From 2462921681d7967d4f9079e5ba050a1a044ad087 Mon Sep 17 00:00:00 2001
From: Qwerasd <qwerasd205@users.noreply.github.com>
Date: Mon, 14 Oct 2024 14:53:09 -0400
Subject: [PATCH] font/sprite(underline): make undercurl segment symmetrical

Previously was biased half a pixel to one side.
---
 src/font/sprite/underline.zig | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/font/sprite/underline.zig b/src/font/sprite/underline.zig
index 3a7d5cb98..29bf7b251 100644
--- a/src/font/sprite/underline.zig
+++ b/src/font/sprite/underline.zig
@@ -187,7 +187,9 @@ fn drawCurly(alloc: Allocator, width: u32, thickness: u32) !CanvasAndOffset {
     // follow Xiaolin Wu's antialias algorithm to draw the curve
     var x: u32 = 0;
     while (x < width) : (x += 1) {
-        const t: f64 = @as(f64, @floatFromInt(x)) * wave_period;
+        // We sample the wave function at the *middle* of each
+        // pixel column, to ensure that it renders symmetrically.
+        const t: f64 = (@as(f64, @floatFromInt(x)) + 0.5) * wave_period;
         // Use the slope at this location to add thickness to
         // the line on this column, counteracting the thinning
         // caused by the slope.