From 8fee4d1a6642079ecd1936423d7da5e551a16213 Mon Sep 17 00:00:00 2001
From: Grant Sanderson <grant@3blue1brown.com>
Date: Fri, 20 Jan 2023 17:35:36 -0800
Subject: [PATCH] Fix straight line fill anti-alias issue

---
 manimlib/shaders/quadratic_bezier_fill/geom.glsl | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/manimlib/shaders/quadratic_bezier_fill/geom.glsl b/manimlib/shaders/quadratic_bezier_fill/geom.glsl
index eaeb6d6f..256ac4d5 100644
--- a/manimlib/shaders/quadratic_bezier_fill/geom.glsl
+++ b/manimlib/shaders/quadratic_bezier_fill/geom.glsl
@@ -62,6 +62,12 @@ void emit_pentagon(
     float angle = acos(clamp(dot(t01, t12), -1, 1));
     is_linear = float(angle < ANGLE_THRESHOLD);
 
+    if(bool(is_linear)){
+        // Cross with unit z vector
+        p0_perp = normalize(vec3(-t01.y, t01.x, 0));
+        p2_perp = p0_perp;
+    }
+
     bool fill_inside = orientation > 0.0;
     float aaw = anti_alias_width * pixel_size;
     vec3 corners[5] = vec3[5](p0, p0, p1, p2, p2);
@@ -71,8 +77,7 @@ void emit_pentagon(
         corners[0] += aaw * p0_perp;
         corners[2] += 0.5 * aaw * (p0_perp + p2_perp);
         corners[4] += aaw * p2_perp;
-    }
-    if(!fill_inside || bool(is_linear)){
+    } else{
         // Add buffer inside the curve
         corners[1] -= aaw * p0_perp;
         corners[3] -= aaw * p2_perp;