02 Drawing shapes with math

length: distance makes a circle

Closer to the centre than the radius means inside the circle.

Centre X0.50

Centre Y0.50

Radius0.24

0.030.45

ExerciseTune values0%

Put a small circle at the top left

Runs once per pixelDrag numbers, or edit the code

void main() {
  vec2 p = (gl_FragCoord.xy - uResolution * vec2(0.500, 0.500))
           / min(uResolution.x, uResolution.y);
  float d = length(p);
  float mask = 1.0 - smoothstep(0.240, 0.240 + 0.008, d);
  vec3 color = mix(vec3(0.06, 0.10, 0.14), vec3(0.96, 0.43, 0.30), mask);
  gl_FragColor = vec4(color, 1.0);
}

void main() {
  vec2 p = (gl_FragCoord.xy - uResolution * vec2(0.500, 0.500))
           / min(uResolution.x, uResolution.y);
  float d = length(p);
  float mask = 1.0 - smoothstep(0.240, 0.240 + 0.008, d);
  vec3 color = mix(vec3(0.06, 0.10, 0.14), vec3(0.96, 0.43, 0.30), mask);
  gl_FragColor = vec4(color, 1.0);
}

JS sidegl.uniform2f(uResolution, …, …)

length(p − center) is the distance to the centre; if it is less than the radius, you are inside.

Try it

Move the circle to the top left
Open the pixel debugger: is d exactly the radius on the edge?