James Monger James Monger - 1 year ago 193
Javascript Question

Fake CRT effect on Canvas

I want to be able to make my canvas look as though it's on a curved and rounded CRT screen, similar to these two images:

Image one
Image two

How would I do this to an HTML5 canvas? I would be happy to do it within the canvas drawing itself, or with a CSS style applied to the canvas.


Answer Source

A quick search gave me this website: WebGL Fake CRT Effect for HTML5 Games, it uses the glfx.js library.

It is a library meant to apply realtime WebGL effects to an image, but the canvas tag can actually be addressed as an image source...

The demo uses this image to overwrite onto the canvas.

And the code to actually create the effect:

// Make sure you've included the glfx.js script in your code!

// Here I load a PNG with scanlines that I overwrite onto the 2D game's canvas.
// This file happens to be customized for the demo game, so to make this a
// general solution we'll need a generic scanline image or we'll generate them
// procedurally.
// Start loading the image right away, not after the onload event.
var lines = new Image();
lines.src = 'http://i.imgur.com/TAJ0Zkw.png';

window.addEventListener('load', fakeCRT, false);

function fakeCRT() {
    var glcanvas, source, srcctx, texture, w, h, hw, hh, w75;

    // Try to create a WebGL canvas (will fail if WebGL isn't supported)
    try {
       glcanvas = fx.canvas();
    } catch (e) {return;}

    // Assumes the first canvas tag in the document is the 2D game, but
    // obviously we could supply a specific canvas element here.
    source = document.getElementsByTagName('canvas')[0];
    srcctx = source.getContext('2d');

    // This tells glfx what to use as a source image
    texture = glcanvas.texture(source);

    // Just setting up some details to tweak the bulgePinch effect
    w = source.width;
    h = source.height;
    hw = w / 2;
    hh = h / 2;
    w75 = w * 0.75;

    // Hide the source 2D canvas and put the WebGL Canvas in its place
    source.parentNode.insertBefore(glcanvas, source);
    source.style.display = 'none';
    glcanvas.className = source.className;
    glcanvas.id = source.id;
    source.id = 'old_' + source.id;

    // It is pretty silly to setup a separate animation timer loop here, but
    // this lets us avoid monkeying with the source game's code.
    // It would make way more sense to do the following directly in the source
    // game's draw function in terms of performance.
    setInterval(function () {
        // Give the source scanlines
        srcctx.drawImage(lines, 0, 0, w, h);

        // Load the latest source frame

        // Apply WebGL magic
            .bulgePinch(hw, hh, w75, 0.12)
            .vignette(0.25, 0.74)
    }, Math.floor(1000 / 40));

Usually works, although I should warn you that Safari 6 managed to freeze my entire system for a minute at least a couple times... but not every time. You might be happier in Chrome.

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