Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!david
From: david@sun.uucp (David DiGiacomo)
Newsgroups: comp.graphics
Subject: Re: Sun pixrect rops
Message-ID: <57247@sun.uucp>
Date: 20 Jun 88 19:36:01 GMT
References: <11115@agate.BERKELEY.EDU>
Organization: Sun Microsystems, Inc. - Mtn View, CA
Lines: 21

In article <11115@agate.BERKELEY.EDU> chernoff@cartan.berkeley.edu (Paul
R. Chernoff) writes:
>I would like to know why the Sun pixrect operation
>for copying a "vertical" rectangle of a given size
>(say 1 by 500) takes much longer than the corresponding
>operation for copying a "horizontal" rectangle (say 500x1).

It's because successive horizontal pixels are part of the same memory word
and can be manipulated as a unit.  On monochrome frame buffers ("bw2") 32
horizontally adjacent pixels can be read or written with one memory access.
On 8 bit color frame buffers there are 4 ("cg4") or effectively 2, 4, or 16
("cg2" frame buffers with rasterop chips) pixels per memory word.

Reading a 500 pixel vertical stripe from a monochrome frame buffer
requires 500 memory accesses as opposed to only 16 accesses for a
horizontal stripe.  (Operating on a single bit within a word also requires
additional shift/mask effort, but the most significant factor is the
number of memory accesses.)

-- 
David DiGiacomo, Sun Microsystems, Mt. View, CA  sun!david david@sun.com