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

[Øyvind Kolås <pippin gimp org>]

On Thu, Aug 23, 2012 at 12:33 AM, Graeme Gill
> 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.
>
> 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.

Thank you both, I've pushed changes that should make babl adhere to
the current standard.

/Øyvind K.

commit
1cec819c3070c21d90eac9a7ab27717499e75239
Author: Øyvind Kolås <pippin gimp org>
Date:   Thu Aug 23 10:26:29 2012 +0200

    Adjust constants used for sRGB gamma

    Spotted by Elle Stone and clarified by Graeme Gill, now updated to
Current IEC
    specification values.


commit 2675787f341ce259f9abec3e326b5efece45ecbf
Author: Øyvind Kolås <pippin gimp org>
Date:   Thu Aug 23 10:30:32 2012 +0200

    tests: adjust float to 8bit regression test for new sRGB precision