Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!wind!naughton
From: naughton%wind@Sun.COM (Patrick Naughton)
Newsgroups: comp.graphics
Subject: Re: Sun pixrect rops
Summary: Bits or Bytes?
Keywords: ROP
Message-ID: <57237@sun.uucp>
Date: 20 Jun 88 17:58:25 GMT
References: <11115@agate.BERKELEY.EDU>
Sender: news@sun.uucp
Reply-To: naughton@Sun.COM (Patrick Naughton)
Followup-To: comp.graphics
Distribution: na
Organization: Sun Microsystems, Mountain View
Lines: 59

Paul Chernoff (chernoff@cartan.berkeley.edu) wrote:

>
> 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).

and Chuck Brunow (loci@csccat.uucp) replied:

>	Probably because a raster is "horizontally" oriented in RAM.
>	Writeing 500 pixels across means memory in sequence, whereas
>	vertical writes must do a mapping computation for each line.
>
>	At least that's the way that other machines work (unix-pc,
>	various ibm graphics. etc.) but I'm not sure about Sun's.

 While it is true that the address calculation for the next scanline
takes longer than the calculation of the next byte,
(y += bytes_per_scanline vs.  y++) the more significant reason for the
speed difference between a "flip horizontal" and a "flip vertical" is
the bit operations, (rotate and shift) that must be performed on the
horizontal version. 

 To flip vertically, all operations are on byte (or other machine word)
sized objects.  These operations are "cheap" as they move 8 (or more)
pixels to the desired position in one instruction. 

 To flip horizontally, the bits in the byte (or other word) must be
reversed.  On most architectures, this requires more than one
instruction, and in fact, usually a loop of n, where n is the machine
word size. 

 It is important to note that this is only an issue with bit-mapped
displays.  That is, each bit in a given display byte represents a pixel,
which will be light or dark depending on its value, (1 or 0).  If you
are dealing with a color display such as a Sun CG2, where each pixel is
represented by a byte, which is in turn a pointer to a 256 entry RGB
color LUT, there will be no performance difference between horizontal
and vertical flips since no bit operations are required.


  Patrick J. Naughton				ARPA: naughton@Sun.COM
  Window Systems Group				UUCP: ...!sun!naughton
  Sun Microsystems, Inc.			AT&T: 415/336-2979
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