Re: pseudo transparency

[Sasha Vasko <sasha aftercode net>]

Owen Taylor wrote:

> Olivier Chapuis <olivier chapuis free fr> writes:
>
>
> > About the E method. Here there is a de facto standard which
> > uses the ESETROOT_PMAP_ID and the _XROOTPMAP_ID atoms. Maybe,
> > this can be documented in the wm-spec? If I well understand
> > this method, applications should use the _XROOTPMAP_ID and
> > setroot programs should destroy the pixmap if
> > 	ESETROOT_PMAP_ID == _XROOTPMAP_ID
> > before setting the background with its own pixmap (and
> > then set _XROOTPMAP_ID and also ESETROOT_PMAP_ID if its
> > own pixmap can be destroyed). That's it?
>
> A program that wants to set the background should:
>
>  - Create a persistant pixmap by calling to the server with:
>
>     dpy = XOpenDisplay(display_name);
>     XSetCloseDownDisplay (dpy, RetainPixmap);
>     pmap = XCreatePixmap (dpy, DefaultRootWindow (dpy), width, height,
>                            DefaultDepthOfScreen (DefaultScreenOfDisplay (dpy)));
>     XCloseDisplay (dpy)
>
>  - Grab the server
>
>  - See if ESETROOT_PMAP_ID is set
>  - If so, call XKillClient(xdisplay, *(Pixmap*)data_esetroot)


I'd like to see the result of that, if current root pixmap was created 
by window manager.

>  - Change ESETROOT_PMAP_ID and _XROOTPMAP_ID to point to the
>    new pixmap
>  - Set the pixmap as the background for the root window

>  - Ungrab the server
>  
> There is no point in checking to see if ESETROOT_PMAP_ID is
> equal to _XROOTPMAP_ID ... if ESETROOT_PMAP_ID is set, it
> will point to a pixmap that must be killed before changing
> ESETROOT_PMAP_ID.


Iportant problem here is that you leave shared resource unprotected and 
unmanaged. Its as if you allowed client apps to set current colormap as 
they like.

Colormaps are actually another part of this particular issue, since it 
would be desirous for clients to obtain the colormap/visual with which 
 such root pixmap was rendered.


> > Now one problem with the E method is that it may need
> > a lot of memory (this depends on the memory you have and
>
> It takes _no_ extra memory. The pixmap is already referenced
> by the server, all the E method does is keep around a 
> XID pointing to the pixmap.


Not exactly correct. When you do XSetWindowBackgroundPixmap, what server 
does is it creates a protected copy of the pixmap. X protocol does not 
have clear requirement for this, but I'm yet to find a server that does 
not do that. So you normally use twice as much memory as needed with 
above approach.

Now workaround for this is to not set root background, but instead track 
Expose events on root window and do XCopyArea on it or XFillRectangle.
This requires app to be present at all times, and breaks ParentRelative 
 approach.

And still, there is this issue of transfering data from the server and 
onto the client in order to tint/shade it - note that pure root 
background is almost useless.

Also, it is quite possile that no app wil actually need transparency and 
you'll be wasting all that memory for nothing.

Note that selection method allows for all kinds of scenarios, and it is 
not all that difficult to implement, and could be even easier and more 
error-proof then current way of tracking sanity of _XROOTPMAP_ID.


> Regards,
>                                         Owen

Sasha.