[Lcms-user] Re: "extended" profiles

[Marti M. <ma...@li...>]

>I'm sorry for bombarding you with half a dozen questions ;-)

No... I am who should be sorry for not being able to answer all lcms 
related questions, so apologizes to anybody who got no response. 
But lcms is a non-commercial project, and despite the kind sponsoring 
I'm getting from HP, I must earn some money too :-) So, most times I'm 
short of time and leave questions already discussed without response. 
Oh, well.

>An extended profile is a scanner profile created with a Level 
>adjustment in the target scan. The target scan can be extended in the 
>highlights or in the shadows or in both.
>The target scan is lighten/darken up a little bit prior to feeding it to 
>the Profiling software. The reference file IS NOT CHANGED though.
>If extending the shadows, for instance RGB=10 is mapped to 0. As a 
>result, the profiling software will be fooled into believing that the 
>scanner is inherently dark and generate a profile which tends to lighten 
>the image substantially."

This is exactly what "push endpoints" option is doing. But one must be carefull in 
doing that, any displacement greater than 5% would probably introduce artifacts. 

Regarding the gamma 1.0 question, first make sure to use 16 bits TIFF when 
doing so. Gamma 1.0 + 8 bits = posterization. Also, since Lab exhibits a 
gamma of 3.0, so the "impedance mismatch" is less when using gamma encoded 
images. My advice would be to use gamma 2.2 or even more where possible,
and leave gamma 1.0 for very special cases, like camera RAW, for example.

Marti.




----- Original Message ----- 
From: "T E Schmitz" <ma...@nu...>
To: "lms mailing list" <lcm...@li...>; "Marti Maria" <ma...@li...>
Sent: Wednesday, February 18, 2004 2:53 PM
Subject: "extended" profiles


Hello Marti,
I'm sorry for bombarding you with half a dozen questions ;-)

Here's another one and I'd be interested in your opinion on the topic:

I have seen the following technique for so-called extended profiles 
described:

"An extended profile is a scanner profile created with a Level 
adjustment in the target scan. The target scan can be extended in the 
highlights or in the shadows or in both.
The target scan is lighten/darken up a little bit prior to feeding it to 
the Profiling software. The reference file IS NOT CHANGED though.
If extending the shadows, for instance RGB=10 is mapped to 0. As a 
result, the profiling software will be fooled into believing that the 
scanner is inherently dark and generate a profile which tends to lighten 
the image substantially."

I have played with a shadow extended profile but while it indeed 
lightens a dark image it also very often introduces posterization.

-- 


Regards,

Tarlika Elisabeth Schmitz