shape
         } else {
-            let shadow = shadow.tessellate(outer_rect, rounding);
-            let shadow = Shape::Mesh(shadow);
-            Shape::Vec(vec![shadow, frame_shape])
+            let shadow = shadow.as_shape(outer_rect, rounding);
+            Shape::Vec(vec![Shape::from(shadow), frame_shape])
         }
     }
 }

crates/egui/src/widgets/image.rs

@@ -762,6 +762,7 @@ pub fn paint_texture_at(
                 rounding: options.rounding,
                 fill: options.tint,
                 stroke: Stroke::NONE,
+                blur_width: 0.0,
                 fill_texture_id: texture.id,
                 uv: options.uv,
             });

crates/egui/src/widgets/slider.rs

@@ -739,14 +739,8 @@ impl<'a> Slider<'a> {
                     };
                     let v = v + Vec2::splat(visuals.expansion);
                     let rect = Rect::from_center_size(center, 2.0 * v);
-                    ui.painter().add(epaint::RectShape {
-                        fill: visuals.bg_fill,
-                        stroke: visuals.fg_stroke,
-                        rect,
-                        rounding: visuals.rounding,
-                        fill_texture_id: Default::default(),
-                        uv: Rect::ZERO,
-                    });
+                    ui.painter()
+                        .rect(rect, visuals.rounding, visuals.bg_fill, visuals.fg_stroke);
                 }
             }
         }

crates/epaint/src/shadow.rs

@@ -1,5 +1,3 @@
-use emath::NumExt as _;
-
 use super::*;
 
 /// The color and fuzziness of a fuzzy shape.
@@ -37,11 +35,10 @@ impl Shadow {
         color: Color32::TRANSPARENT,
     };
 
-    pub fn tessellate(&self, rect: Rect, rounding: impl Into<Rounding>) -> Mesh {
+    /// The argument is the rectangle of the shadow caster.
+    pub fn as_shape(&self, rect: Rect, rounding: impl Into<Rounding>) -> RectShape {
         // tessellator.clip_rect = clip_rect; // TODO(emilk): culling
 
-        use crate::tessellator::*;
-
         let Self {
             offset,
             blur,
@@ -50,40 +47,9 @@ impl Shadow {
         } = *self;
 
         let rect = rect.translate(offset).expand(spread);
+        let rounding = rounding.into() + Rounding::same(spread.abs());
 
-        // We simulate a blurry shadow by tessellating a solid rectangle using a very large feathering.
-        // Feathering is usually used to make the edges of a shape softer for anti-aliasing.
-        // The tessellator can't handle blurring/feathering larger than the smallest side of the rect.
-        // Thats because the tessellator approximate very thin rectangles as line segments,
-        // and these line segments don't have rounded corners.
-        // When the feathering is small (the size of a pixel), this is usually fine,
-        // but here we have a huge feathering to simulate blur,
-        // so we need to avoid this optimization in the tessellator,
-        // which is also why we add this rather big epsilon:
-        let eps = 0.1;
-        let blur = blur.at_most(rect.size().min_elem() - eps).at_least(0.0);
-
-        // TODO(emilk): if blur <= 0, return a simple `Shape::Rect` instead of using the tessellator
-
-        let rounding_expansion = spread.abs() + 0.5 * blur;
-        let rounding = rounding.into() + Rounding::same(rounding_expansion);
-
-        let rect = RectShape::filled(rect, rounding, color);
-        let pixels_per_point = 1.0; // doesn't matter here
-        let font_tex_size = [1; 2]; // unused since we are not tessellating text.
-        let mut tessellator = Tessellator::new(
-            pixels_per_point,
-            TessellationOptions {
-                feathering: true,
-                feathering_size_in_pixels: blur * pixels_per_point,
-                ..Default::default()
-            },
-            font_tex_size,
-            vec![],
-        );
-        let mut mesh = Mesh::default();
-        tessellator.tessellate_rect(&rect, &mut mesh);
-        mesh
+        RectShape::filled(rect, rounding, color).with_blur_width(blur)
     }
 
     /// How much larger than the parent rect are we in each direction?

crates/epaint/src/shape.rs

@@ -668,6 +668,14 @@ pub struct RectShape {
     /// The thickness and color of the outline.
     pub stroke: Stroke,
 
