Aspect Ratio Fix Shader

vec2 ps = vec2(1.0) / iResolution.xy;
float ar = iResolution.x / iResolution.y;
const vec2 image_res = vec2(3840, 2160);
const float image_ar = image_res.x / image_res.y;

vec2 scaleUV(vec2, vec2);
bool crop(vec2);
vec2 correctAspect(vec2);

void main() {
    vec2 uv = gl_FragCoord.xy * ps;
    vec2 image_uv = correctAspect(uv);
    vec3 col = texture2D(iChannel0, image_uv).rgb;
    col = crop(uv) ? col : vec3(0.0);

    gl_FragColor = vec4(col, 1.0);
}

vec2 scaleUV(vec2 uv, vec2 scale) {
    return (uv - vec2(0.5)) * scale + vec2(0.5);
}

bool crop(vec2 uv) {
    bool x_or_y = ar > image_ar;
    float ar_result = x_or_y ? (image_ar / ar) :
                               (ar / image_ar);
    float mask = (1.0 - (ar_result)) * 0.5;
    return x_or_y ? (uv.x > mask && uv.x < 1.0 - mask) :
                    (uv.y > mask && uv.y < 1.0 - mask);
}

vec2 correctAspect(vec2 uv) {
    bool x_or_y = ar > image_ar;
    vec2 ar_fix = vec2(1.0);
    ar_fix = x_or_y ? vec2(ar / image_ar, 1.0) :
                      vec2(1.0, image_ar / ar);
    uv = scaleUV(uv, ar_fix);
    return uv;
}