Ben Goren wrote: > That brought the middle gray patch to roughly R=G=B=100 and the Teflon patch > to > R=G=B=250 (about as bright as I could get it without clipping in places I > cared > about). I then made a profile with ``colprof -u,'' and got a clean white and > good results overall. Hi, Note that -u triggers special extrapolation code, which uses a matrix style model to extrapolate some fake neutral axis/white points. -un disables this and is pure cLUT/regular spline extrapolation. See <http://www.argyllcms.com/doc/colprof.html#un>. The extrapolation code is far from perfect, but it does seem to improve many situations. > I can understand how they might not provide the greatest amount of fidelity. > What I can't understand is why they'd go completely bonkers, way off into > left > field. Shouldn't they reasonably assume that, if the brightest available > patch > is already right on the neutral axis, the extrapolation should remain close > to > the neutral axis and end at L=100 a=0 b=0? They are just algorithms, and the parametric/regular spline is a local algorithm with smoothing - that's part of it's attraction, that it does a local fit to the sample points. But away from sample points all that controls it is the smoothness constraints. So all it needs is for there to be a slight "tilt" near the edge of the gamut, and the smoothness constraint will happily go extrapolating in that direction. It has no "knowledge" or "expectation" about what's reasonable for a profile or device. The special extrapolation code in -u attempts to address this. > But I would still expect the magnitude of the error of the results to be in > the > same range as the magnitude of the error of the input...and I wouldn't expect > minor lightness errors within the target gamut to result in radical hue > errors > outside of it. But that is the nature of extrapolation. Any errors within or at the edge of the measured points get magnified through extrapolation. > I think I know enough to come up with a workflow to get good results...but I > still think there's something very worng with how colprof is doing its > extrapolation. I'm happy to take a look at any .ti3 that triggers a poor input profile result using colprof -u. Graeme Gill.