[argyllcms] Re: RGB Printer profiling and ColorSavvy CM2C

Greg Sullivan schrieb:

I am using the raw "no colour adjustments" mode of the Epson 2200 RGB driver, 
which is the same mode which Epson advises to use for their own media-specific profiles.

Greg, OK, you're already using it.

(I don't know whether it meets that 1 dE per unit RGB requirement - it's very 
non-linear so perhaps it doesn't)

Non-linear is IMO not a problem per se, as long as the non-linearity is *smooth* enough, such that the profile can follow the curvature accurately enough. And the partial derivatives dE/dRGB should not be too steep, since the RGB numbers are not floating point, but quantized 8-bit numbers (I think, it's not possible to feed 16-bit RGB into a Windows GDI driver, is it?). If you want to be able to print smooth color gradients, without visible steps, then IMO the color should not change more than 1dE, when the RGB numbers are increased by one step. And if the gradients happen to be too steep at/near the gray axis, you will not be able to achieve a good, neutral gray axis at all (e.g. assume you have a green cast, reduce the green value by one step, and end up with a magenta cast. You would need 1/2 step, but you can only control RGB from 0-255 in integral steps).

Of course you also need enough patches to be able characterize a strong non-linearity accurately enough. A multi-pass approach, using "targen -c preliminary_profile.icc ..." for the 2nd and all subsequent passes is likely helpful for strongly non-linear devices as well, since with the -c option, targen will place more points in highly curved areas of the color space, and will distribute the points according to perceptuall criteria within the color space, instead of distributing them evenly in device space.

Regards,
Gerhard





Other related posts: