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

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, -4.0);
    vec3 rd = normalize(vec3(p, 1.5));

    float d = 0.0;

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

    for(int i = 0; i < 24; i++)
    {
        vec3 pos = ro + rd * d;

        float ds = map(pos);

        float marker =
            0.015 / abs(length(pos.xy) - 1.0);

        color += vec3(1.0, 0.2, 0.8) * marker;

        d += ds;

        if(ds < 0.001) break;
    }

    fragColor = vec4(color, 1.0);
}