Richard Hughes wrote: > Maybe a really stupid question, but why don't we just use a LUT for > displays? My gut instinct is that the profiles would be larger (and > thus take more processing power to use), and that the RGB display > would match really well to a matrix. Except when is doesn't, and the > user has to specify a LUT type for a screen that doesn't work well > with a -aS profile. Assuming by "LUT" you mean cLUT (3D LUT), then there are tradeoffs. Putting aside the size issue, the main issue is smoothness and accuracy. A cLUT is a piecewise linear function, so it isn't smooth at the fine level, whereas a power or 1DLUT + matrix is very much smoother. This can be of more subjective significance than it appears, if the device is highly non-linear, or you need to deal with a large dynamic range (although the per channel curves of a cLUT profile can ameliorate this to some degree.) If the underlying device response is additive then a 1D curves + matrix function will be more accurate in regions of the response that are not close to points in the measurement sample set. If on the other hand the device response is not additive, then a cLUT has the potential to have lower gross errors in those areas, but it will only be more accurate if there is sufficient density of measurement samples. I guess it may be possible to assess after the fact which profile is "better" (for a given weighting of quality metrics), but I'm not aware that anyone has done much work on this. Maybe something could be done before profiling by evaluating how additive the sample set is, and using some heuristic threshold. An evaluation of the sample distribution would (ideally) also feed into the decision. In practice I think the predominance of matrix display profiles is due to their small size, easy & fast evaluation, the additive nature of good quality displays, and the lack of support amongst some CMM's for cLUT type profiles. One quandary is that in the ICC way of doing things, you can only have the (baked in) gamut mapping with cLUT type profiles. Use of matrix profiles tends to imply gamut per component clipping, which is not very elegant. Graeme Gill.