Saturday, February 23, 2008

A Descreening follow-up

kw: instructions, images, tools

Trust a spate of writing to get the juices flowing! After writing the prior post, I tinkered around a bit with the scanner. I found some better settings and procedures.

Most scanners have a Twain or similar driver screen, and an Advanced mode, where you'll find color tuning and various filtering operations such as Unsharp Masking, often including Descreening. Preview runs fast, so I could pre-outline the area to scan (my older scanner did this more "up front", it took until today to find it in the new one). After setting the area desired, Zoom lets you refine it if needed (an example appears later).

I Previewed the image of the girl and her dogs and Selected just that area, set 150 dpi for final output, and then clicked Descreen and Scan.

First, these clips illustrate the differences between the automatic method and my multi-step "hand" method (you can jump down and skim the prior article—or click here if you're viewing single posts; use Back to return—to get familiar with the method).

The clips, as before, have the corner of the girl's mouth at the upper right. The 30x30 shown here is the result of Auto-Descreening, and the 120x120 is a 4x resize to see the pixels. The two clips below it show the result of my 5- or 6-step method, which begins with a scan, unfiltered, at 600 dpi. The result of either method is an image with 150 dpi resolution, and the halftone dots and their Moiré pattern removed.

The unsharpened version is closest to the Auto result, but a little inferior. The sharpened version has more visual interest for me, but take a look at the white dot in the lower-right clip, on the girl's neck near the collar. That may be real, but he faint ring around it is a sharpening artifact. To my judgment, the Auto process gave slightly better results.

This is the full image of the picture from the book. It is very nearly the same physical size on my screen as the picture in the book (its size on your monitor will depend on its own dpi). Click on the image to see all that its 150 dpi resolution can show. Blogger limits images to about 400 dots vertically, and the 150 dpi image is 752 dots tall. It will slightly overfill the browser window (or the left half of it) on a 1024x768 screen.

This points up a practical matter. No matter how nice your original image is, if you are going to use it in a blog be sure it looks "pretty good" at that size. I once had a graphic with lots of details, and the lines were so thin, they mostly vanished. I had to redraft it and simplify it. It just wouldn't do to say, "Click on this critter to see it better...sorry for the mess you see right now".

So, after all the Sturm und Drang, I've obtained as good a copy of the book's photo as technology (that I can afford) currently allows. I decided to try a full-color picture from a magazine. I needed something small...

I happened to have a recent issue of AARP The Magazine around, and saw an article by Dr. Andrew Weil, with his image in the byline. This is the Preview/Zoom screen view of an early stage in capturing a picture of Dr. Weil. A small (¾ x 1 inch) face-photo accompanies his articles in many venues. The halftone dots are quite clear, particularly around the eyes.

This image is about what you'd see in a raw 300 dpi scan. The larger one that a click will reveal is close to 360 dpi. This is a screen capture image, and its resolution depends on the way the scanner driver runs its Zoom routine.

I had two ways I could proceed. I first used the Auto-Descreen from the scanner, then I loaded the screen capture image and, without any blurring at all, resized it to 0.24 to match the 150 dpi image (it has a boundary the other image doesn't have.) I used the Lanczos filter for resizing, the slowest and best filter offered in IrfanView.

The small image on the left is the Auto-Descreened result. It is very good. The other is the shrunken screen capture shot. There is a bit more background noise, but to a quick glance, the images are very similar in quality.

I reckon I'll keep fooling around with hand descreening, but I've determined the Twain driver does a fine job, and I'm switching to it for production.

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