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 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);
}