AWARE SYSTEMS
TIFF and LibTiff Mail List Archive

Thread

2000.10.19 19:27 "reading LAB files", by Michael O'Rourke
2000.10.19 21:11 "Re: reading LAB files", by Joris Van Damme
2000.10.20 08:11 "RE: reading LAB files", by Mart? Mar?ay7WE=
2000.10.20 18:20 "Re: reading LAB files", by Joris Van Damme
2000.10.20 19:02 "RE: reading LAB files", by Mart? Mar?ay7WE=
2000.10.20 22:25 "RE: reading LAB files", by Bill Radcliffe

2000.10.20 19:02 "RE: reading LAB files", by Mart? Mar?ay7WE=

> Martí María wrote:

I'm sorry but this is not correct for my testfiles. I substract 256 from the a,b parts IF there bit 8 is set, otherwise I leave them alone. I have tested that. Also, the example Michael is giving here seems to support my findings.

Yes, sorry. I forgot to mention "if greater that 127".

The only fast way I've found to convert Lab to RGB and the other way around is using a LUT of about 2 megs big. And that reduces accuracy to about 6 bits per channel, which is fine for fast results, but I do want 16 bits per channel accuracy in the 'perfect' conversion routines. If you know a better way to do either the fast or the perfect conversion, I'd LOVE (!!!) to hear more about it.

I use trilinear interpolation and ICC profiles. This is relatively fast, it gives 16 bits of precission, (dE < 5 on worst case) and needs about 300K for 3D LUT. If you want some code http://www.lcms.coloraid.de

In the demo is also a profile that does convert directly from/to encoded Lab TIFF.

Regards,

Marti Maria
The little cms project
http://www.abaforum.es/martim/lcms.htm
mailto:martim@abaforum.es