[argyllcms] Re: DIY ColorChecker chart ?
- From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 7 Aug 2015 11:45:23 -0700
On Aug 7, 2015, at 10:35 AM, Lars-Daniel Weber <Lars-Daniel.Weber@xxxxxx> wrote:
Wouldn't it be possible to print a target with large swatches, profile it and
use it when starting the shooting?
Yes and no.
The ColorChecker is made with a number of pigments with a variety of spectral
characteristics. Your printer might have all of four pigments, maybe six. (Even
eight-color printers typically don't have eight pigments; at least one or more
of the inks is a "watered down" version of a standard one to provide the
ability to print more delicate highlights.)
If you're going to print your own target, your best bet would be to use Argyll
to generate a set of patches for you. Even better is to use that plus patches
you paint on with quality artist paints (Golden Fluid Acrylics are superb for
this).
And if you're looking to create a copy of a ColorChecker...take one to your
local home paint store and have them match the paints. There's nothing special
about the pigments, save that you don't have them in your printer. Paint stores
have X-Rite spectrometers and software that are amply up to the task of coming
up with a spectral match for a ColorChecker. (With a caveat: some older pigment
palettes can't match the full ColorChecker gamut; go to the newest paint store
in town just to be safe.) If you can get half-pint sized sample paints from
them, the cost will be about as much as you'd pay for a ColorChecker...and
you'll be able to make many, many, many, many ColorCheckers with that much
paint.
A couple other points worth making.
First, I cannot highly enough recommend the ColorChecker Passport. You're not
going to find a better target for field use.
Next...you're not going to make a good color profile with a photograph made
outside of ideal studio conditions. It's just not possible, due to stray light
if nothing else.
But I still recommend the ColorChecker Passport. If you've already got a good
profile, you can use the Passport as a reference for normalizing exposure and
white balance -- essentially serving as a proxy for both a light meter and a
color meter. Explaining how is beyond the scope of an email such as this.
Fundamental to the problem of general-purpose profiles is that the scene
illuminant gets "baked into" the profile. In other words, a profile is only
valid for the light source used to photograph the target.
The way around that is to create a spectrally-based model of the camera's
sensor response. You can then feed it a measurement of any illuminant you like
and generate a profile based on the combination of camera and illuminant.
Determining the scene illuminant ideally requires a spectrometer, but you can
do more than "good enough" for daylight and incandescent by comparing the raw
RGB values in a photograph of a ColorChecker (or other reference) with those
predicted by the model for various illuminants.
With all that in mind...you might want to step back a moment and consider what
you really want to do with the ColorChecker in the first place...there may well
be a better solution entirely. (With one of the best being -- no joke -- a
disposable styrofoam coffee cup.)
Cheers,
b&
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