[Public WebGL] WebGL back buffer contents

Kenneth Russell [email protected]
Wed Jan 27 10:45:32 PST 2010

On Wed, Jan 27, 2010 at 10:11 AM, Tim Johansson <[email protected]> wrote:
> On 2010-01-27 18:51, Gregg Tavares wrote:
>> On Wed, Jan 27, 2010 at 9:35 AM, Tim Johansson <[email protected]
>> <mailto:[email protected]>> wrote:
>>    On 2010-01-27 18:22, Gregg Tavares wrote:
>>        On Wed, Jan 27, 2010 at 12:28 AM, Tim Johansson
>>        <[email protected] <mailto:[email protected]> <mailto:[email protected]
>>        <mailto:[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.
>>    Yes, I am aware of that. That also means that the viewport would
>>    never change without the script resizing the canvas though, and if
>>    the script is doing something special (such as rendering to an
>>    fbo) it can handle that before or after causing the resize.
>>    Rendering directly to the canvas is probably the most common case,
>>    and having that "just work" without special handling would IMO be
>>    better and probably cause less confusion in total. As long as it
>>    is documented that the viewport will change on resize I don't
>>    think it will be such a big problem. The 2d context clears all
>>    state on a resize and that has not cause so much problem.
>> I'm getting lost. If the canvas can not be resized except by user code
>> then why does the viewport need to automatically be updated?  I could see an
>> argument if the browser could magically resize the back buffer but it can't.
>> I just want to make sure the 2 resize cases are not getting confused.
>> Case #1: Resizing the back buffer (ie. canvas.width) can only be done by
>> code
>> Case #2: Resizing the canvas element display size (ie. canvas.style.width)
>> It's only case #1 that has anything to do with the viewport setting and
>> case #1 can only be done in code.
>> So, why should
>> canvas.width = new_width;
>> suddenly change the viewport settings? This doesn't happen in DesktopGL
>> It would basically be making this code invalid
>> // Set the viewport to draw to the top corner of the screen
>> gl.Viewport(0, 0, new_width / 2, new_height / 2);
>> canvas.width = new_width;
>> canvas.height = new_height;
>> Why should that code be invalid?
> What you will want in most cases is to call something like:
> canvas.width=foo;
> gl.viewport(...);
> Since that is the most common case it would make sense to me if it did that
> viewport call automatically. You would have to write extra code in the
> uncommon case that I want to keep the old viewport instead of having to
> write extra code in the very common case that you want to change the
> viewport to match the canvas.
> I'm not saying you would never want to change the viewport, just that it
> will be more common to render to the entire canvas. I don't think it should
> be more difficult to do the common thing that most people will want to do
> than it is to handle a special case.
> In desktop GL the window size can change without the code causing the
> resize, so they can't automatically change the viewport since the code don't
> know when it will happen. In WebGL the script does know when the resize
> happens so it would be possible to change it automatically.

In recent conversations with people at NVIDIA they were pretty adamant
that we not mess with the OpenGL state, in particular the viewport,
upon changing the canvas size. Gregg's render target example is a good
reason to not automatically call glViewport() behind the scenes.
Another point is that it isn't obvious exactly when the implicit call
to  glViewport() would be made -- whether it would be synchronous with
the setting of the width / height properties or whether it would
somehow be done lazily, before the first draw call after the canvas is

I used to think that doing this automatically would be a good idea but
because of the semantic details I don't any more.


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