Prajwal_7 Prajwal_7 - 1 year ago 225
Javascript Question

canvas.toDataURL() is not a function - error in javascript code running in node.js server

Currently I am trying to convert browser based client side volume rendering code to server side pure javascript based rendering.

I use node-webgl at server side. My main goal is to send the canvas content of server to client, later this image data is displayed on the client.

But before this I need to check whether the rendering is proper. So I used canvas.toDataURL() to extract canvas data and want to display it on a separate window. But I am facing the error.

Below is my code:


var nodejs=true,
WebGL = require('./node_modules/node-webgl/index'),
document = WebGL.document();

requestAnimationFrame = document.requestAnimationFrame;

//Read and eval library
eval(fs.readFileSync(__dirname+ '/sylvester.js','utf8'));
eval(fs.readFileSync(__dirname+ '/glUtils.js','utf8'));
eval(fs.readFileSync(__dirname+ '/glAux.js','utf8'));
eval(fs.readFileSync(__dirname+ '/volumercserver.js','utf8'));


and volumerc_main is contained in volumercserver.js (including here only required code)

var Image = require("node-webgl").Image;

var canvas = document.createElement("canvas"); = 'canvas_win';
canvas.width= 512;
canvas.height= 512;
var gl;

var canvas1 = document.createElement("canvas");
canvas1.width= 512;
canvas1.height= 512;
var ctx = canvas1.getContext('2d');

try {
gl = canvas.getContext("webgl", {premultipliedAlpha: false}) || canvas.getContext("experimental-webgl",{premultipliedAlpha: false});
} catch (e) {
if (!gl) {
alert("Could not initialise WebGL, sorry :-(");

// All computations take place here

var dataURLstring = canvas.toDataURL();

var img1 = new Image;
img1.src = dataURLstring;
// img1.onload = function(){
ctx.drawImage(img1,0,0); // Or at whatever offset you like
// };

Now, when I run node npm.js I get the error,

C:\Users\z003npra\Desktop\node>node exp.js
Status: Using GLEW 1.13.0
Status: Using GLEW 1.13.0
var dataURLstring = canvas.toDataURL();

TypeError: canvas.toDataURL is not a function
at drawVolume (eval at <anonymous> (C:\Users\z003npra\Desktop\node\exp.js:15
:9), <anonymous>:443:30)
at Timer.listOnTimeout (timers.js:92:15)

Log of canvas just befor I get error at toDataURL()

{ type: 'nodeGLFW',
ratio: 1,
setTitle: [Function],
setIcon: [Function],
flip: [Function],
getElementById: [Function],
createElement: [Function],
createWindow: [Function],
getContext: [Function],
on: [Function],
addEventListener: [Function],
removeEventListener: [Function],
requestAnimationFrame: [Function],
drawingBufferWidth: 800,
width: 512,
drawingBufferHeight: 800,
height: 512,
canvas: [Circular],
id: 'canvas_win',
onmousedown: [Function: handleMouseDown],
onmouseup: [Function: handleMouseUp],
onmousemove: [Function: handleMouseMove] }

Please help me to solve this.

Answer Source

.toDataURL() function is not defined in node-webGL wrapper. An alternative method is to make use of gl.readPixels (suggested by a member of this group).

Here is the way to use it.

var pixels = new Uint8Array(canvas.width * canvas.height * 4);
gl.readPixels(0, 0, canvas.width, canvas.height, gl.RGBA, gl.UNSIGNED_BYTE, pixels);

after the execution of gl.readPixels, the buffer data is present in pixels. The data is of ImageData() format. With this data, further required process can be done.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download