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#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);
}
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