How to add undo-functionality to HTML5 Canvas?

I have a Sketching app done in all HTML5 and Javascript, and I was wondering how I would create an Undo Button, so you could undo the last thing you drew. Any idea?

Answers:

Answer

You have to store all modifications in a datastructure. Then you can delete the latest modification if the user wants to undo it. Then you repaint all drawing operations from your datastructure again.

Answer

On http://arthurclemens.github.io/Javascript-Undo-Manager/ I have a working example of undo with a canvas element. When you make a modification, you feed the undo manager the undo and redo methods. Tracking of the position in the undo stack is done automatically. Source code is at Github.

Answer

The other option, if you need to manipulate objects is to convert your canvas to SVG using a library that preserves the Canvas API preventing a rewrite.

At least one such library exists at this time (November 2011): SVGKit

Once you have SVG, it is much easier to remove objects and much more without the need to redraw the entire canvas.

Answer

Here is a solution that works for me. I have tried it in the latest versions of Firefox and Chrome and it works really well in those two browsers.

var isFirefox = typeof InstallTrigger !== 'undefined';
var ctx = document.getElementById('myCanvas').getContext("2d");
var CanvasLogBook = function() {
    this.index = 0;
    this.logs = [];
    this.logDrawing();
};
CanvasLogBook.prototype.sliceAndPush = function(imageObject) {
    var array;
    if (this.index == this.logs.length-1) {
        this.logs.push(imageObject);
        array = this.logs;
    } else {
        var tempArray = this.logs.slice(0, this.index+1);
        tempArray.push(imageObject);
        array = tempArray;
    }
    if (array.length > 1) {
        this.index++;
    }
    return array;
};
CanvasLogBook.prototype.logDrawing = function() { 
    if (isFirefox) {
        var image = new Image();
        image.src = document.getElementById('myCanvas').toDataURL();
        this.logs = this.sliceAndPush(image);
    } else {
        var imageData = document.getElementById('myCanvas').toDataURL();
        this.logs = this.sliceAndPush(imageData);
    }
};
CanvasLogBook.prototype.undo = function() {
    ctx.clearRect(0, 0, $('#myCanvas').width(), $('#myCanvas').height());
    if (this.index > 0) {
        this.index--;
        this.showLogAtIndex(this.index);
    }
};
CanvasLogBook.prototype.redo = function() {
    if (this.index < this.logs.length-1) {
        ctx.clearRect(0, 0, $('#myCanvas').width(), $('#myCanvas').height());
        this.index++;
        this.showLogAtIndex(this.index);
    }
};
CanvasLogBook.prototype.showLogAtIndex = function(index) {
    ctx.clearRect(0, 0, $('#myCanvas').width(), $('#myCanvas').height());
    if (isFirefox) {
        var image = this.logs[index];
        ctx.drawImage(image, 0, 0);
    } else {
        var image = new Image();
        image.src = this.logs[index];
        ctx.drawImage(image, 0, 0);
    }
};
var canvasLogBook = new CanvasLogBook();

So every time you draw any thing you will there after run function canvasLogBook.logDrawing() to store a snapshot of the canvas and then you can call canvasLogBook.undo() to undo and canvasLogBook.redo() to redo.

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.