Simple line drawing

mainImage.glsl

// https://www.shadertoy.com/view/XtlXz7

#define resolution vec2(500.0, 500.0)
#define Thickness 0.03

float drawLine(vec2 p1, vec2 p2) {
  vec2 uv = gl_FragCoord.xy / resolution.xy;

  float a = abs(distance(p1, uv));
  float b = abs(distance(p2, uv));
  float c = abs(distance(p1, p2));
  
  float d = sqrt(c*c + Thickness*Thickness);

  if ( a >= d || b >= d )
  {
  	if (distance(p1, uv) <= Thickness ||
        distance(p2, uv) <= Thickness)
        return 1.0;
    else
        return 0.0;
  }

  float p = (a + b + c) * 0.5;

  // median to (p1, p2) vector
  float h = 2.0 / c * sqrt( p * ( p - a) * ( p - b) * ( p - c));
    
  if (h <= Thickness)
  {
      return 1.0;
  }
  else
  {
      	return 0.0;
  }

  //return mix(1.0, 0.0, smoothstep(0.5 * Thickness, 1.5 * Thickness, h));
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
  vec2 p1 = vec2(0.9 + sin(iTime)*0.15, 0.1);
  vec2 p2 = vec2(0.9 + cos(iTime)*0.15, 0.8);
  fragColor = vec4(drawLine(p1, p2));
}