[argyllcms] Re: Homebrew ColorChecker
- From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sat, 13 Feb 2010 10:59:18 -0700
On 2010 Feb 13, at 7:38 AM, Pascal de Bruijn wrote:
> Even while being advised against it, I still went ahead and
> tried to make my own ColorChecker:
Since you're printing this on an inkjet, there's no advantage to
using the same colors as on the MacBeth ColorChecker. You might as
well use targen to create your own set of patches -- and probably
a (much) larger set, to boot.
I've been absolutely slammed the past week, and this coming week
is looking to be no better. But I did get a chance to snag a
bunch of paint chips from HomeDepot and I've compared a few
measurements against a ColorChecker. Perhaps not too surprisingly
in retrospect, chips with a similar visual appearance have a very
similar spectral response. I'd be willing to bet a beer that the
automated color matching system at paint stores these days should
be able to come up with at least a 90% match, if not actually
within manufacturer tolerances.
What's more, there are some chips that have a purer visual
appearance than the corresponding ColorChecker matches, and
``cleaner'' spectra to boot. The process yellow is one example;
the paint chip had a lower high-frequency response, a higher
low-frequency response, a sharper transition, and everything was
much flatter (fewer small bumps and wavy lines in the graph).
And so I've come to the tentative conclusion that there's nothing
especially magical about the ColorChecker, either in pigment
composition or color choice, for the purposes of profiling a
digital camera. Sure, the colors are designed to be representative
of typical scenes, but the choices were made looooong before the
days of digital spectral colorimetric imaging.
I'm starting to work towards the following for my own homebrew
chart:
* The BabelColor White-Balance Target arrived in the mail
the other day. It'll be on the chart.
* I've already mentioned that I'll be making a simple
black trap that I'll be including.
* Once I get a chance to inhale, I'll be getting advice
from a local art instructor on what media / pigments to
choose. Aside from practical factors such as durability
and (lack of) specular reflections, I'll be looking for
the following:
* Ideally, at least one pigment each with a single
tight spectral peak for each named color of the
rainbow. I realize this might not be realistic.
* One pigment each for a reasonable spectral
match for the 18 colored squares on the
ColorChecker; this is to match the stated aim
of including ``real-world'' colors. (Actually,
there're patches on the ColorChecker with very
similar spectral responses to each other, so
I'll probably reduce the 18 to a subset of just
a dozen or so.)
* One each of every pigment she's had troubles
with when doing art reproduction in the past.
* I haven't had a chance to test it yet, but, at
least in theory, I should be able to get
neutrals that are flat enough for these purposes
out of my Canon iPF8100. If not, I'll work with
the artist on that, as well.
* Once I've got the pigmented and neutral patches decided
upon, I'll count 'em up and subtract that from however
many patches I think I can fit on a reasonably-sized
target. I'll feed that number to targen. (Tentative
guess: a couple dozen pigments, a dozen neutrals, a few
dozen from targen.)
* The whole thing gets laid out in either Photoshop or
Illustrator, and then printed (with lots of empty
spaces) on Canon Watercolor Paper on the iPF8100. I cut
out holes for the white target and the black trap, apply
the pigments according to the artist's instructions,
mount it, and the rest of the usual stuff.
* Then, of course, comes measurements and tests....
Of course, I'll post all details of whatever I come up with,
including exactly what pigments I used and their measured spectra.
Anyway, if I were going to create a homebrew ColorChecker, I'd
take a real ColorChecker to the paint store. If doing it on the
cheap, I'd just get paint chips that seemed like good candidates
for matches, and measure them at home with the i1 to pick the
winners. Then, just use scissors and tape to assemble them into a
chart. If I had money to burn (and, for whatever perverse reason,
didn't want to spend it on a real ColorChecker), I'd hand the
ColorChecker to the person behind the counter and ask for a pint
of each color in flat exterior paint. (It might be a good idea to
compare the spectra of the various base white paints first....)
You know, I just remembered: I have a not-too-terribly-distant
uncle who's worked as a chemist for a paint company. I'll have to
get in touch with him, too....
Cheers,
b&
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