Path: utzoo!attcan!uunet!lll-winken!ames!oliveb!apple!bloom-beacon!EXPO.LCS.MIT.EDU!jim
From: jim@EXPO.LCS.MIT.EDU
Newsgroups: comp.windows.x
Subject: Re: Contents of windows
Message-ID: <8905051439.AA06274@expo.lcs.mit.edu>
Date: 5 May 89 14:39:57 GMT
References: <4841@umd5.umd.edu>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: X Consortium, MIT Laboratory for Computer Science
Lines: 25


>> A bit of backing store? Lets see, my default xterm window size is
>> 80x60 using an 8x13 font so that is 499200 pixels. On a machine with
>> 32 bits/pixel (yes, such machines exist and run X) thats 1,996,800
>> bytes or 1.95 megabytes.
> 
>         I do not know how xterm is handled but when you specify backing
> store, you also specify the number of planes to back. Given that xterm
> is monocrome, it uses 1 color plane. So a 80x60 xterm using an 8x13 font
> that is 499200 pixels and at 1 bit per pixel thats 62400 bytes.

Nope, the previous poster was correct.  You're mixing up the number of colors
(and you forgot one) with the number of planes it takes to display a color.
The VT window in xterm can have up to three separate pixel values in it:
foreground, background, and text cursor.  These colors are represented in the
frame buffer by arbitrary pixel values, so backing store would have to save
the planes representing the union of all three pixel values.  Since a fair
number of frame buffers are "Z-oriented" (they store all of the bits of a pixel
contiguously in memory) instead of "XY-oriented" (they store each plane
separately), trying to save a subset of planes isn't guaranteed to be any
cheaper than saving all of them.


							Jim Fulton
							MIT X Consortium