0-1 or -1 to 1 ...

experiments/draw_poly/test.frag

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+#ifdef GL_ES
+precision mediump float;
+#endif
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_time;
+
+#define PI 3.14159265359
+
+const vec3 uAColor = vec3(.5,.5,.5);
+const vec3 uBColor = vec3(.5,.5,.5);
+const vec3 uCColor = vec3(1.,1.,1.);
+const vec3 uDColor = vec3(.0,.33,.67);
+
+//get colors from http://dev.thi.ng/gradients/
+vec3 cosPalette(float t) { return uAColor + uBColor*cos(6.28318*(uCColor*t+uDColor)); }
+
+float dot2(vec2 v) {
+  return dot(v,v);
+}
+
+
+float fill(float x, float size) { return 1.-step(size, x); }
+
+void main() {
+  vec2 st = (gl_FragCoord.xy*2.-u_resolution.xy)/u_resolution.y;
+  vec3 color = vec3(0.0,0.0,0.0);
+  float alpha = 1.0;
+
+  gl_FragColor = vec4(color, alpha);
+}