[argyllcms] Re: Determining proper error value for -r

Ben Goren wrote:

Anyway, it  occurred to  me that  the best way  to figure  out the
proper error value to use for the -r option of profile would be to
print out  a chart  with a  bunch of patches  of the  same colors,
measure it,  and then  figure out what  the standard  deviation is
(and convert  that into a  percentage). So, that's exactly  what I
did: I created  a 39-patch  chart, did  a copy /  paste in  a text
editor (with some editing) to make  eight copies of the patch set,
printed it, measured  it, imported the results  into an OpenOffice
spreadsheet, and fiddled around with it until I came up with about
a third  of one  percent as  the error--which is  in line  with my
eyeball guesses. (I'll be  doing some more testing  later today to
further confirm and explore all this.)


This is a pretty reasonable way to go about things. It's
tedious though, and tells you about the level of error at only one
point in the colorspace.

I'm also pretty sure that that value  would be much too low for my
parents' cheap  color laser printer,  and probably a bit  too high
for ``real'' paper  (I did this on plain paper  just to figure out
if it even made sense to  do in the first place). I'm also curious
to learn just how much of an impact patch size has on accuracy.


The most sensitive "real world" test I stumbled across, was
to simply make up the profile, and eyeball the gamut surface.
I found quite noticeable changes in the smoothness of the gamut
surface, as I varied the -r factor in profile. Too small,
and the surface was noticeably bumpy. As the number increased,
the surface got visibly smoother, and looked more like
one would expect for a well behaved device. The self fit
errors rise as the -r factor goes up too, so I stopped at
a suitable "knee" point.

Now, whether this is a good way of doing it, or the only way
of doing it, I'm not sure.

There'd be a new flag to  targen, similar to the existing -e flag,
that tells  how many times to  repeat /all/ patches, not  just the
white ones. When profile is fed  a chart with repeated patches, it
calculates the  appropriate value for  -r itself, uses  that, then
creates a  profile and  reports the recommended  value for  -r for
future use. You  can then  use the generated  profile with  the -c
option of targen or just toss it entirely.


I' not sure such a thing is worth doing, unless there is an indication
that such a calculated number is useful and meaningful. You've
already expressed doubt that the number you came up with for
your printer seems realistic. How does it compare with the number
you arrive at in getting a smooth looking gamut surface ?
Do the numbers agree ?

What sort of color was your test patch ? It would be interesting
to do the same test with a quite light, and quite dark color,
to see if the errors have a different magnitude.

cheers,
        Graeme Gill.


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