From 8602326af5884b988e69dc22a090d13882d1aac2 Mon Sep 17 00:00:00 2001
From: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
Date: Tue, 22 Nov 2022 13:42:52 +0100
Subject: [PATCH] Plot: round hlines, vlines, and grid lines to pixels to avoid
 aliasing

---
 crates/egui/src/widgets/plot/items/mod.rs | 24 +++++++++++++++++------
 crates/egui/src/widgets/plot/mod.rs       |  3 +++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/crates/egui/src/widgets/plot/items/mod.rs b/crates/egui/src/widgets/plot/items/mod.rs
index aa24e60a..3be7e9f1 100644
--- a/crates/egui/src/widgets/plot/items/mod.rs
+++ b/crates/egui/src/widgets/plot/items/mod.rs
@@ -176,7 +176,7 @@ impl HLine {
 }
 
 impl PlotItem for HLine {
-    fn shapes(&self, _ui: &mut Ui, transform: &ScreenTransform, shapes: &mut Vec<Shape>) {
+    fn shapes(&self, ui: &mut Ui, transform: &ScreenTransform, shapes: &mut Vec<Shape>) {
         let HLine {
             y,
             stroke,
@@ -184,9 +184,15 @@ impl PlotItem for HLine {
             style,
             ..
         } = self;
+
+        // Round to minimize aliasing:
         let points = vec![
-            transform.position_from_point(&PlotPoint::new(transform.bounds().min[0], *y)),
-            transform.position_from_point(&PlotPoint::new(transform.bounds().max[0], *y)),
+            ui.ctx().round_pos_to_pixels(
+                transform.position_from_point(&PlotPoint::new(transform.bounds().min[0], *y)),
+            ),
+            ui.ctx().round_pos_to_pixels(
+                transform.position_from_point(&PlotPoint::new(transform.bounds().max[0], *y)),
+            ),
         ];
         style.style_line(points, *stroke, *highlight, shapes);
     }
@@ -286,7 +292,7 @@ impl VLine {
 }
 
 impl PlotItem for VLine {
-    fn shapes(&self, _ui: &mut Ui, transform: &ScreenTransform, shapes: &mut Vec<Shape>) {
+    fn shapes(&self, ui: &mut Ui, transform: &ScreenTransform, shapes: &mut Vec<Shape>) {
         let VLine {
             x,
             stroke,
@@ -294,9 +300,15 @@ impl PlotItem for VLine {
             style,
             ..
         } = self;
+
+        // Round to minimize aliasing:
         let points = vec![
-            transform.position_from_point(&PlotPoint::new(*x, transform.bounds().min[1])),
-            transform.position_from_point(&PlotPoint::new(*x, transform.bounds().max[1])),
+            ui.ctx().round_pos_to_pixels(
+                transform.position_from_point(&PlotPoint::new(*x, transform.bounds().min[1])),
+            ),
+            ui.ctx().round_pos_to_pixels(
+                transform.position_from_point(&PlotPoint::new(*x, transform.bounds().max[1])),
+            ),
         ];
         style.style_line(points, *stroke, *highlight, shapes);
     }
diff --git a/crates/egui/src/widgets/plot/mod.rs b/crates/egui/src/widgets/plot/mod.rs
index 33331d11..8367d9ab 100644
--- a/crates/egui/src/widgets/plot/mod.rs
+++ b/crates/egui/src/widgets/plot/mod.rs
@@ -1416,6 +1416,9 @@ impl PreparedPlot {
                 let mut p1 = pos_in_gui;
                 p0[1 - axis] = transform.frame().min[1 - axis];
                 p1[1 - axis] = transform.frame().max[1 - axis];
+                // Round to avoid aliasing
+                p0 = ui.ctx().round_pos_to_pixels(p0);
+                p1 = ui.ctx().round_pos_to_pixels(p1);
                 shapes.push(Shape::line_segment([p0, p1], Stroke::new(1.0, line_color)));
             }