float scene(vec3 p)
{
    return length(p) - 1.0;
}

float march(vec3 ro, vec3 rd, int steps)
{
    float t = 0.0;

    for(int i = 0; i < 128; i++)
    {
        if(i >= steps) break;

        vec3 p = ro + rd * t;

        float d = scene(p);

        if(d < 0.001) return 1.0;

        t += d;

        if(t > 20.0) break;
    }

    return 0.0;
}

void mainImage(out vec4 fragColor, in vec2 fragCoord)
{
    vec2 uv = fragCoord.xy / iResolution.xy;

    vec2 p = uv * 2.0 - 1.0;

    p.x *= iResolution.x / iResolution.y;

    vec3 ro = vec3(0.0, 0.0, -3.0);
    vec3 rd = normalize(vec3(p, 1.5));

    vec3 color = vec3(0.02, 0.04, 0.08);

    if(uv.x < 0.5)
    {
        float hit = march(ro, rd, 16);

        color += vec3(0.2, 0.7, 1.0) * hit;
    }
    else
    {
        float hit = march(ro, rd, 96);

        color += vec3(1.0, 0.3, 0.8) * hit;
    }

    float divider =
        1.0 - smoothstep(0.0, 0.003, abs(uv.x - 0.5));

    color += divider * vec3(1.0);

    fragColor = vec4(color, 1.0);
}