Elena [service address] wrote: Hi,
I saw there're many research papers around on this topic. For example: http://ee.washington.edu/research/guptalab/publications/2009-CIC-Smooth-LUTsWithAnimation.pdf
Hmm. I don't think this article makes any reference to the topic at hand. It instead seems to be dealing with setting up the A2B table. The approach has many similarities to what Argyll already uses for forward profile creation, the regular spline, which is based on much earlier research articles.
http://cilab.knu.ac.kr/seminar/Seminar/2009/20090516%20Accuracy-Preserving%20Smoothing%20Of%20Color%20Transformation%20LUTs.pdf
Interesting work, but it is in the area of post smoothing the lookup table. The problem at hand (I think) is not colorimetric smoothness of the table, but the lack of smoothness of the separation (K channel). This is likely to be a result of topological inconsistencies due to the extra degrees of freedom when there are more than 3 colorants. I've not often come across any articles that touch on this. I suspect that this is either because few people are aware of the issue, or that those who are aware of it regard any solutions as proprietary information. A lot of academic articles (ie. student papers) fall into the first category. CMYK is seen as a messy detail, and it's only people dealing directly with devices that are faced with actually coping with such issues !
This is how I see the problem, definitely. Assume the simple case, when I for testing started with -kx and I looked at what happened. The K channel is turning down at about 90% density, an that's is fine, since I understood why and that made a sense at the end. The problem is, I shouldn't being noticing it visually as a bluish bump or other irregularities. If I notice it, I assume that colprof has not enough sample points (patches) around that zone to work out a reliable result, i.e to resolve this even abrupt transition in a way which is colorimetrically acceptable and without high errors (to the eye, at least). At the end, I could well be WANTING such a K curve, and I assume that if colprof had enough patches in that zone, it could make out an abrupt transition which is however visually precise and without visible discontinuities. I just can't figure out how to accomplish that in a simple fashion. I would have, perhaps, to add more patches in that zone with xicclu, but it would require a dos script at least and I can't play with scripts.
The level of accuracy of the forward (A2B) characterisation of the device is quite important of course, but it comes down to judging where the color errors are being introduced. "Bumpiness" in the darker areas, particularly near the gamut edges, with a visibility that is heavily influenced by the B2A grid resolution and black generation curve level, are a hint that there are black topology problems. Currently, grid point accuracy and maximum gamut size have priority over the black curve, and it is only the black curve smoothness that results in B2A table smoothness of K value. So nothing prevents sudden transitions in the black level at grid points at the gamut edge, and the resulting device value interpolations of the B2A table can reveal that the device value interpolated color is very far from the color of the two grid points that are being interpolated between. Currently the best that can be done is to choose a black generation curve that minimises sudden transitions at the gamut edges. Ink limits (total and black) may also have an influence.
improvement in handling that area, i.e less noticeable visual discontinuity. Unfortunately that was not the case, or if so, the improving is very very little.
Yes. That is a way of proving that the problem must lie elsewhere !
I hope I could explain my concern with my poor English (I'm Italian).
Your English is very good, and infinitely better than any attempt of mine at Italian !
I never tried the matte black since I heard it actually contains some dyes and may not be durable as pigments.
I'm not sure what the difference is, since I have not investigated such details. Certainly on the inkjet I have (an Epson R1800), the claim is that all the inks are long lasting.
sometimes else the opposite happens: the final profile has curves smoother than predicted. But the issue I see is always the same: the problem is not so much how "good" the K curve looks, rather what the visual result is. If, let's say, the transition in the source space from red to black leads to one graysh bump around 80%, how is it that colprof doesn't notice it and try to fix it ? Perhaps those bumps actually originates at interpolation level, i.e between actual B2A grid points ?
Yes. K smoothness issues result in poor color accuracy between B2A grid points, where it is the device values that are being interpolated between. Graeme Gill.