threejs canvas todataurl is blank

Question

Okay, I know that canvas.toDataUrl() will produce an image in png format. However, when I try to get the image from http://threejs.org/examples/#webgl_lines_sphere. All I see is a black image on chrome. To replicate the steps -

1) Open dev console and select the canvas element. 2) canvas = $0 3) context = canvas.getContext('webgl', {preserveDrawingBuffer: true}) 4) img = canvas.toDataUrl() 5) document.write('<img src="'+img+'"/>')

The image is blank. However, I tried with a different canvas at link http://threejs.org/examples/#canvas_geometry_cube. Please do the following steps to replicate.

1) Open dev console and select the canvas element. 2) canvas = $0 3) context = canvas.getContext('2d', {preserveDrawingBuffer: true}) 4) img = canvas.toDataUrl() 5) document.write('<img src="'+img+'"/>')

This gave the expected result. Why is there a difference and how can this be avoided to retrieve first image too?

Apterygial

Answer 1

I was also getting a solid black image.

My code previously was:

this.renderer = new THREE.WebGLRenderer({premultipliedAlpha: false});

I have changed the parameter in the THREE.WebGLRenderer to:

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

I am getting an image on taking a snapshot.

Hope it helps.

Nupercaine

Answer 2

This is because the first example _{(see sources line 103)} does use a THREE.WebGLRenderer creator, while the second one _{(see sources line 92)} uses a THREE.CanvasRenderer.

Some notes :

• There is no preserveDrawingBuffer contextAttribute in the context2d API, only in the WebGL one.
• You can create only one context per canvas element.
• You can't set the preserveDrawingBuffer flag after context's creation
• With three.js you can simply call renderer.domElement.toDataURL() (you'll need to go to the iframe target to be able to call from the dev tools).
• Another solution, (and better than preserveDrawingBuffer flag) is to call canvas.toDataURL() in the rendering loop itself, before the browser takes controls again.

Lacrimatory

Is there a work around to capture the image after the entire document is rendered and not during the render loop? Assume that I cannot and would not be able to get my hands on the render loop.. – Apterygial

If you have access to the renderer variable, you can call renderer.domElement.toDataURL(), see my edit. – Lacrimatory

It seems renderer.domElement.toDataURL() also does not work. Did it work for you, Kaiido? Also, there is no difference in accessing renderer.domElement and $0 in the console, as it just returns the canvas either way. – Apterygial

@SantanMaddi, you're right, it doesn't work in Chrome... In Firefox however, if you do call twice the toDataURL or readPixel method, then you've got the image... I don't clearly get it... But if you don't have access to the rendering loop, I'll say that you're stuck, and the only way will be to perform a screenshot from outside the page (browser extension or OS app) – Lacrimatory