Using Gimp 2.6.6 on Ubuntu 8.04 (Hardy) Linux. (Working great, NO crashes as some people complain of.)

The answer to this question is not as straight-forward as it sounds.

Problem:

- Scanning (xsane from within gimp) images of canceled postage stamps. (Also tried with Photoshop/Windows using various scanner drivers.) Usually there are 10-30 stamps in each scan (for productivity) which are then cut apart into individual images. We scan up to hundreds per week, thus this is an ongoing issue.

- The stamps' designs are of varying colors, though their paper is usually white-ish and the designs of the stamps usually does _not_ reach the edge of the paper.

- However, the _postmarks_ (usually black) DO reach the edge of the stamps' paper.

- The desired end result is the stamp on a black background, like this:

http://jsa.viewimage.net/jsa/web/Lists/Finland/SpecStamps/Regular/sc0197_used-fvf-superb-pm_142761_r_m.jpg

- We usually scan against a black background and this works well unless the postmark reaches the edge of the stamp paper as in the above image. Even though we are scanning against a "black" background, it is never 100% black and we consider it _critical_ to make it 100% black. Thus we select the "black" background/surrounding area and fill it with black. However, in the process of selecting the background/surrounding area, it is almost impossible for the selection to avoid eating into (and following) the postmark into the design. We thus have to manually, tediously exclude the postmark from the selection before filling with black.

Experiments so far:

We have tried using TV's equivalent of a "blue screen" by which the stamps are against a colored background that is intended to be replaced with a different color.

We have tried using backgrounds of various colors (physically putting colored paper on the scanner back), but invariably, we run into the problem of a "shadow" of whatever the background color is along the stamps' perforations on one or more sides. The result is different from one model of scanner to another, but they all seem to have a "direction of light travel" and thus at least one side has a shadow of whatever color. This seems to be the nature of flatbed scanners.

Removing that color shadow is even more problematic than deselecting the postmark problem areas.

Using a digital camera has not produced the desired results, both from a productivity standpoint and a quality standpoint. Clarity and focus of image quality is absolutely critical. "Good enough" is not good enough. There are lighting problems, distortion (curvature, plane, depth of field) problems, etc, etc., etc. to say nothing of the need to keep the object flat and digital cameras don't like shooting through glass (we have purchased heavy optical glass, but end up seeing the second side of the glass). And that is all before the issue of background color which still remains!

So....

I am hoping for suggestions as to a) how to avoid the color shadow of using a colored background and b) if it cannot be avoided, how to fix it in gimp without a lot of messing around and/or other color distortion problems.

Thanks.

Jay

On 06/13/2009 02:46 PM, Chris Mohler wrote:

On Sat, Jun 13, 2009 at 1:08 PM, Jay Smith wrote: [big snip]

So....

I am hoping for suggestions as to a) how to avoid the color shadow of using a colored background and b) if it cannot be avoided, how to fix it in gimp without a lot of messing around and/or other color distortion problems.

Have you tried putting something heavy on the "colored background" (I tend to use a thick book)? This may reduce or eliminate the shadow.

Have you tried reducing the wand (or select by color) tool's threshold when selecting the black background? I would guess that the postmark color and the background color differ at least slightly.

A couple of the raw scans of the stamps might be useful for analysis.

Chris

Yes, we have weighted the item on the scanning bed.

The scanner is able to pick up the thickness of the postage stamp paper (the shadow caused thereby) because from either the leading or trailing direction, the light source causes a very slight shadow that the scanner detects.

And, yes, we have played extensively with the selection tool's threshold. Doing so solves problems in some spots, but creates problems in others. The shades of black are quite variable.

===

Using a "blue screen" method in which I would use a background of some outrageous color that is not present anywhere in the items to be scanned, what would the best tool/method to use to select and eliminate ALL of that color? Again, the problem is that the background itself can be removed, but at the above described "shadow edge", there is a gradation of color. Of course the catch is that there is no background color will work all the time. I can't simply delete all reds or all blues or all ...whatever.

Ideas?

Jay