[sane-devel] gamma correction procedure

[abel deuring]

Christopher Marshall schrieb:
> All:
> 
> I think I've basically answered my own question,"How should you attempt to deduce a correct gamma
> correction
> value for your scanner with sane?"
> 
> The procedure is this:
>  - open a graphics program like the gimp and make some blocks of solid color (gray scale)
>  - I suggest hitting the following (R,G,B) values
>    (64,64,64)
>    (128,128,128)
>    (192,192,192)
> - print the page
> - scan it 
> - open the scan with a photoshop-like program like the Gimp
> - measure the values from the corresponding blocks in the image.

Christopher,

this way, you measure (and correct to some extent) the colour deviations 
of the *entire chain* image colour values -> printed colour -> scanned 
colour. But both '->', i.e. each "transformation" of the colours into a 
new "representation", introduces new errors. Especially, you must expect 
serious deviations by the print process. This does not help if you want 
to have "true colours" for a scan of a photo, since the photo has not 
been produced by the printer.

I'd recommend to follow Gerard's recommendation to use IT8 targets for 
colour correction. With an IT8 target, you have a sort of a "calibrated 
image"; which you can scan, and using programs from lcms or gcms, you 
can "calibrate" your scanner, i.e. create an ICC profile for it. ICC 
profiles provide a far better way to get something similar to "true 
colours" than the comparatively simple gamma correction.

Abel