[argyllcms] Re: Camera matrix profile, adding ti3 perfect white data set

  • From: Elle Stone <l.elle.stone@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 29 Jan 2010 16:44:44 -0500

On 1/28/10, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:

> I also notice that there is a bug in V1.1.0 for matrix input
> profiles where the correct white point value is not being written
> (this bug was introduced in adding support for the XYZ cLUT
> display + matrix profile type).
>
> Graeme Gill.
>

Could someone please explain what "media white point" means when the
thing being profiled is a digital camera?

I did notice in 1.1.0 that the media white point for a matrix profile
is set to D50, rather than being based on readings from the brightest
patch - normally GS00 in an it8 target, as was the case in V1.0.4 (I
think the brightest patch is what determined the media white point -
yes?). Are you saying this a "bug"? I thought it was a feature. Or are
you talking about something else altogether?

For a digital camera, is "media white point" the color of the light
hitting the target? In which case perhaps the media white point
shouldn't be based on GS00 but rather on the most neutral patch on the
target, which for my particular it8 chart is GS06, assuming my
batch-averaged chart doesn't have a gross error for GS06 which it
doesn't seem to.  In fact I'd take a guess that the neutral patch with
the highest error on my chart is actually GS00 - the L* part always
comes out way off in profcheck and the patch itself is far from being
really neutral.

Using V1.0.4 I did an experiment, removing all data sets from the .ti3
file that had values higher than the RGB values for GS06, and used the
resulting profile's media white point to set an artificial white point
for the full data set to make a "real" profile.

But as the media white point in V1.1.0 was D50 rather than a value
reflecting GS00, I assumed I didn't have a clue what I was doing in
V1.0.4, which is still probably true, even if the change in media
white point really is a bug not a feature. So a mini-education on
"what is media white point for a digital camera" would be most
welcome!

Also, the black point for a matrix profile in V1.1.0 is zero (with or
without an artificial black point added to the data set), rather than
being based on the darkest patch on the it8 target as it was in
V1.0.4. I had already decided to get rid of the non-zero black point
in my camera profile, simply to prevent software from doing unwanted
black point compensation (I'd rather do my own). So is this new zero
black point in V1.1.0 a feature?

Elle

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