Imagine/main.js

let canvas;
let ctx;
let imports;
let settings = {
  brightness: {},
  saturate: {},
  contrast: {},
  blur: { filter: value => value * canvas.width / 100 + 'px' }
};
const img = new Image();

// wait for site to be parsed so element can be found
document.addEventListener("DOMContentLoaded", function() {
  canvas = document.getElementById("myCanvas");
  ctx = canvas.getContext("2d");
  imports = document.getElementById("imports");

  // bind listeners
  document.getElementById("take-picture").addEventListener("click", take_picture);
  document.getElementById("upload-image").addEventListener("change", upload_image)

  document.getElementById("save").addEventListener("click", save_image)
  document.getElementById("share").addEventListener("click", share_image)

  window.addEventListener("resize", () => draw(true))

  for (let setting in settings) {
    // make an array out of an iterable
    const elements = [...document.getElementsByClassName(setting)];
    settings[setting].elements = elements;
    // if filter is not definded, use identity function
    settings[setting].filter ||= value => value;
    for (let element of elements) {
      element.addEventListener("input", settings_apply);
    }
  }

})

function take_picture() {
  canvas.classList.remove("is-hidden");
  imports.classList.add("is-hidden");

}

function upload_image() {
  canvas.classList.remove("is-hidden");
  imports.classList.add("is-hidden");
  console.log(this.files[0]);

  img.src = URL.createObjectURL(this.files[0]);
  img.onload = () => draw(true);
}

function save_image() {
  draw(false);

  const dataUrl = canvas.toDataURL("image/png");
  // downloading only works with links but not window.open
  const link = document.createElement('a');
  link.href = dataUrl;
  link.download = 'imagine.png';
  link.click();
}

function share_image() {

}

function settings_apply(event) {
  const changed_setting = event.target.id;
  const new_value = event.target.value;

  // update all inputs for that setting (mobile and desktop) to the new value
  for (let element of settings[changed_setting].elements) {
    if (element == event.target) continue;
    element.value = new_value;
  }

  draw(true);
}

function length(vec) {
  return Math.sqrt(vec.reduce((acc, e) => acc + e * e, 0))
}
function mul(vec, scalar) {
  return vec.map(e => e * scalar);
}
function add(vec_a, vec_b) {
  return vec_a.map((e, i) => e + vec_b[i]);
}
function mix(vec_x, vec_y, scalar) {
  return add(mul(vec_x, 1 - scalar), mul(vec_y, scalar));
}

// https://www.shadertoy.com/view/ldSXWK
function lensflare(uv, pos) {
  const intensity = 1.5;
  // const main = add(uv, mul(pos, -1));
  const uvd = mul(uv, length(uv));

  // let dist = length(main); dist = Math.pow(dist, .1);

  // const f1 = max(0.01-Math.pow(length(add(uv,mul(pos,1.2))),1.9),.0)*7.0;

  const f2 = Math.max(1.0 / (1.0 + 32.0 * Math.pow(length(add(uvd, mul(pos, 0.8))), 2.0)), .0) * 0.1;
  const f22 = Math.max(1.0 / (1.0 + 32.0 * Math.pow(length(add(uvd, mul(pos, 0.85))), 2.0)), .0) * 0.08;
  const f23 = Math.max(1.0 / (1.0 + 32.0 * Math.pow(length(add(uvd, mul(pos, 0.9))), 2.0)), .0) * 0.06;

  let uvx = mix(uv, uvd, -0.5);

  const f4 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.4))), 2.4), .0) * 6.0;
  const f42 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.45))), 2.4), .0) * 5.0;
  const f43 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.5))), 2.4), .0) * 3.0;

  uvx = mix(uv, uvd, -.4);

  const f5 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.2))), 5.5), .0) * 2.0;
  const f52 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.4))), 5.5), .0) * 2.0;
  const f53 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, 0.6))), 5.5), .0) * 2.0;

  uvx = mix(uv, uvd, -0.5);

  const f6 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, -0.3))), 1.6), .0) * 6.0;
  const f62 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, -0.325))), 1.6), .0) * 3.0;
  const f63 = Math.max(0.01 - Math.pow(length(add(uvx, mul(pos, -0.35))), 1.6), .0) * 5.0;

  let c = [
    f2 + f4 + f5 + f6,
    f22 + f42 + f52 + f62,
    f23 + f43 + f53 + f63
  ];

  c = mul(c, 1.3).map(e => e - length(uvd) * .05);

  return mul(c, intensity);
}

function draw(viewport_scale) {
  const filter = Object.entries(settings)
    .map(([setting, { elements, filter }]) => `${setting}(${filter(elements[0].value)})`)
    .join(" ")

  // set the resolution to the original and then scale down to the viewport to
  // only calculate the filter on pixels the user can see. 
  canvas.height = img.naturalHeight;
  canvas.width = img.naturalWidth;
  if (viewport_scale) {
    canvas.height = canvas.clientHeight;
    canvas.width = canvas.clientWidth;
  }
  ctx.filter = filter
  console.log(filter);
  ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
  ctx.filter = '';

  // for (let y = 0; y < canvas.height; y++) {
  //   for (let x = 0; x < canvas.width; x++) {
  //     let uv = [x / canvas.width, y / canvas.height];
  //     // uv[0] *= canvas.width / canvas.height;
  //     // let color = lensflare(uv, [.5,.5]);
  //     ctx.fillStyle = `rgb(${[...uv,0].map(e => Math.floor(e * 255)).join()})`;
  //     ctx.fillRect(x,y,1,1);
  //   }
  // }

  var imgdata = ctx.getImageData(0, 0, canvas.width, canvas.height);
  const pixel_count = imgdata.data.length / 4;
  for (let i = 0; i < pixel_count; i++) {
    let x = i % canvas.width;
    let y = i / canvas.width;
    let u = (x / canvas.width - .5) * canvas.width / canvas.height;
    let v = y / canvas.height - .5;

    // imgdata.data[4*i] = u * 255;
    // imgdata.data[4*i+1] = v * 255;
    // imgdata.data[4*i+2] = 0
    // imgdata.data[4*i+3] = 255; 

    let color = lensflare([u, v], [0.3, 0.3]);
    imgdata.data[4 * i] = color[0] * 255;
    imgdata.data[4 * i + 1] = color[1] * 255;
    imgdata.data[4 * i + 2] = color[2] * 255;
    imgdata.data[4 * i + 3] = 255;
  }
  ctx.putImageData(imgdata, 0, 0);

}