Trying to customize the screen

• Don't forget to fill your page being created not only with objects (buttons/sliders/other), but also with headings with explanations. And sometimes with tables to align the text
• How many headings and object groups to make is up to you. I usually group by purpose rather than by similarity of objects

<h1>Green Screen Web Page</h1>

<script src="https://www.dukelearntoprogram.com/course1/common/js/image/SimpleImage.js"> </script>

<canvas id="can1"></canvas>
<canvas id="can2"></canvas>

<p>
Foreground: <input type="file" multiple="false" accept="image/*" id="fgfile" onchange="loadForegroundImage()">
</p>

<p>
Background: <input type="file" multiple="false" accept="image/*" id="bgfile" onchange="loadBackgroundImage()">
</p>

<input type="button" id="button" value="Create composite" onclick="doGreenScreen()">

<input type="button" id="button" value="Clear canvas" onclick="clearCanvas()">

• If there is a standard object whose size and color suit you, I advise you not to explicitly set parameters for it. Save time
• Also, if you are not making a new project, but a continuation, you can save the selected settings and simply copy from .CSS to .CSS file

h1 {
  font-size: 20pt;
  font-family: Arial;
  color: #4B0082;
}

body {
  background-color: #FAEBD7;
}

p {
  font-size: 13pt;
}

canvas {
  width: 380px;
  background-color: #7B68EE;
  border: 2px solid #A9A9A9;
}

input {
  font-size: 12pt;
}

Here's a fairly simple but illustrative page design:

• It is better to encapsulate all work with objects in functions to separate the areas of global and local variables
• Identify values ​​with default values (null) ​​- the best programming rule I have ever heard. Errors -100

var fgimage = null;
var bgimage = null;
var can1 = null;
var can2 = null;

function loadForegroundImage(){
  can1 = document.getElementById("can1");
  var imgFile = document.getElementById("fgfile");
  fgimage = new SimpleImage(imgFile);
  fgimage.drawTo(can1);
}

function loadBackgroundImage(){
  can2 = document.getElementById("can2");
  var imgFile = document.getElementById("bgfile");
  bgimage = new SimpleImage(imgFile);
  bgimage.drawTo(can2);
}

function doGreenScreen(){
  if (fgimage==null || !fgimage.complete()) {
    alert("Foreground not loaded");
    return;
  }
  if (bgimage==null || !bgimage.complete()) {
    alert("Background not loaded");
    return;
  }
  clearCanvas();

  var output = new SimpleImage(fgimage.getWidth(), fgimage.getHeight());

  for (var pixel of fgimage.values()) {
    if (pixel.getGreen()> pixel.getRed() + pixel.getBlue()) {
      var x = pixel.getX();
      var y = pixel.getY();
      var bgpixel = bgimage.getPixel(x, y);
      output.setPixel(x, y, bgpixel);
    }
    else {
      output.setPixel(pixel.getX(), pixel.getY(), pixel);
    }
  }
  output.drawTo(can1);
}

function clearCanvas(){
  can1 = document.getElementById("can1");
  can2 = document.getElementById("can2");

  var context1 = can1.getContext("2d");
  var context2 = can2.getContext("2d");
  context1.clearRect(0,0,can1.width,can1.height);
  context2.clearRect(0,0,can2.width,can2.height);
}