I am having some problems with a slight posterization in clear, blue, Sky areas. The area affected is where a dark, polarized, sky starts to blend into the lighter Sky areas near the horizon. I see this problem whether I scan from negatives, or prints; with and without ICC profiles being used. I also see this posterization in the blue scale of an IT8 target when I scan it. If anyone has any suggestions as to the cause, I would appreciate hearing from you.

Thanks, Dennis Edson

(DennisWCR@email.msn.com)

-- Dennis Edson (DennisWCR@email.msn.com), July 09, 1999

Answers

I don't know whether this will help or not, but I'll give it a shot.

Technically speaking, posterization is the process of specifying the number of tonal levels (or brightness values) for each channel in an image and then mapping the pixels to the closest matching level. This can be done deliberately (as with the Posterization command in Photoshop) or it can occur involuntarily when the number of pixels in the image is not adequate for its size.

A type of posterization also occurs when scanning a halftone since the pattern of halftone dots (the so-called "moire" pattern) must be interpreted to form pixels with specific tonal or brightness values.

Unfortunately, from your description I can't really assign any of the foregoing causes to your particular problem.

In any event, when faced with this problem I try one (or both) of the following: (1) Use the Despeckle command in Photoshop. (2) Using the Lasso tool, select the sky area that is posterized and feather it (I typically use 5 pixels). Then apply a Gaussian blur on the area (between 3 and 6 pixels).

-- Albert J. Klee (aklee@fuse.net), July 10, 1999.

The human eye is far more sensitive to variations in hue in the blue region of the spectrum than anywhere else. Unfortunately, blue is just the color that silicon CCD sensors have the most trouble with! Thus, if you're going to see any quantization (the technical term for "posterizing", it'll be in gently gradated blue objects.

Besides the options for dealing with this listed above, here's another one, a bit counter-intuitive: Add noise! - There's a function in Photoshop that lets you do this, and you'd be surprised by the results! - Just add it in the area where you're having the posterization problems, and don't use a lot. A fairly small amount of noise does a nice job of breaking up the posterizing, without losing any edge detail in surroundin

-- Dave Etchells (detchells@imaging-resource.com), July 17, 1999.

You don't say whether you see the posterization on your screen or on printout.

If on screen, it could be your display screen settings. If you look at a perfectly good image with your video display set to 256 colors (the stupid Windows default) it will APPEAR posterized although the intrinsic image may not be.

You can change the settings (in Windows 95 or 98) by clicking on Start, then Settings, then Control Panel. Then double-click on Display, select the Settings tab, and adjust Color Palette (if not already so set) to 24-bit or "True Color."

-- bruce komusin (bkomusin@bigfoot.com), July 25, 1999.