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Re: Advice on extending CanvasRenderingContext2D to support Depth Streams

From: Rob Manson <roBman@buildAR.com>
Date: Wed, 05 Nov 2014 10:04:24 +1100
Message-ID: <54595B78.1020109@buildAR.com>
To: Rik Cabanier <cabanier@gmail.com>, "Kostiainen, Anssi" <anssi.kostiainen@intel.com>
CC: "public-canvas-api@w3.org" <public-canvas-api@w3.org>, "Hu, Ningxin" <ningxin.hu@intel.com>
Hi Rick,

First, I'd say it's definitely worth checkout out the presentation[1] 
and video of the demos[2] that were presented at TPAC.

> It's also not clear to me how these API's will be used.
> Can you write some very high pseudo-code that shows how a typical author
> would get/set the data and present it to the user?

Here's some pseudo code examples[1] in our initial Use Cases document.

Here's the one for a 2d canvas context (although it should probably use 
rAF() instead of setInterval()).
NOTE: This obviously assumes the 2d drawing context is already setup.

   setInterval(function() {
     context.drawImage(video, 0, 0, w, h);

     var depthData = context.getDepthData(0, 0, w, h);
     // depthData.data is an Uint16Array for depth stream.
     console.log(depthData.data instanceof Uint16Array); // prints "true"
     var depthMap = imageData.data;

     // the depthMap is an array of 16-bit unsigned integers
     for (var i = 0; i < depthMap.length; i++) {
       var depth = depthMap[i];
       // process the 16-bit unsigned depth value
   }, 1000 / fps);

So you can see this is designed based on the existing 
canvas2dcontext.drawImage(video, ...)/getImageData(...) model that is 
already widely used (see discussion below).

>     Can you explain why you would like to see this integrated with canvas?

Well canvas objects deliver the only pipeline available to Computer 
Vision related media stream processing on the Web Platform today.

<video>  ->  <canvas(2d/3d)>  ->  Typed Array  ->  js  ->  *
              (using .drawImage)   (using .getImageData)

So this pattern seemed like an obvious starting point for us.

BTW: On a related note, I'm just about to circulate the results of a 
test framework we've been running across all the main browser/OS 
combinations that looks at the overall performance of these types of 
media stream processing pipelines - especially contrasting the 2dcanvas 
and the webgl canvas pipelines. I'll post info to this list soon and 
would appreciate any feedback.

>     It seems that the extension to Canvas 2D could be implemented as a
>     new standalone object that works in conjunction with a media element
>     (that is possibly drawn to a canvas)

I'm not quite clear exactly how you see this might work. Could you 
please sketch this idea out a little bit more?

BTW: Initially, I used to think we needed a way to decode the video 
frames without touching other DOM elements like canvas at all. But after 
running our test framework it seems that some of the interactions 
between <video> and <canvas> have already been really well optimised.

Of course, I and the other authors of this depth extension are very open 
to suggestions for better solutions though.

>     The model states that this should taint the canvas [1].
>     Is it possible today to draw a local video stream from your camera
>     to a 2d context and read the pixels?

Yes. The model we have applied so far is that we pack the Uint16Array 
data into as few channels as possible (in WebGL we're using 3 in a 5_6_5 
structure) so they can be extracted and recombined in as few steps as 

There is also some existing work on how to pack these uint16's into 
existing codecs - see reference 8[2] on our Use Cases page.

Our thoughts are that the getDepthData() method would take the data from 
the RGBA channels from the Uint8Array in the depth video and recombine 
them in an agreed way to pack them into a Uint16Array and return that.

I think the behaviour would be exactly parallel to how writing <video> 
frames to a 2d canvas drawing context works today.


[2] https://www.youtube.com/watch?v=cZiSPsaZ_Dc
[3] https://www.w3.org/wiki/Media_Capture_Depth_Stream_Extension#Examples
[4] https://www.w3.org/wiki/Media_Capture_Depth_Stream_Extension#References
Received on Tuesday, 4 November 2014 22:57:45 UTC

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