Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!decwrl!spar!hunt
From: hunt@spar.SPAR.SLB.COM (Neil Hunt)
Newsgroups: comp.lang.c
Subject: Re: Shifting question
Message-ID: <1818@spar.SPAR.SLB.COM>
Date: 19 Jul 88 20:48:34 GMT
References: <705@bnr-rsc.UUCP> <11556@steinmetz.ge.com> <60290@sun.uucp>
Reply-To: hunt@spar.UUCP (Neil Hunt)
Organization: SPAR - Schlumberger Palo Alto Research
Lines: 51

In article <60290@sun.uucp> guy@gorodish.Sun.COM (Guy Harris) writes:
>	7.5 Shift operators
>	...
>	The result is undefined if the right operand is negative, or greater
>	than or equal to the length of the object in bits.
>ANSI C:
>	3.3.7 Bitwise shift operators
>	...If the value of the right operand is negative or is greater than or
>	equal to the width in bits of the promoted left operand, the behavior
>	is undefined.

This has been a pet peeve of mine for a long time: does the following function
work for applying a shift (left or right) to all pixels in a large image ?

shift_pixel(image, count)
struct image *image;
int count;
{
	int i, j;

	for(j = 0; j < image->rows; j++)
		for(i = 0; i < image->cols; i++)
			image->pixels[i][j] >>= count;
}

No, it doesn't; the last line has to be:

			if(count > 0)
				image->pixels[i][j] >>= count;
			else
				image->pixels[i][j] <<= -count;

because of the stupid undefined rule about shifts.

An alternative solution is

	rshift = count > 0 ? count : 0;
	lshift = count < 0 ? -count : 0;

	for(j = 0; j < image->rows; j++)
		for(i = 0; i < image->cols; i++)
			image->pixels[i][j] =
			  ((image->pixels[i][j] >> rshift) << lshift);

We are talking factors of two in execution time for these functions.
What a pain !!

Neil/.
-- 

	hunt@spar.slb.com
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