+    /// If larger than zero, the edges of the rectangle
+    /// (for both fill and stroke) will be blurred.
+    ///
+    /// This can be used to produce shadows and glow effects.
+    ///
+    /// The blur is currently implemented using a simple linear blur in sRGBA gamma space.
+    pub blur_width: f32,
+
     /// If the rect should be filled with a texture, which one?
     ///
     /// The texture is multiplied with [`Self::fill`].
@@ -695,6 +703,7 @@ impl RectShape {
             rounding: rounding.into(),
             fill: fill_color.into(),
             stroke: stroke.into(),
+            blur_width: 0.0,
             fill_texture_id: Default::default(),
             uv: Rect::ZERO,
         }
@@ -711,6 +720,7 @@ impl RectShape {
             rounding: rounding.into(),
             fill: fill_color.into(),
             stroke: Default::default(),
+            blur_width: 0.0,
             fill_texture_id: Default::default(),
             uv: Rect::ZERO,
         }
@@ -723,18 +733,32 @@ impl RectShape {
             rounding: rounding.into(),
             fill: Default::default(),
             stroke: stroke.into(),
+            blur_width: 0.0,
             fill_texture_id: Default::default(),
             uv: Rect::ZERO,
         }
     }
 
+    /// If larger than zero, the edges of the rectangle
+    /// (for both fill and stroke) will be blurred.
+    ///
+    /// This can be used to produce shadows and glow effects.
+    ///
+    /// The blur is currently implemented using a simple linear blur in `sRGBA` gamma space.
+    #[inline]
+    pub fn with_blur_width(mut self, blur_width: f32) -> Self {
+        self.blur_width = blur_width;
+        self
+    }
+
     /// The visual bounding rectangle (includes stroke width)
     #[inline]
     pub fn visual_bounding_rect(&self) -> Rect {
         if self.fill == Color32::TRANSPARENT && self.stroke.is_empty() {
             Rect::NOTHING
         } else {
-            self.rect.expand(self.stroke.width / 2.0)
+            self.rect
+                .expand((self.stroke.width + self.blur_width) / 2.0)
         }
     }
 }

crates/epaint/src/shape_transform.rs

@@ -37,6 +37,7 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             rounding: _,
             fill,
             stroke,
+            blur_width: _,
             fill_texture_id: _,
             uv: _,
         })

crates/epaint/src/tessellator.rs

@@ -1503,9 +1503,10 @@ impl Tessellator {
     pub fn tessellate_rect(&mut self, rect: &RectShape, out: &mut Mesh) {
         let RectShape {
             mut rect,
-            rounding,
+            mut rounding,
             fill,
             stroke,
+            mut blur_width,
             fill_texture_id,
             uv,
         } = *rect;
@@ -1524,6 +1525,29 @@ impl Tessellator {
         rect.min = rect.min.at_least(pos2(-1e7, -1e7));
         rect.max = rect.max.at_most(pos2(1e7, 1e7));
 
+        let old_feathering = self.feathering;
+
+        if old_feathering < blur_width {
+            // We accomplish the blur by using a larger-than-normal feathering.
+            // Feathering is usually used to make the edges of a shape softer for anti-aliasing.
+
+            // The tessellator can't handle blurring/feathering larger than the smallest side of the rect.
+            // Thats because the tessellator approximate very thin rectangles as line segments,
+            // and these line segments don't have rounded corners.
+            // When the feathering is small (the size of a pixel), this is usually fine,
+            // but here we have a huge feathering to simulate blur,
+            // so we need to avoid this optimization in the tessellator,
+            // which is also why we add this rather big epsilon:
+            let eps = 0.1;
+            blur_width = blur_width
+                .at_most(rect.size().min_elem() - eps)
+                .at_least(0.0);
+
+            rounding += Rounding::same(0.5 * blur_width);
+
+            self.feathering = self.feathering.max(blur_width);
+        }
+
         if rect.width() < self.feathering {
             // Very thin - approximate by a vertical line-segment:
             let line = [rect.center_top(), rect.center_bottom()];
@@ -1566,6 +1590,8 @@ impl Tessellator {
 
             path.stroke_closed(self.feathering, stroke, out);
         }
+
+        self.feathering = old_feathering; // restore
     }
 
     /// Tessellate a single [`TextShape`] into a [`Mesh`].