[Public WebGL] WebGL back buffer contents

Gregg Tavares [email protected]
Wed Jan 27 09:22:05 PST 2010


On Wed, Jan 27, 2010 at 12:28 AM, Tim Johansson <[email protected]> wrote:

> On 2010-01-27 01:45, Chris Marrin wrote:
>
>> On Jan 26, 2010, at 3:37 PM, Oliver Hunt wrote:
>>
>>
>>
>>> On Jan 26, 2010, at 3:28 PM, Chris Marrin wrote:
>>>
>>>
>>>
>>>> On Jan 26, 2010, at 10:00 AM, Vangelis Kokkevis wrote:
>>>>
>>>>
>>>>
>>>>>
>>>>> ...
>>>>> I think we should settle on having the contents of the WebGL drawing
>>>>> buffer be persistent, like the 2D context. Having a completely new
>>>>> back buffer potentially swapped in unexpectedly will be too surprising
>>>>> to the programmer.
>>>>>
>>>>> -Ken
>>>>>
>>>>> I like that idea although I'm not sure how we would handle resizing of
>>>>> drawing surface.  What's the expectation then?
>>>>>
>>>>>
>>>> Here is what the Canvas element says:
>>>>
>>>>        When the canvas element is created, and subsequently whenever the
>>>> width and height attributes are
>>>>        set (whether to a new value or to the previous value), the bitmap
>>>> and any associated contexts must
>>>>        be cleared back to their initial state and reinitialized with the
>>>> newly specified coordinate space
>>>>        dimensions.
>>>>
>>>>        When the canvas is initialized, its bitmap must be cleared to
>>>> transparent black.
>>>>
>>>> That seems like an appropriate definition for us.
>>>>
>>>>
>>> My only concern with this exact definition is that the 2d canvas is
>>> completely reset - all state is clobbered, applying the same logic to webgl
>>> would imply that all shaders, etc would be unloaded as well, which seems a
>>> little extreme.
>>>
>>>
>> Yeah, I didn't fully read the "and any associated contexts" part. I think
>> that in the past we agreed that we should not even mess with the viewport
>> coordinates on a size change. So perhaps better wording would be:
>>
>>        When the canvas element is created, and subsequently whenever the
>> width and height attributes are
>>        set (whether to a new value or to the previous value), the bitmap
>> must be cleared to transparent black.
>>        Furthermore upon creation of the context its initial state shall be
>> as described in the OpenGL ES 2.0
>>        specification [GLES20].
>>
>>        Setting the width and height attributes after initialization shall
>> not change the context state.
>>
>>
>>
> I thought we said we should change the viewport when resizing, or maybe
> that changed later and I missed it?
>
> I think it would be less confusing, at least in the most common case, if
> the viewport did change on resize.
>
>
http://www.khronos.org/bugzilla/show_bug.cgi?id=245

Setting the viewport automatically could cause problems.

Here's one example.

--psuedo code-
// Set to render to a render target
...
// Set the viewport to match the render target
gl.Viewport(/* rendertarget dimensions */)

// Change the dimensions of the canvas
canvas.width = new_width;
canvas.height = new_height;

// Render something to render target.
// The user should not expect the viewport setting to have changed
gl.drawElements(...);

-------------

Just to clarify. There are 2 dimensions to a canvas

canvas.width and canvas.height set the dimensions of the back buffer. These
can only be set by code.

canvas.style.width and canvas.style.height set the dimensions to display.
These can change without code by setting them to percentages.

There is no case where the browser is going automatically to change the back
buffer dimensions. Only user code can do that and so user code can set the
viewport.




> //Tim
>
>
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