How to interact with HTML behind a canvas?

Question

The Scenario:
I have a website, currently hosted locally and I want to be able to draw on a canvas over the top of the website, as illustrated below. However, not only do I want to allow a user to draw on the website but also then have the drawing remain on the that specific webpage whilst the user interacts with the rest of that page. E.g. filling out forms or interacting with drop downs, etc.
NB: The HTML Canvas covers the entire page (not just the viewport). Additionally, there would be a button to distinguish between drawing and interacting with HTML



The specific problem:
How to interact with normal HTML elements on a web page whilst a canvas is present. I know the canvas will reset from page to page, and that is fine - I'm not trying to solve that. I'm only trying to solve for a single page.

Methods tried
I've tried playing with transparency, zIndex and other css properties.

Alternative solutions
1. I know that this can be solved using SVG's and just overlaying them in a transparent DIV over the whole webpage. But I'd prefer not to use this method and stick with finding a solution using canvas if possible.

2. Currently the HTML Canvas covers the entire page (not just the viewport) so I suppose I could have a dynamically resizing canvas dependent on how much was draw on the page. However, then I might as well just use SVG's.

Thoughts? I'm open to libs/snippets, anything.

Answer 1

One way to solve this is:

• Set default point-events to none using CSS
• Enable pointer events for elements you want to use for clicks etc. including event catcher (background, body or an element)
• Use body or an element depending on what your design requires, as event-catcher for events that aren't consumed above.
• Put canvas on top of everything with pointer-events set to none

Any click that isn't consumed by a clickable element (or an element you want to use for events) will now end at the back. Simply transfer the coordinates from this backing element to canvas to draw.

Demo

Demo isn't flawless (ie. doesn't consider things like native touch events/gestures, it draws dots not lines etc.) but it should be enough to show the basic principle and get you going.

We simply use body here to catch any events that wasn't consumed. Scroll down to draw and to click the second link in the text.

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var isDown = false;

setup();
window.onresize = setup;
window.onmousedown = function(e) {
  isDown = true;
  draw(e);
};
window.onmousemove = function(e) {if (isDown) draw(e);};
window.onmouseup = function() {isDown = false};

function setup() {
  canvas.width = window.innerWidth;
  canvas.height = parseInt(getComputedStyle(document.body).getPropertyValue("height"), 10);
  ctx.fillStyle = "rgba(0,160,255,0.4)";
}

function draw(e) {
  var rect = canvas.getBoundingClientRect();
  ctx.beginPath();
  ctx.arc(e.clientX - rect.left, e.clientY - rect.top, 15, 0, 6.28);
  ctx.fill();
}

* { /* optimize by being more specific, here just for demo */
  pointer-events:none;
  }
a {
  color:#00c;
  pointer-events:auto;
  cursor:pointer;
  text-decoration:underline;
  }
html, body {
  height:3500px;
  width:100%;
  }
body {
  pointer-events:auto;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  }
#poster {
  width:80%;
  margin:50px auto 0 auto;
  background:rgba(0,0,0,0.2);
  padding:20px;
  box-sizing:border-box;
  z-index:1;
  }
#canvas {
  position:absolute;
  left:0;
  top:0;
  z-index:1000;
  }

<h1>Demonstration</h1>

<div id="poster">
  <h2>Drawable page</h2>
  <a onclick="alert('Was clicked');return false">Click me</a>
</div>

<h3>The standard Lorem Ipsum passage, used since the 1500s</h3><p>"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."</p><h3>Section 1.10.32 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3><p>"Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, vel illum qui dolorem eum fugiat quo voluptas nulla pariatur?"</p>
<h3>1914 translation by H. Rackham</h3>
<p>"But I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system, and expound the actual teachings of the great explorer of the truth, the master-builder of human happiness. No one rejects, dislikes, or avoids pleasure itself, because it is pleasure, but because those who do not know how to pursue pleasure rationally encounter consequences that are extremely painful. Nor again is there anyone <b><a onclick="alert('yeehaw');return false">who loves</a></b> or pursues or desires to obtain pain of itself, because it is pain, but because occasionally circumstances occur in which toil and pain can procure him some great pleasure. To take a trivial example, which of us ever undertakes laborious physical exercise, except to obtain some advantage from it? But who has any right to find fault with a man who chooses to enjoy a pleasure that has no annoying consequences, or one who avoids a pain that produces no resultant pleasure?"</p>
<h3>Section 1.10.33 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3>
<p>"At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga. Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus. Temporibus autem quibusdam et aut officiis debitis aut rerum necessitatibus saepe eveniet ut et voluptates repudiandae sint et molestiae non recusandae. Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat."</p>
<h3>1914 translation by H. Rackham</h3>
<p>"On the other hand, we denounce with righteous indignation and dislike men who are so beguiled and demoralized by the charms of pleasure of the moment, so blinded by desire, that they cannot foresee the pain and trouble that are bound to ensue; and equal blame belongs to those who fail in their duty through weakness of will, which is the same as saying through shrinking from toil and pain. These cases are perfectly simple and easy to distinguish. In a free hour, when our power of choice is untrammelled and when nothing prevents our being able to do what we like best, every pleasure is to be welcomed and every pain avoided. But in certain circumstances and owing to the claims of duty or the obligations of business it will frequently occur that pleasures have to be repudiated and annoyances accepted. The wise man therefore always holds in these matters to this principle of selection: he rejects pleasures to secure other greater pleasures, or else he endures pains to avoid worse pains."</p>

<canvas id="canvas"></canvas>