Work-in-progress - 2009-01-24

Work on GPLin is still progressing, albeit a little more slowly now the new year's underway.

I've found that the typical gamma correction needed for inkjet printers is pretty extreme, and the infinite gradient of the gamma function at x=0 can be a problem - so GPLin now uses a modified gamma function, somewhat similar to the one used in the sRGB colour space, though with slightly different weightings and adjustments.

Using the RSPL library from Argyll (which is probably overkill!) I now have GPLin successfully creating correction curves to linearize channels, and the "printdevicen" utility can now make use of them.  To avoid wasting too much decent media I've been experimenting on some pretty awful cheap plain copy paper - quite a useful test since ink-limiting is pretty critical for such awful paper!  Here's a plot of my R285's response on this paper - a straight input -> L* plot at the top, at the bottom a plot with the points' positions adjusted using the calculated linearization curve.  (Note, the yellow points don't fit the curve quite so well because the Yellow linearization is calculated in respect of b, not L*)

Plain paper, CMYK mode.

I have also created a CMYK profile for this plain paper, using the linearization curve depicted above, with pretty reasonable results given the limitations of the medium.  The fitting error reported by Argyll's colprof command is nice and low (peak err = 2.717121, avg err = 0.862201) - though this was with just a single sheet of A4, so under 400 patches.  The greys appear to be spot on, too.  Encouraging results so far.

I shall make a new pre-release available in the near future so anyone who wants to experiment can do so.