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Re: [Public WebGL] Help with the WebGL Context Lost specification

Ok, thanks for the info. With that change and the fixes to context-lost-restored.html, our implementation conforms, unless there are bugs or mistakes that we somehow passed.



----- Original Message -----
From: "Gregg Tavares (wrk)" <gman@google.com>
To: "Glenn Maynard" <glenn@zewt.org>
Cc: "Benoit Jacob" <bjacob@mozilla.com>, "Douglas Sherk" <dsherk@mozilla.com>, "Kenneth Russell" <kbr@google.com>, "public webgl" <public_webgl@khronos.org>, "Chris Marrin" <cmarrin@apple.com>, "Cedric Vivier" <cvivier@mozilla.com>
Sent: Wednesday, December 14, 2011 11:07:20 AM
Subject: Re: [Public WebGL] Help with the WebGL Context Lost specification

On Wed, Dec 14, 2011 at 9:07 AM, Glenn Maynard < glenn@zewt.org > wrote: 

(Another gigantic email...) 

On Wed, Dec 14, 2011 at 11:54 AM, Benoit Jacob < bjacob@mozilla.com > wrote: 

The current spec says (IIUC) that the first WebGL call, and only the first call, following a context loss, should generate an WEBGL_CONTEXT_LOST error. 

All it says is: 
GLenum getError() ( OpenGL ES 2.0 ยง2.5 , man page ) If the context's webgl context lost flag is set, returns CONTEXT_LOST_WEBGL the first time this method is called. Afterward, returns NO_ERROR until the context has been restored. The ED puts it differently, but has the same effect. 

So IIUC it requires the implementation to keep track of whether a WebGL call was already made since the context loss. 

It only requires keeping track of whether getError has been called. Calling most functions while the context is lost is a complete no-op: steps 1.2 of [1] returns without doing anything. 

[1] https://cvs.khronos.org/svn/repos/registry/trunk/public/webgl/specs/latest/index-context-lost.html#5.14 @ r16355 

It doesn't require any of the that. Most implementations either have a set of error flags or a last error. When context lost is detected, in the error flags case you do this 

error_flags = CONTEXT_LOST_WEBGL_BIT; 

which effectively clears all the bits except 1. In the last error case you do 

last_error = CONTEXT_LOST_WEBGL 

After that since every function is a no-op, and since getError always clears the error or the bit for a specific error you'll only get one CONTEXT_LOST_WEBGL error. No checking needed 

Maybe you want want to change the spec to make that clearer? 

As for bugs in the context lost conformance test Douglas is correct, it's a bug. At the time I fixed the test there was no spec compliant WEBGL_lose_context extension so there was no way to test it. The line mentioned 

shouldGenerateGLError(gl, gl.NO_ERROR, "extension.loseContext()"); 

should be changed to 


Similarly, most likely, any other calls to extension. should not be wrapped in a shouldGenerateGLError 

That's just an oversight from copying and pasting lines and not having the ability to test. 

Glenn Maynard 

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