From a429a146c723493291633d1b3eadca94cbe9c4d7 Mon Sep 17 00:00:00 2001
From: Grant Sanderson <grant@3blue1brown.com>
Date: Sat, 9 Jan 2021 10:30:11 -0800
Subject: [PATCH] Fix how stroke is scaled for perspective

---
 manimlib/shaders/quadratic_bezier_stroke/geom.glsl | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/manimlib/shaders/quadratic_bezier_stroke/geom.glsl b/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
index 91c74e54..0c768be4 100644
--- a/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
+++ b/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
@@ -216,7 +216,8 @@ void main() {
     for(int i = 0; i < 3; i++){
         float sf = perspective_scale_factor(controls[i].z, focal_distance);
         if(bool(flat_stroke)){
-            sf *= abs(dot(v_global_unit_normal[i], vec3(0.0, 0.0, 1.0)));
+            vec3 to_cam = normalize(vec3(0.0, 0.0, focal_distance) - controls[i]);
+            sf *= abs(dot(v_global_unit_normal[i], to_cam));
         }
         scaled_strokes[i] = v_stroke_width[i] * sf;
     }