With colleagues, we have written a long journal paper about our CLUT compression technique
(the one that is implemented in G’MIC, with filters
Colors / Color Presets and
Colors / Film Simulation), which is an extended version of our previous research report: https://hal.archives-ouvertes.fr/hal-02066484. And we have submitted it to TOG (ACM Transactions on Graphics).
We received the reviews a few weeks ago, and the paper has been rejected. Some of the reviewers made very useful comments, and we already has worked on improving the paper with the suggestions they made (mainly considering other colorspaces than sRGB for the compression / decompression steps).
However, one of the reviewers was very negative, with comments that make me say that he is not impartial. I think he may be one of the authors of a “competing” paper that performs CLUT compression (a lossless compression technique, so with compression rates far much lower than those we obtain with our method).
I don’t have no clear evidences of this assertion, but a reviewer who tells that a bunch of papers from the same authors are missing in the bibliography is generally not good, particularly when there is already one paper from them already cited in the bibliography.
i’d really like to have your thoughts on some of the remarks he made. I’m not sure what he says is 100% correct. But I may be wrong. So your opinions are welcome.
"In general, I would like to summarize my thought that this paper is not really about compression, rather it focuses on the reconstruction of a color look-up table using a discrete set of points by iterative placement. I would encourage the authors to look at the use of ICC profiles which are an industry standard for communication of color transforms.
One of the key differences between this work and ICC profiles is that ICC profiles use a regularly gridded structure, whereas this work uses a sparse distribution of keypoints located to characterize the curvatures of the color space.
On line 164, the statement is made that “Usually a CLUT is stored either as an ASCII zipped file …, or as a PNG image”. This statement is not correct. Although the examples that you have shown are stored in that manor, in my 20+ years of experience, color tables are commonly stored using ICC profiles, or binary formats."
So my first question is : Have you ever seen an image retouching software proposing color-modification filters based on the use of ICC ? I was not aware of such software. In our paper, this is quite clear in the introduction that we are interested in the “artistic” side of color modifications.
I thought ICC profiles were mainly used for color corrections for devices (printers, scanners, monitors, etc.).
On line 172, the statement is made “typical to sizes 32^3, 48^3, 64^3”. As an engineer that has implemented many 3D CLUTs over my career, I would never use an even sampling of the input color space. When using an even number ( 16, 32, etc ) it means that the domain is sampled in non-integer increments. For example, if using a 16^3 CLUT, you only have 15 subdivisions, resulting in an increment of 17.066 per node spacing. This is not desirable, so it is common to use an odd number that is one greater than a power of two. For example, 17 or 33 such that the increment will be an integer value.
I would say that 17^3 is a ridiculously low resolution for most of the CLUTs. Do you think 17^3 is acceptable in any way for general CLUTs ? I’ve already processed CLUTs with a higher level of details (variations) in them. No way they can be subsampled accurately with a 17^3 resolution.
Also I’m not sure I agree with the fact that 2^n + 1 resolution is preferable. I don’t really see the point. What I know is that when using 2^n resolutions, you can often generate a 2D image by unrolling the values of the 3D CLUT, and indeed save it as a .png file, which is really convenient (the CLUT set from RawTherapee is provided as 1728x1728 .png files, I don’t think this is a wrong approach).
Maybe the guy has only designed CLUTs for printers or devices with limited memory ?
It is clear that the authors themselves are not up-to-date on the current standard used in the industry. ICC profiles have been around for over 20 years and provide a very efficient mechanism for describing color appearance transforms.
In general, I would suggest that the authors spend a significant amount of additional time learning about ICC profiles, color science and color tables. That will provide a solid foundation upon which to attempt to re-write this paper.
So here again, I’m asking : have you ever used software that are mainly based on the use of ICC profiles to apply color transformations to images ? Every CLUT pack I’ve downloaded from the net only provide CLUTs, as .cube or .png files. Never seen any .icc for this purpose.