Having recently had to perform some fairly intensive comparisons of
Terminal Server 2000 (which is either the same as or very close to the
Citrix ICA protocol/system) and VNC, I have discovered the following:
1. TS2000 is *extremely* fast. We can use it to remotely control video
cameras and see the video feed in almost real-time (slight tearing) on a
10Mbps ethernet.
2. TS2000 is 8bit palettized *ONLY*. It will not support 16bit or higher.
3. When VNC is also limited (at the SERVER side) to 8bit, it becomes very
fast as well.
4. Checking the "Restrict to 8 bit depth" on the VIEWER will not result in
as much of a speedup as changing the SERVER depth to 8bit. It is still
useful over slow links, but it increases the server CPU load and thus slows
down overall frame rate over a good link.
5. TS2000/Citrix have the advantage of working much more like the Xvnc
server. They generate a virtual screen that does not exist anywhere except
in memory and on the remote terminal. The server is immediately notified
of all graphics operations, and (as Wez mentioned) it can simply forward
the calls directly to the viewer which can perform the operation there.
For instance, text can be transmitted as a context number and the string in
TS2000. Cross platform transmission requires transmitting the actual pixel
values, which takes quite a bit more (although Tight Encoding can help
quite a bit over a slow link -- not as useful over a fast link)
6. WinVNC serving an 8bit screen appears to have some palette handling
oddities if the hardware palette is being changed extremely rapidly (video
camera, anim playback, etc). This may be a misconfiguration on my part.
Overall, if you set your server to 8bit (which is the only valid way to
compare), WinVNC can perform at only around 3-4 times slower than TS2000
for graphically intensive work, and at almost the same speed for
conventional application work. This is fairly impressive for a free,
no-licensing, cross platform system. Slower links will benefit from Tight
Encoding and maintaining the newest TridiaVNC server/viewer. Also, if you
have a fast connection, Hextile encoding appears to be the fastest option.
I can't give any direct suggestions yet about when to switch to Tight
Encoding and jacking up the compression levels.
BTW, if anyone has a suggestion to avoid the palette mangling on VNC, I'd
like to hear it. Seems like there was a palette problem on an X based CAD
program that changed the palette a lot. Seems like a palette change
triggers a screen update because thats the only way to guarantee the colors
are all right. Really fast palette changes cause the palette to change
faster than the screen updates can be sent, which is probably the source of
the problem.
Hope that others can benefit from my results,
Mac
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