1 files changed, 53 insertions, 0 deletions

diff --git a/res/shaders/glsl330/quadratic-bezier.fs b/res/shaders/glsl330/quadratic-bezier.fs
new file mode 100644
index 0000000..cac602f
--- /dev/null
+++ b/res/shaders/glsl330/quadratic-bezier.fs
@@ -0,0 +1,53 @@
+#version 330 core
+
+uniform float time;
+uniform vec2 resolution;
+uniform vec2 mouse;
+
+out vec4 color;
+
+float
+quadratic_bezier (float x, vec2 a)
+{
+  float epsilon = 0.00001;
+  a.x = clamp (a.x, 0, 1);
+  a.y = clamp (a.y, 0, 1);
+  if (a.x == .5)
+    a += epsilon;
+
+  // solve t from x (an inverse operation)
+  float om2a = 1 - 2*a.x;
+  float t = (sqrt (a.x*a.x + om2a*x) - a.x)/om2a;
+  float y = (1 - 2*a.y)*(t*t) + 2*a.y*t;
+  return y;
+}
+
+float
+line_segment (vec2 p, vec2 a, vec2 b)
+{
+  vec2 pa = p - a;
+  vec2 ba = b - a;
+  float h = clamp (dot (pa, ba)/dot (ba, ba), 0, 1);
+  return smoothstep (0, 1/resolution.x, length (pa - ba*h));
+}
+
+void
+main ()
+{
+  vec2 st = gl_FragCoord.xy/resolution;
+  float px = 1/resolution.y;
+
+  // control point
+  // vec2 cp = mouse/resolution;
+  vec2 cp = vec2 (cos (time), sin (time))*0.45 + 0.5;
+  float l = quadratic_bezier(st.x, cp);
+  vec3 c = vec3 (smoothstep (l, l + px, st.y));
+
+  // draw control point
+  c = mix (vec3 (.5), c, line_segment (st, vec2 (0), cp));
+  c = mix (vec3 (.5), c, line_segment (st, vec2 (1), cp));
+  float d = distance (cp, st);
+  c = mix (vec3 (1, 0, 0), c, smoothstep (.01, .01 + px, d));
+
+  color = vec4 (c, 1);
+}