Unless I'm mistaken, in order for an app to experience this problem, it
would have to:
(1) Obtain vis1 using some means other than glXChooseVisual():
(a) Using XGetVisualInfo(), in which case the 3D app obviously
doesn't care about the visual's 3D capabilities. In this case,
_MatchConfig() will assign a reasonable FB config.
(b) Iterating through the available visuals using glXGetConfig().
In this case, _MatchConfig() will assign an FB config to the visual on
the first call to glXGetConfig() (further discussion on this once I look
at your other patch.)
(2) Create a GLX context using vis1.
(3) Call glXChooseVisual() to obtain a visual with specific
capabilities. Assuming that the visual returned has the same ID as
vis1, VirtualGL will re-hash the visual ID to an FB config that fits the
desired GLX attributes.
(4) Specifically check the visual ID of vis1 against the visual ID of
vis2 and avoid calling glXCreateContext() if they match.
I know of at least one app that does (1)(b), (2), and (3), but since the
app subsequently calls glXCreateContext() with vis2, it still works
fine. I wouldn't want to print a warning unless the app was doing (4),
and there's no way to ascertain that.
On 2/25/11 2:23 PM, Nathan Kidd wrote:
> An app would almost have to try on purpose for this to be a problem,
> but it could be hard to track down:
> ctx1 = glXCreateContext(vis1); (_MatchConfig assigns to config1,
> depthbuf 24)
> then
> vis2 = glXChooseVisual(some attribs, depth 0 ); (assigns config2
> depthbuf 0)
>
> Then notice vis1.visualid == vis2.visualid, therefore no need to create
> a new context to draw with a 0-size depth buffer, but though visualids
> are the same they are mapped to very different server configs.
>
> Ideally MatchConfig and glXChooseVisual would use the same mechanism so
> they could never disagree, but because one uses arbitrary attribs, the
> other doesn't, and both are subject to the whims of glXChooseFBConfig I
> think the only way to guarantee a match is to do the config selection
> ourselves; too much code for just a theoretical edge case.
>
> Why not just make glXChooseVisual return what _MatchConfig already
> chose? It could be done, but I left a warning instead, thinking the
> greater danger is the app getting a config it doesn't like if we ignore
> the explicit attributes passed in. (__vglMatchVisual() is fairly loose.)
>
> -Nathan
>
>
>
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