I have already noted the limitations of gamma and L*a*b encodings as documented below, and you are quoting me out of context in an attempt to prove your point. The same limitation applies to a log encoding, but then zero luminance rarely occurs in practical photographic situations and the minimum value possible in a log encoding is sufficiently close to zero for practical use. Log encodings are successfully used for HDR along with floating point. Did you take the trouble to read the article by Greg Ward?

Regards,

Bill

I'm not uncomfortable with log scaling. It's the basis for floating point representation in essentially all computer architectures. And it does provide consistent, relative error performance. I just don't believe it is necessary in photography or printing.

You were the one that pointed out that the slope of gamma curves becomes infinite at 0 as if that was significant. I was just noting that log scales have no value, let alone slope, at 0. At some point you have to truncate (or clip) a log scale. Neither of these facts impairs the ability of either to function. It would be interesting to construct an analysis of log scale (say, using 16 bit floats) v 16 bit, gamma 4 over possible HDR ranges. I think both would serve quite well.

And yes, gamma curves do not have a constant relative error per step change. Certainly 8 bit discrete, gamma encoding is not going to work for HDR work. Either in synthetic images or real image captures. But 16 bits does. With or without a linear front end ramp though I do not like linear front end ramps on a gamma scale as a simple scale factor change can alter colors and that is not the case with pure gamma encoding.

Constant relative error is a useful property but it does not represent actual light physics. Shot noise magnitude, for instance, tracks the square root of luminance thus intrinsically will not produce constant relative error. Multi exposure HDR techniques can produce more constant relative error which allows wider adjustments in post. Still, it would be a rare 16 bit gamma encoded image where even an HDR image could not be encoded with errors below the physical shot noise limit. Unlike L* or sRGB scales, which are mixed linear/gammas, both log and gamma scales provide easy luminance scaling without shifting color. Gamma scales by simply multiplying RGBs by a factor while log scales accomplish the same thing by adding a factor.