VNC request

Kenneth Albanowski kjahds "at" kjahds.com
Sat, 17 Oct 1998 21:04:03 +0000


On Thu, 15 Oct 1998, kandalva wrote:

> 
> 
> >I expect this will not help, for two reasons. First, the VNC protocol has
> >no ability to transmit pixels in less then a byte, so 8-bits per pixel is
> >the minimum. (If I'm wrong, I'd like to hear about it.) 
> 
> It may be realized by packing 8 B/W pixels into 1 byte(8 bit) and send
> as 8 bit mode,and client may extract it and draw B/W graphics on screen.

Yes, of course, if the protocol is changed, it would be easy to support
sub-byte packings.

> >However, you should be able to use this already, though without most of
> >the speed gain: just go in to the Mac's Screen control panel, and switch
> >the display to 1-bit B&W. Assuming the VNC server understands this (I'm
> >not sure), you'll get mostly undithered B&W, which should compress quite
> >well.
> 
> Nops,recent Macintosh have no B&W color mode. ;-) 

Pity!

> I think PC server can perform B&W operation.
>      1.First make two offscreen,one is the same depth of screen,the other
>           is the B&W
>      2.When the screen update occurred,convert only updated region to B&W
>           with dither algorism
>      3.Pack these B&W bits into 8 bit packet and send it.
> 
> The algorithm of conversion any depth color mode to B&W mode  with dither
> is popular,so it's equipment is not so tough.

The problem is that this is inefficient: sending dithered B&W across the
connection may end up compresing worse then a paletted or true-color mode,
and dithering in this fashion could make the screen very difficult to read
depending on the colors and dither patterns.

If the compression is actually worse (I've not tested this), then it makes
more sense to have the client do the dithering. But that doesn't remove
the problem that dithering down a color screen to B&W will produce worse
results then starting with a B&W screen in the first place. It may still
be possible to produce this on the Mac. 

> Actually the source code that sort of argorhism is included in XV
> (unix graphic application),NIH Image(Macintosh application) and 
> these applications make their surce code in public.

There's not just a single dithering algorithm: there are many, and many
ways to use all of them. None are always perfect.

-- 
Kenneth Albanowski (kjahds "at" kjahds.com, CIS: 70705,126)



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