I had assumed that is a help to confirm the correct distribution of light on the surface of the target/document. Salud Jose Bueno 2013/5/25 Roger Breton <graxx@videotron.ca>

Take a peek at the RGB values of those multiple white, gray and black patches around the perimeter of the SG target and you will get an idea of their value...

Best / Roger

-----Original Message----- From: colorsync-users-bounces+graxx=videotron.ca@lists.apple.com [mailto:colorsync-users-bounces+graxx=videotron.ca@lists.apple.com] On Behalf Of Alex Jamison Sent: Saturday, May 25, 2013 1:37 PM To: colorsync-users@lists.apple.com Subject: ColorChecker Digital SG question

Hello,

Does anyone know the reason for the multiple white, gray and black patches around the perimeter of the SG target? Are they necessary for successful profiling or are they redundant? Does profiling software make use of them to equalize color or illumination across the field perhaps?

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On May 25, 2013, at 7:22 PM, Thomas Lianza wrote:

The patches there to allow software to correct for uneven lighting on targetŠ.

Works only if the target is perfectly flat. If it is not, and it is usually the case, such attempt of correction may do more harm than good. -- Best regards, Iliah Borg

Am 26.05.2013 01:58, schrieb Iliah Borg:

On May 25, 2013, at 7:22 PM, Thomas Lianza wrote:

...

The patches there to allow software to correct for uneven lighting on targetŠ. Works only if the target is perfectly flat. If it is not, and it is usually the case, such attempt of correction may do more harm than good.

Well, but what's the difference, eventually? Regardless whether uneven illumination, and/or a non-flat object surface, and/or vignetting, all of them lead to uneven intensity captured by the camera. So the correction of the uneven intensity should deal with all these cases, independent of the root cause. Whether or not the amount and distribution of the multiple white patches is sufficient in order to establish a sufficiently accurate model for the spatial relative intensity distribution across the extent of the image of the target is a different question... And I've also doubts whether the characterization of the spatial intensity distribution can be always easily separated from the color characterization of the camera, particularly if one does not make any a priori assumptions about the camera (i.e. if the camera behavior might be arbitrarily non-linear). One has certainly better cards in the hand if one can already rely on some a priori knowledge about the camera (e.g. if we know that it is granted that the raw data are sufficiently linear and that the R, G, and B channels are sufficiently independent). Best Regards, Gerhard

On May 26, 2013, at 3:22 AM, Gerhard Fuernkranz <nospam456@gmx.de> wrote:

Whether or not the amount and distribution of the multiple white patches is sufficient in order to establish a sufficiently accurate model for the spatial relative intensity distribution across the extent of the image of the target is a different question...

Seems like a suboptimal tool for the job, especially when the right answer is so easy: make another exposure in the exact same setup with the chart replaced by a uniform white reference -- it could even be the back side of the chart or its cover or whatever if you're making your own chart -- and use that to normalize the scene illumination. You can do that in Photoshop, but Robin Myers's EquaLight does it better. If it's just for profiling, Iliah's RawDigger can do it when extracting patch values, but you'll still want EquaLight (or equivalent) for the same purpose when shooting the art. You'd be much better off using those patches on the SG as normal patches, giving you that many more data points...and the patches would have been better designed to have sampled a larger portion of the color space -- either more saturated colors outside the existing gamut or more finely sampling the interior space of the gamut. Even a better distribution of neutral and near-neutral patches (such as what takes up most of the additional patches on the ColorChecker Passport) would be more useful; that's a very critical part of color space and oversampling it is a good idea. b&