On 10/19/07, Hal V. Engel <[EMAIL PROTECTED]> wrote: > On Friday 19 October 2007 08:04:07 [EMAIL PROTECTED] wrote: > > * 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. > > This can be an issue in light that does not have a smooth distribution of > light across the spectrum. So for profiling in florescent lighting in > particularly it might be advantageous to use one of the GretagMacbeth targets > since florecent light has significant spikes in its spectrum.
It's also a problem if you're using the camera to image objects with 'spikey' reflectance spectra (obviously). The IT8 is good for film scanners where you are profiling the film process and the scanner electronics as a (horribly complicated) pair. You can run into problems with metamerism if you use it for profiling a digital camera. Put another way, you should profile your device with something similar to the object you will be imaging. A film scanner should be calibrated with a film target (the IT8), a digital camera with something that has 'real world' reflectances. > > 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. > > The 24 patch targets are not well suited to profiling and should be avoided if > possible. These targets simply do no have enough patches to do anything > other than a matrix/shaper profile. But that's all you need for a digital camera. They are very simple devices -- three colour filters and a linear CCD. Your profile accuracy is limited by the physics of the filters: how close are they to XYZ? They are the exact (almost) complement to a CRT. If you're calibrating your CRT all you need is a 3x3 matrix and three LUTs. Now if you're making a scanner + film profile, then yes, you need a very large number of patches, because you are trying to model the chemical processes in the film as well. > If you are going to use a GretagMacbeth > target it should be either the ColorChecker DC or the ColorChecker SG. I've not had good experiences with the DC, but perhaps I've been dumb. In my experience, if your camera has sensible filters, you can get very reasonable (and predictable) accuracy with a 3x3 matrix and a set of offsets. The last camera project I was on got an average of about 4 dE on real world objects. 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
