Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!decwrl!fernwood!shograf!jack
From: jack@shograf.com (Jack Ritter)
Newsgroups: comp.graphics
Subject: Re: Gamma
Summary: gamma correction for non linear phosphors
Message-ID: <1991Apr24.160214.911@shograf.com>
Date: 24 Apr 91 16:02:14 GMT
References: <1991Apr21.185211.29293@wpi.WPI.EDU>
Organization: SHOgraphics
Lines: 38


> 	I am writing a graphics program in C and one of the routines has to
> apply the gamma function to do some lightening.  The program is in C on an

If "gamma function" is to mean gamma correction, here's
code I use. GAMMA should be set to the tube's gamma
correction factor (usually around 2.5). The routine
converts a (linear) real from 0.0 -> 256.0 to 
a gamma-corrected int 0 -> 255.

#define GAMMA 2.5
int gammacorrect(intensity)
real intensity;
    {
    int ival;
    real dval;
    /* scale to 0-1 range */
    dval = intensity/255.0;
    if (dval > 1.0)
        dval = 1.0;
    if (dval<=0.0)
        dval = 0.0001; /* log(0) blows up */
    /* do gamma correction */
    dval = exp ( log(dval) / GAMMA );
    /* convert to integer range: 0-255 */
    dval *= 255.0;
    ival = (int) (dval+0.5);
    if (ival > 255) ival = 255;
    return(ival);
    }



-- 
"The traffic lights, they turn blue tomorrow"
     Jimi Hendrix

Jack Ritter