[argyllcms] Re: Ideal camera target
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 03 Feb 2010 15:21:20 +1100
Ben Goren wrote:
So, based on the recent (and past) threads about camera profiles,
I'm now seriously thinking about building my own ``ultimate''
target. What I've come up with so far:
* A white ``patch'' that's essentially a piece of paper
backlit by a $15 slave flash that will establish the white
point (should be R=G=B=255 in the native camera output).
I think you need to think that though a bit more. Since you are
attempting to characterize the camera, why should 255,255,255 be
some sort of anchor point ? What XYZ does it correspond to ?
Looking at a flash is highly likely to add to glare, particularly
from anything surrounding the chart or from elements within the lens.
Some sensors might behave very badly (streaking etc.), and some may
even be damaged.
* A selection of hand-applied artist's pigments (perhaps
pastels?) that I'll select and apply with the help of a
local art instructor; the selection will include colors
spectrally similar to those of a ColorChecker plus any
that the instructor considers difficult or especially
important to reproduce. If I can find any pigments that
are close to monochromatic primaries, I'll be sure to
include those.
The problems with this are mainly:
Uniformity - if it isn't uniform, you may get large
discrepancies between instrument measurements and
camera measurements.
Stability - it not very useful if you have to keep
creating reference files because the pigments change
too rapidly with time.
Statistical representation - implicit in creating a calibration,
particularly a matrix, is that the test chart spectra
represent similar statistics to the colors of images in the real
world. If they don't, then the profile's fit compromise will
be tilted in a way that isn't optimal for real world images.
ie., to do this well requires some work.
A possible cheat is to use paint swatch samples, since they
use a variety of pigments and are reasonable uniform. There
may still be problems with stability, and there will certainly
be problems covering a reasonable gamut, since popular
pain colors are typically rather muted.
* If I can't get a spectrally-flat gray out of the
iPF8100, then either a pigment or cut-up gray card or
something-or-other to use as a gray balance reference.
Something carefully selected is needed for spectrally flat neutrals.
Pure K print on a good substrate may be OK, but it depends on
the black ink involved. CMY etc. is not going to be flat enough.
* And a glossy black something-or-other (piece of plastic,
or maybe printed black with a glossy clear coat applied)
to give an indication if glare is a problem.
I'm not sure how that will help. It may simply add glare. The
black trap may give a better indication.
First, does that sound useful / sane?
It's a lot of work to do right. Buying a ColorChecker SG
is sounding faster and cheaper :-)
Building a black trap, obtaining a good white reference,
and establishing uniform illumination seem worthwhile.
Graeme Gill.
Other related posts:
