Rasterization: which pixels a triangle covers

float edge(vec2 a, vec2 b, vec2 p) {
  return (b.x - a.x) * (p.y - a.y) - (b.y - a.y) * (p.x - a.x);
}
float segment(vec2 p, vec2 a, vec2 b) {
  vec2 pa = p - a, ba = b - a;
  float h = clamp(dot(pa, ba) / dot(ba, ba), 0.0, 1.0);
  return length(pa - ba * h);
}

void main() {
  float cellSize = uResolution.x / 18.000;
  vec2 p = gl_FragCoord.xy / cellSize;        // in units of pixel cells
  vec2 center = floor(p) + 0.5;               // the sample point of this cell
  vec2 size = uResolution / cellSize;
  vec2 A = vec2(0.10, 0.12) * size;
  vec2 B = vec2(0.90, 0.30) * size;
  vec2 C = vec2(0.450, 0.90) * size;
  float area = edge(A, B, C);
  float w0 = edge(B, C, center) / area;       // barycentric coordinates
  float w1 = edge(C, A, center) / area;
  float w2 = edge(A, B, center) / area;
  float covered = step(0.0, min(w0, min(w1, w2)));
  vec3 mixed = w0 * vec3(1.0, 0.35, 0.3) + w1 * vec3(0.3, 0.85, 0.5) + w2 * vec3(0.3, 0.55, 1.0);
  vec3 fill = mix(vec3(0.96, 0.43, 0.30), mixed, 0.000);
  vec3 color = mix(vec3(0.06, 0.10, 0.14), fill, covered);
  vec2 local = fract(p);
  float gridLine = step(0.93, max(local.x, local.y));
  color = mix(color, vec3(0.3, 0.38, 0.42), gridLine * 0.5);
  color = mix(color, vec3(1.0), (1.0 - smoothstep(0.06, 0.1, length(local - 0.5))) * 0.6);
  float outline = min(segment(p, A, B), min(segment(p, B, C), segment(p, C, A)));
  color = mix(color, vec3(1.0, 0.85, 0.4), (1.0 - smoothstep(0.02, 0.07, outline)) * 1.000);
  gl_FragColor = vec4(color, 1.0);
}