Wayland - Beyond X (The H)

Posted Feb 14, 2012 2:59 UTC (Tue) by aliguori (subscriber, #30636)
In reply to: Wayland - Beyond X (The H) by nix
Parent article: Wayland - Beyond X (The H)

VNC only sends updates (1) to the dirty region of the framebuffer (2) at the request of the client.

As it turns out, in real life, the framebuffer changes constantly so the use of a refresh rate for something like VNC is to batch updates. But only dirty portions of the screen change.

Where VNC lacks compared to modern remote display protocols is things like offscreen image buffers and z-ordering.

to post comments

Wayland - Beyond X (The H)

Posted Feb 14, 2012 11:19 UTC (Tue) by nix (subscriber, #2304) [Link] (3 responses)

The framebuffer changes constantly, but when textual output is being displayed it normally changes to display the same bits of stuff over and over again (or the same bits of stuff over and over again, alpha-composited onto a constant background or a constant background pixmap). If the remoting model used by VNC represented this properly, it would be very low-bandwidth indeed. I hope the remoting model used by Wayland, when it eventually gets one, can do the same.

(Yeah, this won't handle the hard cases where people are playing any game, at least any game by Jeff Minter, or where people are displaying text in dancing fonts atop animated shaded backgrounds, but text display is a lot of what X, or any windowing system, is *for*. Forget xterms -- think IRC clients or web browsers displaying any textual article. GlyphSets *rule* for bandwidth reduction: no content-unaware compression algorithm could get remotely the same size reduction for the same low CPU use. Discarding that would be terrible.

Given that the inventor of GlyphSets is involved in Wayland I suspect that it will not be forgotten.)

Wayland - Beyond X (The H)

Posted Feb 14, 2012 21:53 UTC (Tue) by aliguori (subscriber, #30636) [Link] (2 responses)

>The framebuffer changes constantly, but when textual output is being displayed it normally changes to display the same bits of stuff over and over again (or the same bits of stuff over and over again, alpha-composited onto a constant background or a constant background pixmap). If the remoting model used by VNC represented this properly, it would be very low-bandwidth indeed.

It doesn't. All modern tool kits do their own font rendering (this is what Pango provides).

Wayland - Beyond X (The H)

Posted Feb 14, 2012 22:59 UTC (Tue) by nix (subscriber, #2304) [Link] (1 responses)

Yeah. They do their own font rendering, rendering to glyphs which are then sent to X as a GlyphSet, right?

Wayland - Beyond X (The H)

Posted Feb 16, 2012 0:05 UTC (Thu) by aliguori (subscriber, #30636) [Link]

Oh, I was not aware of the GlyphSets extension. VNC only has a very simple bitmap based protocol that can provide updates to the bitmap at a rectangle granularity.

It has no concept of offscreen bitmaps, alpha blending, or anything like that.

Wayland - Beyond X (The H)

Posted Feb 14, 2012 11:40 UTC (Tue) by Cato (guest, #7643) [Link]

There is a way to do offscreen image buffers with VNC, though it's a bit of a hack. You x11vnc's client-side caching feature, which allocates a huge long screen buffer, and tell the client that the screen is less tall than it appears. See http://en.wikipedia.org/wiki/X11vnc#Client-side_caching

X11VNC lets you VNC to the main X screen as well, while SSVNC is a handy Linux client that does the tunnelling for you.

I find that UltraVNC on Windows works fine as a client to x11vnc, and TightVNC viewer on Linux is OK too.

iSSH on iPhone almost works but is currently rather crashy for VNC. If anyone else can recommend solid VNC-over-SSH clients for iOS I'd really like to know them.