Triangle primitive.

src/component.rs

+// See component directory. This file exists to make the individual components accessible to the
+// rest of the code.
+
 pub mod arrow;
-/// See component directory. This file exists to make the individual components accessible to the
-/// rest of the code.
 pub mod code;
 pub mod grid;
+pub mod triangle;

src/component/arrow.rs

@@ -4,6 +4,7 @@ use eframe::emath::Rot2;
 use std::cmp::Ordering;
 
 /// Pointy line used to emphasize something.
+#[must_use = "must call show"]
 pub struct Arrow {
     pub origin: Pos2,
     pub tip: Pos2,

src/component/triangle.rs

+use crate::egui::{Pos2, Stroke, Ui};
+use eframe::egui::{Color32, Shape};
+
+/// Three-point polygon.
+#[must_use = "must call show"]
+pub struct Triangle {
+    pub points: [Pos2; 3],
+    pub stroke: Color32,
+    pub stroke_width: f32,
+    pub fill: Color32,
+}
+
+impl Default for Triangle {
+    fn default() -> Self {
+        Self {
+            points: [Pos2::ZERO; 3],
+            stroke: Color32::BLACK,
+            stroke_width: 1.0,
+            fill: Color32::TRANSPARENT,
+        }
+    }
+}
+
+impl Triangle {
+    pub fn show(&self, ui: &mut Ui) {
+        ui.painter().add(Shape::convex_polygon(
+            self.points.into(),
+            self.fill,
+            Stroke::new(self.stroke_width, self.stroke),
+        ));
+    }
+}

src/slide.rs

 use eframe::egui::{Context, Ui};
 
-/// See slide directory. This file exists to make the individual slides accessible to the
-/// rest of the code.
+// See slide directory. This file exists to make the individual slides accessible to the
+// rest of the code.
 pub mod s1_title;
 pub mod s2_introduction;
 pub mod s3_complexity;

src/slide/s7_mosaic.rs

-use crate::egui::Ui;
+use crate::component::triangle::Triangle;
+use crate::egui::{Color32, Ui};
 use crate::slide::Slide;
 use eframe::egui::style::Margin;
-use eframe::egui::Frame;
+use eframe::egui::{Frame, Pos2};
 
 #[derive(Default)]
 pub struct Mosaic {}
@@ -13,6 +14,18 @@ impl Slide for Mosaic {
             ui.heading("Going Further");
             ui.add_space(8.0);
             ui.label("TODO: Triangle mosaic");
+
+            Triangle {
+                points: [
+                    Pos2::new(200.0, 150.0),
+                    Pos2::new(500.0, 220.0),
+                    Pos2::new(300.0, 400.0),
+                ],
+                stroke_width: 0.0,
+                fill: Color32::RED,
+                ..Triangle::default()
+            }
+            .show(ui)
         });
     }
 }