[Public WebGL] Updating the Spec W.R.T. preserveDrawingBuffer = false

Gregg Tavares [email protected]
Mon Jun 10 22:53:16 PDT 2013


I don't think we have the luxury to break existing apps which is why I
suggested that using the canvas as a source only uses the buffer being
composited until a draw call is made.

That way code like this



   // progressively capture as I draw
   gl.drawXXX();  // draw stuff
   gl.drawXXX();  // draw more stuff
   (new Image()).src = gl.canvas.toDataURL();
   gl.drawXXX();  // draw more stuff
   (new Image()).src = gl.canvas.toDataURL();





On Mon, Jun 10, 2013 at 10:30 PM, Mark Callow <[email protected]>wrote:

>  On 2013/06/11 9:28, Kenneth Russell wrote:
>
> This proposed preserveDrawingBuffer change probably would not decrease
> WebGL's efficiency in the majority of browsers and on the majority of
> OSs. The browser can still use DiscardFramebufferEXT for the depth and
> stencil buffers when presenting the WebGL canvas to the compositor. In
> my understanding, it's never been possible to discard the color
> buffer, because the page compositor needs it in case the page needs to
> be redrawn using the same WebGL results from the previous frame. Since
> the color buffer is always available after compositing, it can be made
> available to toDataURL and other APIs at no cost.
>
>  I think you are slightly misunderstanding both preserveDrawingBuffer and
> DiscardFramebufferEXT.
>
> IIUC, all browsers render into framebuffer objects. They use the resulting
> framebuffer for compositing with the rest of the web page. The browser must
> either
>
>    1. maintain 2 or more buffers, one for drawing and one for
>    compositing, or
>     2. ensure that the WebGL application's draw functions are not called
>    until the buffer is no longer needed by the compositor.
>
> In the former case pDB=true requires copying the data from the buffer
> received for compositing to the next one handed to the WebGL application to
> draw into. In the latter case the value of pDB is immaterial. Data is
> preserved.
>
> The purpose of DiscardFramebufferEXT is to tell a tiling GPU that is does
> not need to copy the data corresponding to the current tile from the
> framebuffer of the previous frame into the tile memory prior to rendering.
> *It has no effect whatsoever on the content of a framebuffer*.
> Copy-to-tile only needs to be done when pDB==true. When pDB==false, the
> browser can always call DFExt before calling the WebGL app's draw function.
>
> I suspect most browsers use the multiple buffer model. So it seems
> sensible to define that toDataURL and drawImage receive the data from the
> current compositing buffer rather than the current drawing buffer whose
> content is likely still a work in progress. It makes sense for the
> tex*Image2D calls too. The proposed change means that tex*Image2D(...,
> canvas), where canvas is the current canvas, would actually capture the
> content of the previous frame. I think this is okay. If the app wants to
> capture the content of the current drawing buffer, it can use copyTexImage
> which is much more efficient anyway.
>
> Regards
>
>     -Mark
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