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Re: [Public WebGL] Depth+stencil extension



On Thu, Apr 1, 2010 at 4:41 PM, Chris Marrin <cmarrin@apple.com> wrote:
>
> So we believe the problem with failing tests on the WebKit build bots has to do with the fact that many are headless servers and are therefore using the software OpenGL driver, which does not support the depth+stencil extension.
>
> This makes me think that we should only fail to create a context on such systems when they actually ask for both a depth and stencil buffer. That will allow most tests to continue to run on headless systems and will allow us to carefully segregate tests that use both so they can be skipped when we detect that the test machine does not support it.
>
> This raises the question of letting the author know about such situations. Is there a way today that an author can know that the reason a context was not created was because it has depth+stencil? I hope we don't force authors to first try depth+stencil and then depth only just in case that's why it failed.

The problem isn't with the creation of the WebGL back buffer; we can
always ignore the request for a stencil buffer and still be spec
compliant. The issue is that WebGL (and OpenGL ES 2.0) require that
client code be able to create a framebuffer object (FBO) and attach
both depth and stencil renderbuffers. FBO support isn't optional in
WebGL or OpenGL ES 2.0.

On desktop GL, to the best of my knowledge, using the
EXT_packed_depth_stencil extension is the only widely supported way of
attaching both depth and stencil renderbuffers to an FBO. If this
extension isn't available, then if user code happens to use
framebuffers and try to create a depth+stencil renderbuffer, the call
will fail with an OpenGL error. I don't think this is an acceptable
failure mode. If we return a non-null context from
getContext("webgl"), then it needs to support all of the core
functionality. Therefore unfortunately, if we're running on desktop
OpenGL and EXT_packed_depth_stencil isn't available, I think we need
to refuse to run any WebGL content.

-Ken

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