Created attachment 2222 [details] [review]
xreaddisplay-symbols-2.patch

Export three functions from dix/events.c and one variable from dix/globals.c. 
I'm never happy to be adding SYMVARs but this one appears to be safe, and I
don't see any other way to get its value.

ajax:
What is XReadDisplay() ?

aiui, it allows you to read back the colors actually displayed on the screen,
regardless of the visual used in each drawable, (possibly) including overlays.

i don't know of an open source implementation, or even if the spec is published
anywhere; google isn't being particularly helpful.  SGI ships it with their
XFree86-based servers though, and will ship it against Xorg too.

any open source implementation would need these symbols too, since the cursor
could be part of the displayed image.

(In reply to comment #3)
> aiui, it allows you to read back the colors actually displayed on the screen,
> regardless of the visual used in each drawable, (possibly) including overlays.
> 
> i don't know of an open source implementation, or even if the spec is published
> anywhere; google isn't being particularly helpful.  SGI ships it with their
> XFree86-based servers though, and will ship it against Xorg too.

Any idea how this is different from the |XReadScreen()| function in Solaris (see
http://docs.sun.com/app/docs/doc/802-2011/6i60mct66?a=view) ?

yes, XReadDisplay() is a superset of XReadScreen():

extern XImage *XReadDisplay (
#if NeedFunctionPrototypes
        Display *       /* dpy */,
        Window          /* src */,
        int             /* x */,
        int             /* y */,
        unsigned int    /* w */,
        unsigned int    /* h */,
        unsigned long   /* hints */,
        unsigned long * /* hints_return */
#endif
);

where hints, in addition to whether to include the cursor, also allows for alpha
channel readback and selection of arbitrary sets of individual overlay/underlay
planes.

applied, closing.

(In reply to comment #6)
> applied, closing.

Is there a bug yet to add that API to Xlib ?

no.  nor will there be, the existing ABI puts the ReadDisplay functions in
libXSGIext.so, not libX11.so.