[Public WebGL] WebGL back buffer contents

Tim Johansson [email protected]
Tue Jan 26 00:59:48 PST 2010


On 1/25/10 11:43 PM, Chris Marrin wrote:
> Generally, these are all good questions. More inline...
>
> On Jan 25, 2010, at 2:11 PM, Gregg Tavares wrote:
>
>    
>> Something that's not clear to me still is how WebGL knows what's in the backbuffer.
>>
>> If I do something like
>>
>> window.setInterval(drawNextTriangle, 1);
>>
>> function drawNextTriangle() {
>>    ctx.drawElements(...);
>> };
>>
>> Then what will I see on the screen? Is it defined?
>>      
>    
I think that is not defined, but it probably should be because authors 
will do such things.
> I'm not sure what you mean here. This should draw a new triangle every ms, right? Depending on how fast your browser can service the setInterval, it will draw a new triangle at that rate. Theoretically every ms, but I believe most browsers will throttle that down to something much lower.
>
>    
Right, but after each draw call the content could theoretically be reset 
to a different back buffer or cleared to black since that behavior is 
not specified as far as I know.
You would get either all triangles, every second triangle (switching if 
it is even or odd numbered triangles visible) or you could get just the 
last triangle.
>> I posted the 3rd example because section 2.2 of the spec says after compositing the contents of the backbuffer are undefined which suggests to me that second drawImage call will have undefined results? Am I mis-interpreting the spec?
>>      
>
> The "undefined" bit was put there so that different swapping models could be afforded. But since direct rendering of the WebGL content is not really possible (it has to be composited by HTML to have the proper layering), I would have no trouble saying that the contents of the drawing buffer are valid until changed by a clear() or further drawing calls.
>
>    
I think a specific behavior will be required to be compatible with web 
content anyway, so I am not against specifying this in some way.

//Tim

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