Newsgroups: comp.sys.amiga.programmer
Path: utzoo!utgpu!watserv1!watdragon!rose.waterloo.edu!ccplumb
From: ccplumb@rose.waterloo.edu (Colin Plumb)
Subject: Re: best way to reverse a bitplane?
Message-ID: <1991Apr29.202206.240@watdragon.waterloo.edu>
Sender: news@watdragon.waterloo.edu (News Owner)
Organization: University of Waterloo
References: <1991Apr29.101302.1@atmo2.atmo.arizona.edu>
Date: Mon, 29 Apr 1991 20:22:06 GMT
Lines: 30

leuthold@atmo2.atmo.arizona.edu wrote:
>What is the best way to reverse a bitplane?  A brute force method would be 
>to reverse the bytes and then reverse the bits in each byte.  The only way I
>can think of to reverse the bits in each byte is to mask each bit off then
>shift it to its new place and add it to the new byte.  Is there a better way?

Yes, table look-up.  A 256 byte table wouldn't take up much more room than
your bit-reversing code, and would be a lot faster.

Oh, and the best way to reverse a bitplane is to swap bytes in from the 
ends.  In C,

UBYTE *p1, *p2;
UBYTE c1, c2;
extern UBYTE table[256];

p1 = <starting address of row>;
p2 = p1 + <bytes per row>;	/* Note: assumed even, and >0 */
do {
	c1 = *p1;
	c2 = *--p2;
	*p1++ = table[c2];
	*p2 = table[c1];
} while (p1 != p2);

Translating this into assembler is easy.  Just keep the high parts
of the registers holding c1 and c2 clear, and use the indexed addressing
mode.
-- 
	-Colin