On 10/18/07, Michael Shneier <[EMAIL PROTECTED]> wrote: > I am a new lcms user. I want to calibrate the color of a camera for use > in image processing.
I have a 2c to add as well, for what it's worth: * In my opinion the IT8 is not very suitable for camera calibration as the patches are formed from combinations of only three dyes. I prefer the Macbeth chart, where (I think) they use mixes of 18 dyes to make patches with spectral curves similar to those you find in real objects. Gretag sell a postcard-sized 'mini-macbeth' (the standard one is about A4) which is small enough that you can include the target in your scene without obscuring too much other stuff. * Camera calibration is actually very simple. All you need to do is remove the camera gamma (if any), fix the black point, and use a 3x3 matrix to go to XYZ. You can find the matrix using least-mean-squares from the Macbeth colours, or if you are feeling brave, do a non-linear optimisation to get a matrix that is minimal with respect to errors in CIELAB. You can do this in just a few lines of matlab (or equivalent). * Getting nice, repeatable results is hard though :-( as other posters have said, things like lighting are really difficult. You'll find cameras are rather sensitive to temperature, illumination uniformity, lens vignetting, aperture, the list goes on and on. A particularly awful gotcha is scatter in the optics. You'll find a white patch next to a black patch 'bleeds' a lot of light into the black, significantly altering its colour. This is a very serious problem with trying to use any lens to do absolute colorimetry. I have a HOWTO on my website: http://www.vips.ecs.soton.ac.uk/index.php?title=Colour_calibration_with_nip2 nip2 is quite fun for playing with this stuff. It has an LCMS menu, so you can also import and export with ICC profiles. John ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
