canvas.toDataURL() for large canvas

Question

I have a problem with .toDataURL() for large canvas. I want to enconde in base64 and decode on php file but if I have a large canvas the strDataURI variable is empty.

My code:

var strDataURI = canvas.toDataURL();
strDataURI = strDataURI.substr(22, strDataURI.length);
$.post("save.php",
{ 
   str: strDataURI
};

Is there any alternative to .toDataURL() or some way to change the size limit?

Thanks.

Answer 1

I'm not sure if there are limitation to canvas dimensions, but data urls have limitations depending on the browser: Data URL size limitations.

What you could try is using Node.js + node-canvas (server side) to recreate the canvas. I've been using these for creating printable images from canvas elements, and didn't have any problems/limitations using toDataURL so far.

Are you using the fabric.js library? I noticed you posted on their forum as well. Fabric.js can be used in Node.js and has a toDataURLWithMultiplier method, which scales the canvas/context allowing you to change the dataurl image size. You can check the method source to see how this is done.

Edit:

Since you're using fabric.js I would suggest using Node.js to handle the canvas to image processing on the server. You'll find more info on how to use fabric.js on Node.js here.

Here is a simple server using Node.js and express:

var express = require('express'),
    fs = require('fs'),
    fabric = require('fabric').fabric,
    app = express(),
    port = 3000;

var allowCrossDomain = function (req, res, next) {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'POST, OPTIONS');
    res.header('Access-Control-Allow-Headers', 'Content-Type');
    next();
}

app.configure(function() {
    app.use(express.bodyParser());
    app.use(allowCrossDomain);
});

app.options('/', function(req, res) {
    res.send(200);
});

app.post('/', function(req, res) {
    var canvas = fabric.createCanvasForNode(req.body.width, req.body.height);
    
    console.log('> Loading JSON ...');
    canvas.loadFromJSON(req.body.json, function() {
        canvas.renderAll();
        
        console.log('> Getting PNG data ... (this can take a while)');
        var dataUrl = canvas.toDataURLWithMultiplier('png', req.body.multiplier),
            data = dataUrl.replace(/^data:image\/png;base64,/, '');
        
        console.log('> Saving PNG to file ...');
        var filePath = __dirname + '/test.png';
        fs.writeFile(filePath, data, 'base64', function(err) {
            if (err) {
                console.log('! Error saving PNG: ' + err);
                res.json(200, { error: 'Error saving PNG: ' + err });
            } else {
                console.log('> PNG file saved to: ' + filePath);
                res.json(200, { success: 'PNG file saved to: ' + filePath });
            }
        });
    });
});

app.listen(port);
console.log('> Server listening on port ' + port);

When the server is running you can send data to it (postData). The server expects json, width and height to recreate the canvas, and a multiplier to scale the data url image. The client side code would look something like this:

var postData = {
    json: canvas.toJSON(),
    width: canvas.getWidth(),
    height: canvas.getHeight(),
    multiplier: 2
};

$.ajax({
    url: 'http://localhost:3000',
    type: 'POST',
    contentType: 'application/json; charset=utf-8',
    data: JSON.stringify(postData),
    dataType: 'json',
    success: function(data) {
        console.log(data);
    },
    error: function(err) {
        console.log(err);
    }
});

Answer 2

You should first consider this: the size of the upload is limited. The limit depends on browser, OS and server environment. You can have a look at this article: http://www.motobit.com/help/scptutl/pa98.htm

In general you can try something like this: first we need a function to convert the dataURI to a blob:

function convertDataURItoBlob(dataURI) {
        'use strict'

        var byteString,
            mimestring

        if(dataURI.split(',')[0].indexOf('base64') !== -1 ) {
            byteString = atob(dataURI.split(',')[1])
        } else {
            byteString = decodeURI(dataURI.split(',')[1])
        }

        mimestring = dataURI.split(',')[0].split(':')[1].split(';')[0]


        var content = new Array();
        for (var i = 0; i < byteString.length; i++) {
            content[i] = byteString.charCodeAt(i)
        }
        var rawContent = new Uint8Array(content),
            returnBlob = new Blob([rawContent], {type: mimestring})

        return returnBlob;

}

and next a function for the upload of the file, using XMLHttpRequest2:

function upload(blob) {
  var xhr = new XMLHttpRequest();
  xhr.open('POST', '/yourServerEndPoint', true);
  xhr.onload = function(e) { ... };

  xhr.send(blob);
}

Now you can pass your strDataURI to the first function and then upload the file with the second function.

You can have a deeper look at XMLHTTPRequest2 here: http://www.html5rocks.com/en/tutorials/file/xhr2/ and about the blob constructor here: https://developer.mozilla.org/en-US/docs/DOM/Blob

Answer 3

You could always just break the image up into smaller sections and save those individually, which probably isn't a bad idea anyway. Basically you'd have a function that's something like

var largeCanvas = document.getElementById('yourGiantCanvas').getContext('2d'),
    slice = document.createElement('canvas').getContext('2d');

slice.canvas.width = 1000;
slice.canvas.height = 1000;

for (var y=0; y < canvas.height; y+=1000){
  for (var x=0; x < canvas.width; x+=1000){
    slice.clearRect(0, 0, slice.canvas.width, slice.canvas.height);
    slice.drawImage(largeCanvas.canvas, x, y, 1000, 1000, 0, 0, 1000, 1000);

    var imagePiece = slice.canvas.toDataURL();

    //Now just save the imagePiece however you normally were planning to
    //and you can build the image again using these slices. You can create 
    //a much better user experience this way too. 
  }
}

Answer 4

Have updated the code to split the canvas into smaller canvas objects. Works pretty good and have added a tracker also:

This Allows for tracking of the upload process and overall I think is better for the user. I use PHP to rejoin at a later stage.

It avoids the issues of size of canvas / browser etc.

My first post so hope it helps!

// pass in type for the file name

function sliceCanvas(type, canvasId){
var largeCanvas = document.getElementById(canvasId).getContext('2d');
var slice = document.createElement('canvas').getContext('2d');

var baseSize = 500;

fileH = largeCanvas.canvas.height / baseSize;
fileW = largeCanvas.canvas.width / baseSize;

slice.canvas.width = baseSize;
slice.canvas.height = baseSize; 
count = 1;

numFiles = Math.ceil(fileH) * Math.ceil(fileW);

for (var y=0; y < largeCanvas.canvas.height; y+=baseSize){
  for (var x=0; x < largeCanvas.canvas.width; x+=baseSize){
    slice.clearRect(0, 0, slice.canvas.width, slice.canvas.height);
    slice.drawImage(largeCanvas.canvas, x, y, baseSize, baseSize, 0, 0, baseSize, baseSize);

    var imagePiece = slice.canvas.toDataURL();

    typeFinal = type + count;

    exportSlice(typeFinal, imagePiece, numFiles);
    count++;


  } 
}
}

Ajax to upload:

function exportSlice(type, dataURL, fileNum){

percent = 0;
percentComplete = 0;

    $.ajax({
         type: "POST",
          url: YourServerSideFiletoSave,
          data: {image: dataURL, type: type}
        })
        .done(function( response ) {
            console.log(response);
            percent++;
            percentComplete = Math.ceil(Number(percent/fileNum*100));
           return true;
         })
          .fail(function(response) {
            console.log("Image FAILED");

            console.log(response);

            return false;

        })
        .always(function(response) {
          console.log( "Always");
        });

  }