Changing sign bit of float number

Posted on 2004-11-18
Medium Priority
Last Modified: 2012-06-22
I have the following code in Win32 application:

float* p = pReferenceImage;

for ( int j = 0; j < nImageSize; j++ )
    if ( *p < 0 )
      *p = -*p;

It sets absolute value for each member of float array. The following code makes the same and seems to give right results:

float* p = pReferenceImage;
DWORD* pDW = (DWORD*) p;

for ( int j = 0; j < nImageSize; j++ )
  *pDW++ &= 0x7FFFFFFF;

Is this 100% right - setting 0 to bit 31 of float makes this number positive? I didn't understand this from float format specification.
Question by:AlexFM
  • 2

Expert Comment

ID: 12613082
Hi AlexFM,

Quoting this page [http://docs.sun.com/source/806-3567/data.represent.html]

      32 bits (four bytes or one word), aligned on a four-byte boundary. A float has a sign bit, 8-bit exponent, and 23-bit fraction.

So yes, it would appear that bit 31 is the sign bit and that clearing it indicates a positive value.

The only possible exception I can see is 0xFFBFFFFF, seeing as 0x7FBFFFFF is defined as NAN.

LVL 16

Accepted Solution

PaulCaswell earned 2000 total points
ID: 12613218
Even if it is 100% correct for that particular implementation you are making yourself implementation specific, compiler specific and all sorts of other stuff that you probably dont want.

Have you tried using fabs?

LVL 48

Author Comment

ID: 12613560
Using fabs is a good idea, it performs like second version (with DWORD), and better than first version. However, I can get better performance using 0x7FFFFFFF way with MMX, this is a reason I ask whether this is correct. Portability, compiler and platform independency are not important for me in this case.
I know that changing of sign bit in integer is wrong, and I am not sure about float. My tests show that this formula gives right results, but I cannot test all possible float values :-)
LVL 48

Author Comment

ID: 12613887
Well, I tested DWORD + MMX version, and it gives only small performance boost relatively to fabs. So, using fabs is optimal.

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