basic: added distance field example; utils: added polygon shape drawer

utilities/shapes.glsl

@@ -4,3 +4,19 @@ vec3 circle(in vec2 _st, in float _radius, vec3 color){
                          _radius+(_radius*0.02),
                          dot(dist,dist)*4.0);
 }
+
+// Inspired by (but modified)
+// http://thndl.com/square-shaped-shaders.html
+#define TAU 6.28318530718
+vec3 polygon(int n, float size, vec2 pos, vec3 color, float rot) {
+  float e = 0.01;
+  vec2 st = gl_FragCoord.xy/u_resolution.xy;
+  st.x *= u_resolution.x/u_resolution.y;
+  st = (st-pos)*2.;
+  float a = atan(st.x,st.y)+rot;
+  float r = TAU/float(n);
+
+  float d = cos(floor(.5+a/r)*r-a)*length(st);
+
+  return color-smoothstep(size-e,size+e,d);
+}