Step 1 - Drawing a circle

fragment-shader.glsl

@@ -0,0 +1,23 @@
+#ifdef GL_ES
+precision mediump float;
+#endif
+
+precision highp float;
+uniform float u_time;
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+
+float PI = 3.14159;
+vec2 center = u_resolution * 0.5;
+float radius = min(u_resolution.x, u_resolution.y) * .5;
+
+float circle(vec2 coord, vec2 center, float radius) {
+  float distanceToCenter = distance(coord, center);
+  return step(distanceToCenter, radius);
+}
+
+void main() {
+  vec2 coord = vec2(gl_FragCoord);
+  float isFilled = circle(coord, center, radius);
+  gl_FragColor = vec4(1. - isFilled);
+}