test.fsh

// Ray intersection point = ro + rd*t | t > 0
float sphere(vec3 center, float r, vec3 ro, vec3 rd)
{
    vec3 ray = ro - center;
    float c = dot(ray, ray) - (r*r);
    float b = dot(rd, ray);
    float d = b*b - c;
    float t = -b - sqrt(abs(d));
    float st = step(0.0, min(t, d));
    return mix(-1.0, t, st);
}

void main()
{
    vec2 uv = (-1.0 + 2.0*gl_FragCoord.xy / iResolution.xy) * vec2(iResolution.x/iResolution.y, 1.0);
    vec3 ro = vec3(0.0, 0.0, -6.0);
    vec3 rd = normalize(vec3(uv, 1.0));

    vec3 sphereCenter = vec3(sin(iGlobalTime)*2.0, 0.0, cos(iGlobalTime)*2.0 + 4.0);
    float sphereRadius = 3.0;

    // Reflect a ray off the sphere
    float t = sphere(sphereCenter, sphereRadius, ro, rd);
    vec3 nml = normalize(sphereCenter - (ro+rd*t));
    rd = reflect(rd, nml);

    gl_FragColor.xyz = mix(vec3(0), rd, step(0.0, t));
}