Conference bulova::decw_jan-89_to_nov-90

3278.0. "XSetClipMask problem ?" by XLIB::MARSON () Wed Aug 29 1990 13:58

I have an ISV who is having a problem with setting a clip mask.  Has anyone
experienced the problem described below?  The client has a DS5000 but I think
he is using an Xcfb from his 3100 which is running an older (pre-release)
version of ULTRIX. Could this be a problem as well.?

Thanks in advance,
Barry Marson


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Configuration: RISC/Ultrix 4.0.0 Rev. 174.
		OSF/Motif and X Windows Application
		X Windows server: Xcfb (DECstation 3100 and 5000/200CX)

Our application assumes (for historical reasons) that every display device
provides a 16-color transparent "overlay" window.  Under X Windows, you
can create this effect by creating a 1-bit-deep pixmap and assigning this
pixmap (with XSetClipMask) to the GC used for copying the overlay to the
display window.  The overlay pixmap is 8 bits deep (matching the depth of
the screen).

XCopyArea is used to copy the overlay pixmap to the drawing_area widget
(our "viewing window").  To create the overlay effect, the clip mask
pixmap has certain bits set to 1 and others set to 0.  XCopyArea will only
copy a pixel from the pixmap when the corresponding mask location is set
to 1; overlay pixels corresponding to mask locations set to 0 are not copied.

In theory, when the entire clip mask is set to 0, no pixels from the overlay
pixmap should be copied.  Not true with Xcfb.  It appears that a completely
zero clip mask is the same as not having a clip mask at all (in other words,
all values from the overlay pixmap are copied into the window, effectively
destroying the window's contents).

On a few other (non-DEC) servers, this problem also appears.  To get around
it, we've made the clip mask pixmap one line longer than the overlay pixmap.
We then set the extra line to all 1's (so at least something is set in the
clip mask).  This workaround does not work with Xcfb.

Help.

Sounds like a bug.  Submit a problem report on it.  I don't know the procedure
for filing QARs on ULTRIX DECwindows.

--PSW