Hey there,
There is a problem here, and that is your assumption that the 18% grey would
be at RGB 117/117/117. RGB is a totally device specific colour space and so
you are completely at the mercy of the video card and monitor and
resolution. The correct value for mid grey would be... whatever the RGB
value you have measured off your monitor that matches mid grey. if you
change your monitor/card/resolution you have to re-measure. If you are under
different lighting - you have to re-measure.
Basically - the only way to do this successfully is to get one of those
monitor measuring devices and profile your monitor, and calibrating your
workflow.
I worked on the front end to Agfa Colourtune a while ago and thus worked
with the colour scientists a lot. I remember one guy saying to me that
somebody had published the RGB values for mid-grey and how this was crazy.
Stuck in my head.
Regards
Ned
>From: "michael shaffer" <rarewolf@roadrunner.nf.net>
>Reply-To: filmscanners@halftone.co.uk
>To: "film scanner list" <filmscanners@halftone.co.uk>
>Subject: filmscanners: creating correction curves from scanned calibration
>chart?
>Date: Fri, 9 Nov 2001 19:22:37 -0330
>
>I have a scan of a photographed color chart, and I also have the logD
>reflectance values for the patchs. I want to create Photoshop curves for
>correcting the scan ... for example, assuming gamma=2.2 working space, I
>calculate a value of 117 for 18% reflectance:
>
>gray_value == (0.18^(1/2.2))*255 = 117 . Similarly,
>
>logD (18%) = .745 implies (1/(10^logD))^(1/2.2)*255 = 117
>
>(sorry ... it looks much better in a spreadsheet)
>
>so ... if my scanned RGB values for 18% gray are 115/118/116, I should
>create a curve for transforming these to 117/117/117. If correct, I have 2
>questions:
>
>(1) Although I may be satisfied with neutralizing gray patchs, if I have
>similar logD numbers for the colored patchs, can I apply the same formula
>to
>each of the RGB channels?
>
>(2) What is the best way to create the curves? For example, I would like
>to use the target for the "curve's" gray eyedropper to change the curve
>such
>that 115/118/116 becomes 117/117/117, ... and for a different gray,
>155/159/154 becomes 157/157/157 ... but the gray eyedropper doesn't seem to
>work this way(?)
>
>tia ... shAf :o)
>
_________________________________________________________________
Get your FREE download of MSN Explorer at
