[sane-devel] gamma correction procedure

[Christopher Marshall]

--- abel deuring <adeuring at gmx.net> wrote:
> 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

I see what you (and Gerard) are saying now.

Assuming I used an IT8 target to calibrate my scanner, could I then use the procedure I outlined
to calibrate my printer?  Or I am still not getting everything you are saying?

I take it there are at least three gamma distortions I need to worry about: my monitor, my
scanner, and my printer.

Using an IT8 target would allow me to separate the scanner's gamma distortion from the other two. 
The procedure I proposed could then let me calculate the gamma distortion for my printer.

Correcting my monitor would be the hardest of all.  The simplest thing to do would be to display
an image of the IT8 target on my screen, hold a physical IT8 target in my hand next to the
monitor, and make adjustements to the monitor's gamma settings until my eye felt the two agreed
largely.

Chris Marshall
 

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