View difference between Paste ID: Gc2gwhAk and XUKCZ9Zn

 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;
 }