[argyllcms] Re: profile: black generation

  • From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 08 Jan 2006 16:53:33 +0100

Hello :)

Graeme Gill wrote:
Klaus Karcher wrote:

[...] one of my favorite gamut mapping test images (the hull of a RGB Cube)

I've actually just written a program to generate such test images, so that I could play with the resolution and pixel depth, as well as generate non-surface versions of the chart. It will be in the next release ...

Great :-)

I noticed that although the composite view of the
result looks smoothly and coherently, viewing each canal separately shows a undulating surface.
[...]
The only way to avoid the problem seems to set black generation to zero or max -- increasing the quality parameter only reduces the wavelength of the oscillation and using profile's new "-r" parameter or varying the "smth" variable in gammap.c seems to have almost no influence on the issue.

This is consistent with it being related to the limited resolution of the CLUT grid (although I'm not sure which grid, the forward, the gamut mapping, or the reverse one. It might even be a "beat" between the two of the grids. It would take a little work changing each grid resolution independently to pin this down)

The source profile was matrix/shaper based.

Were you using device links with icclink -G ? (I would guess not).

You caught me red-handed ;-) ATM my aim is to make profiles to be used straightforward by "ordinary users" (usually in Photoshop).

You will notice that the result using this path is a lot smoother
than using the B2A table of a profile.

Indeed I've noticed that: using your device links, the result is almost unsurpassable :-)

This doesn't explain anything, but it narrows the phenomenon
down to being to do particularly with the nature and resolution
of the B2A table in the destination profile. Some trials
indicates that the resolution of the "bumps" is related
directly to the resolution of the B2A table, and nothing else.

I've added two new examples to http://digitalproof.info/argyll/k-test/ -- one with "icclink -g" and one with "icclink -G"

IMO the new examples seem to strengthen your presumption of a "beat"
between the grids.

[...] Notice that bumpiness is only revealed easily when you examine
the K separate from the CMY, the CMYK combination is quite close to
the target color. I think it would take a detailed analysis of what's
going on along a particular locus down the the surface to point to
an answer to this. I suspect it is desirable to reduce this artefact,
because it will then make the separation more robust, in the face of
device behaviour change, or profile inacuracy.

I think this is of particular importance for press profiles.

Klaus




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