I've noticed that vertical scrolling with Firefox and many other
applications are quite slow, especially when you have a limited bandwidth.
The reason for this is that Xvnc fails to detect the framebuffer update as
a copy, which can be sent as a CopyRect RFB operation. Instead, the entire
screen is resent.
The code in question is here (function vncHooksCopyArea):
http://tigervnc.svn.sourceforge.net/viewvc/tigervnc/trunk/unix/xserver/hw/vnc/vncHooks.cc?view=markup
We have:
REGION_INTERSECT(pScreen, src.reg, src.reg, &((WindowPtr)pSrc)->clipList);
However, when scrolling in Firefox, the clipList region have x1 == x2 and
y1 == y2. (We have a data pointer, but size and numRects are zero.) The
intersection result is empty, and no CopyRects are sent. The problem can
be solved with this patch:
- REGION_INTERSECT(pScreen, src.reg, src.reg, &((WindowPtr)pSrc)->clipList);
+ if (REGION_NOTEMPTY(pScreen, &((WindowPtr)pSrc)->clipList)) {
+ REGION_INTERSECT(pScreen, src.reg, src.reg,
&((WindowPtr)pSrc)->clipList);
+ }
But why is clipList so strange? I've tracked it down to
pixman_set_extents:
if (!region->data)
return;
if (!region->data->size)
{
region->extents.x2 = region->extents.x1;
region->extents.y2 = region->extents.y1;
return;
}
This code is called every now and then. (In our case, it seems to be an
INTERSECT call which calls pixman_set_extents.) Apparently, the idea seems
to be that if the region has a data pointer, then it must have a size as
well, or it should be considered empty/NIL. Then, the question is, what on
earth is setting clipList to such a broken region? It cannot be
miRegionEmpty/REGION_EMPTY because those are directly setting x1 == x2 and
y1 == y2.
Using pixman 0.20.0 and xorg-server 1.5.3. However, the problem seems to
be present even with the Fedora 14 version of Xvnc, which uses more modern
versions. Comments appreciated...
Rgds,
---
Peter Åstrand ThinLinc Chief Developer
Cendio AB http://www.cendio.com
Wallenbergs gata 4
583 30 Linköping Phone: +46-13-21 46 00
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