Re: [Gimp-developer] unusual (babl/babl/util.h) sRGB tone curve values

[Graeme Gill <graeme2 argyllcms com>]

Nicolas Robidoux wrote:
> I stand to be corrected on this, but I believe that there is more than
> one sRGB standard. I also vaguely remember reading that the formulas
> are slightly different for 8 and 16 bit sRGB, meaning that they don't
> exactly agree at the common (scaled) values.

Hi,
	it appears that the util.h values are from a draft of the
IEC sRGB standard, not the final one.

Elle Stone wrote:
> The standard values are: 0.04045 and 0.0031308.
> The util.h values are:   0.03928 and 0.0030402477.

Nicolas Robidoux wrote:
> Again from fading memory, I believe that the original formulas failed
> to be perfectly smooth, which was completely irrelevant in 8-bit but
> could be argued to be significant in 16-bit. I also would not be
> surprised if some 16-bit sRGBs have a slightly wider gamut.
> These tweaks would break the "alignment" of 8-bit sRGB with 16-bit sRGB.

Let's see:

Current IEC specification values:
0.04045 / 12.92               = 0.003130804954
((0.04045 + 0.055)/1.055)^2.4 = 0.003130807229
continuity error of 1 part in 1.3e6

Draft IEC sRGB & util.h values
0.03928 / 12.92               = 0.003040247678
((0.03928 + 0.055)/1.055)^2.4 = 0.003039492412
continuity error of 1 part in 4e3

So the util.h values are a lot less smooth than the sRGB standard values.

Graeme Gill