Hi all, Ive recently started scanning some BW negatives using a new HP Photosmart scanner for eventual web display. I'm obviously a beginner at this game! On about 5% of my scans Ive noticed that some hard edges, both straight and curved, appear very "steppy", at a scale larger than pixels. There does not seem to be anything particularly different from the scans that show the problem and those that don't, although the problem does seem to be mostly related to a fine light tone line against a darker background.

I'm pretty certain that its not a scanner issue, at least in the sense that the scanner does seem able to extract sufficient data, but for some reason it won't always present on the screen as I expect it to when the image is resized to fit on the page.

Anyway, rather than going into a lot of detail here, I've set up a web page that explains the problem, and includes some examples, at:

http://home.primus.com.au/Brindabella/Pageoftests.html

I can partly correct some of these problems using editing software but needless to say I would much rather overcome the problem at the scanning stage.

Does anyone have any suggestions?

Many thanks,

Alan Brisbane Australia

-- Alan Shaw (brindabella@primus.com.au), July 30, 2000

Answers

The way the effect only shows up at certain sizes when depicted onscreen sounds kind of like a moiri pattern effect. Are you familiar with moiri pattern effects like you typically get when scanning things printed in magazines?

Does using a higher, or sometimes lower, scanning resolution make a difference? Are you using 24 bit color for display on your monitor? If it only appears at magnifications lower than 100%, it could be a moiri effect either scanned into the file or from the routine that's averaging pixels together to display at other than 100% resolution...

Just a thought.

Good Luck!

-- Gerald M. Payne (gmp@surferz.net), July 30, 2000.

I've looked at your "problem picture" on the web, and I think it's more to do with the narrow angle presented by the lines.
It's impossible for a computer to show near horizontal lines as anything but jaggies, because the line has to step from one pixel to another. See for yourself by simply drawing a thin line at an angle of about 1 degree to the horizontal in any drawing package.
A thought just occurs to me. Is there a re-sample option in your image editor, as opposed to resizing? If there is, you should use that instead, and this may help.
Here's a little illustration I've just done with Paintshop Pro 4. It shows an angled line, and the same line re-sized, and re-sampled. There's a dramtic difference between the two. More sophisticated image editors, such as Photoshop, generally make a better job of re-sizing.
It doesn't help much, but at least you know that it's a common problem.

-- Pete Andrews (p.l.andrews@bham.ac.uk), July 31, 2000.