Imagine/main.js

let canvas;
let ctx;
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");
  document.getElementById("back").addEventListener("click", () => document.body.classList.remove("editor-enabled"));

  // 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);
    }

    const reset_elements = document.getElementsByClassName(setting + "-reset");
    for (let reset_element of reset_elements) {
      reset_element.addEventListener("click", () => reset_all(setting));
    }
  }
});

function reset_all(setting) {
  console.log("reseting " + setting);
  for (let element of settings[setting].elements) {
    element.value = element.defaultValue;
  }
  draw(true);
}

function take_picture() {
  document.body.classList.add("editor-enabled");

}

function upload_image() {
  document.body.classList.add("editor-enabled");

  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);
}

// https://www.shadertoy.com/view/ldSXWK
function lensflare(pos_x, pos_y) {
  const imgdata = ctx.getImageData(0, 0, canvas.width, canvas.height);
  const pixel_count = imgdata.data.length / 4;
  const aspect_ratio = canvas.width / canvas.height;
  for (let i = 0; i < pixel_count; i++) {
    const x = i % canvas.width;
    const y = i / canvas.width;
    const u = (x / canvas.width - .5) * aspect_ratio;
    const v = y / canvas.height - .5;

    const intensity = 1.5 * 255;
    const uv_len = u * u + v * v;
    const uvd_x = u * uv_len;
    const uvd_y = v * uv_len;
    const uvd_len = uvd_x * uvd_x + uvd_y * uvd_y;

    const uvd_pos_x = uvd_x + pos_x;
    const uvd_pos_y = uvd_y + pos_y;
    let temp = uvd_pos_x * uvd_pos_x + uvd_pos_y * uvd_pos_y
    const f2 = Math.max(1.0 / (1.0 + 32.0 * temp * .64), .0) * 0.1;
    const f22 = Math.max(1.0 / (1.0 + 32.0 * temp * .72), .0) * 0.08;
    const f23 = Math.max(1.0 / (1.0 + 32.0 * temp * .81), .0) * 0.06;

    let uvx_x = u * (1 + 0.5) + uvd_x * 0.5;
    let uvx_y = v * (1 + 0.5) + uvd_y * 0.5;
    let uvx_pos_x = uvx_x + pos_x;
    let uvx_pos_y = uvx_y + pos_y;

    temp = uvx_pos_x * uvx_pos_x + uvx_pos_y * uvx_pos_y
    const f4 = Math.max(0.01 - temp * .16, .0) * 6.0;
    const f42 = Math.max(0.01 - temp * .20, .0) * 5.0;
    const f43 = Math.max(0.01 - temp * .25, .0) * 3.0;

    uvx_x = u * (1 + 0.4) + uvd_x * 0.4;
    uvx_y = v * (1 + 0.4) + uvd_y * 0.4;
    uvx_pos_x = uvx_x + pos_x;
    uvx_pos_y = uvx_y + pos_y;

    temp = uvx_pos_x * uvx_pos_x + uvx_pos_y * uvx_pos_y
    const f5 = Math.max(0.01 - temp * .04, .0) * 2.0;
    const f52 = Math.max(0.01 - temp * .16, .0) * 2.0;
    const f53 = Math.max(0.01 - temp * .36, .0) * 2.0;

    uvx_x = u * (1 + 0.5) + uvd_x * 0.5;
    uvx_y = v * (1 + 0.5) + uvd_y * 0.5;
    uvx_pos_x = uvx_x - pos_x;
    uvx_pos_y = uvx_y - pos_y;

    temp = uvx_pos_x * uvx_pos_x + uvx_pos_y * uvx_pos_y
    const f6 = Math.max(0.01 - temp * .9, .0) * 6.0;
    const f62 = Math.max(0.01 - temp * .1, .0) * 3.0;
    const f63 = Math.max(0.01 - temp * .12, .0) * 5.0;

    imgdata.data[4 * i + 0] += 1.2 * ((f2 + f4 + f5 + f6) * 1.3 - uvd_len * .05) * intensity;
    imgdata.data[4 * i + 1] += 1.5 * ((f22 + f42 + f52 + f62) * 1.3 - uvd_len * .05) * intensity;
    imgdata.data[4 * i + 2] += 1.3 * ((f23 + f43 + f53 + f63) * 1.3 - uvd_len * .05) * intensity;
    imgdata.data[4 * i + 3] = 255;
  }
  ctx.putImageData(imgdata, 0, 0);
}

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 = '';

  // lensflare(.3,.3);
}