precision mediump float;

uniform float time;
uniform vec2 resolution;

float sdSphere (vec3 p, float r) { return length(p) - r; }

float map (vec3 pos) {
  float scene = 1000.;
  scene = min(scene, sdSphere(pos, 1.));
  return scene;
}

vec3 lookAt (vec3 eye, vec3 at, vec2 uv) {
  vec3 forward = normalize(at - eye);
  vec3 right = normalize(cross(forward, vec3(0,1,0)));
  vec3 up = normalize(cross(right, forward));
  return normalize(forward + right * uv.x + up * uv.y);
}

void main () {
  vec2 uv = (gl_FragCoord.xy-.5*resolution.xy)/resolution.y;
  vec3 eye = vec3(0,1,-3);
  vec3 ray = lookAt(eye, vec3(0), uv);
  vec3 pos = eye;
  float shade = 0.;
  for (float i = 0.; i <= 1.; i += 1./30.) {
    float dist = map(pos);
    if (dist < .001) {
      shade = 1.-i;
      break;
    }
    pos += dist * ray;
  }
  vec3 color = vec3(1.);
  color *= shade;
  gl_FragColor = vec4(color, 1);
}