hierophant wip. Kwil zo graag doorwerken maar mijn kopke is empty

learning/PixelSpirit/hierophant.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 rectSDF(vec2 st, vec2 s) {
+  st = st*2.-1.;
+  return max(abs(st.x/s.x),abs(st.y/s.y));
+}
+
+float crossSDF(vec2 st, float s) {
+  vec2 size = vec2(.25, s);
+  return min(rectSDF(st,size.xy),rectSDF(st,size.yx));
+}
+
+float fill(float x, float size) {
+  return 1.-step(size, x);
+}
+
+void main() {
+  vec2 st = gl_FragCoord.xy/u_resolution;
+  vec3 color = vec3(0.0,0.0,0.0);
+  float alpha = 1.0;
+  float rect = rectSDF(st,vec2(1.));
+  color += fill(rect,.5);
+  float cross = crossSDF(st,1.);
+  //i don't understand what happens here
+  //it has to be fract causing this to look like that
+  color *= step(.5,fract(cross*4.));
+  
+  gl_FragColor = vec4(color, alpha);
+}