How do you save an image from a Three.js canvas?

Since the toDataURL is a method of canvas html element, that will work for 3d context too. But you have to take care of couple of things.

1. Make sure when the 3D context is initialized you set preserveDrawingBuffer flag to true, like so:

   var context = canvas.getContext("experimental-webgl", {preserveDrawingBuffer: true});
   

2. Then user canvas.toDataURL() to get the image

In threejs you would have to do the following when the renderer is instantiated:

new THREE.WebGLRenderer({
    preserveDrawingBuffer: true 
});

Also, keep in mind this can have performance implications. (Read: https://github.com/mrdoob/three.js/pull/421#issuecomment-1792008)

This is only for webgl renderer, in case of threejs canvasRenderer though, you can simply do renderer.domElement.toDataURL(); directly, no initialization parameter needed.

My webgl experiment: http://jsfiddle.net/TxcTr/3/ press 'p' to screenshot.

Props to gaitat, I just followed the link in his comment to get to this answer.

I read the conversation posted by Dinesh (https://github.com/mrdoob/three.js/pull/421#issuecomment-1792008) and came up with a solution that won't slow down your application.

    function render() { 
        requestAnimationFrame(render);
        renderer.render(scene, camera);
        if(getImageData == true){
            imgData = renderer.domElement.toDataURL();
            getImageData = false;
        }
    } 

With this you can leave the preserveDrawingBuffer-Flag at false and still get the image from THREE.js. Simply set getImageData to true and call render() and you are good to go.

getImageData = true;
render();
console.debug(imgData);

Hope this helps people like me who need the high fps :)