did until. These last two (fill especially) were harder to wrap my head

around

learning/PixelSpirit/emperor.frag

@@ -0,0 +1,35 @@
+#ifdef GL_ES
+precision mediump float;
+#endif
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_time;
+
+#define PI 3.14159265359
+
+float rectSDF(vec2 st, vec2 s) {
+  st = st*2.-1.;
+  return max(abs(st.x/s.x),abs(st.y/s.y));
+}
+
+float stroke(in float x_coor, float s, float width){
+  float d = step(s,x_coor+width*.5)-step(s,x_coor-width*.5);
+  return clamp(d, 0.,1.);
+}
+
+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 sdf = rectSDF(st, vec2(1.));
+  color += stroke(sdf,.5,.125);
+  color += fill(sdf,.1);
+
+  
+
+  gl_FragColor = vec4(color, alpha);
+}

learning/PixelSpirit/moon.frag

@@ -0,0 +1,30 @@
+#ifdef GL_ES
+precision mediump float;
+#endif
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_time;
+
+#define PI 3.14159265359
+
+float circleSDF(vec2 st) {
+  return length(st-.5)*2.;
+}
+
+
+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;
+  color += fill(circleSDF(st),.65);
+  vec2 offset = vec2(.1,.05);
+  color -= fill(circleSDF(st-offset),.5);
+
+  
+
+  gl_FragColor = vec4(color, alpha);
+}