(This article originally appeared in the first quarter 2007STAR Newsletter. -I)
There’s More to Re-sizing than Meets the Eye
by Mike Etkin
Those of you who process digital images know about resizing. If most of you are like me, your exposure to resizing is probably limited to reducing file sizes to something that would be easily transmitted over the Internet – something on the order of 80 to 100 kilobytes.
My exposure increased considerably when I was getting some images ready for Astronomy Days 2007. Southeastern Camera offered to print some images for us to exhibit at Astronomy Days and the NC Museum of Natural Sciences generously put up some funds for the effort. Most of us who took up the offer were using DSLRs and had 6 to 8 or even 12 megabytes to work with and no need to worry about resizing either up or down. I, on the other hand was capturing planetary images using an unmodified webcam. The CCD chip in my webcam was 640×480 pixels. The resultant image file using uncompressed BMP files was approximately 1 megabyte or slightly less.
The first response from Johnny, the print enlarger technician at Southeastern was: “There are not enough pixels here!” I tried to explain to him how Registax freeware uses features of the image to register and stack frames and how the pixels on each frame are not fixed relative to the image (due to errors in tracking among other things). Thus, the pixel edges themselves were randomly placed relative to the image and for all intents and purposes became noise in the Signal to Noise Ratio world of digital imaging. Well, it turns out that Johnny was right and I was wrong! Certainly a place I’d been to before on many occasions.
What I had overlooked was the fact that after enhancing the Signal to Noise Ratio through registering and stacking, the resultant image is then represented again in the form of pixels. When blowing the 1 megapixel BMP image up to 11×14 you could see the pixels plainly (pixelation). Of course Johnny was right!
Not to be cheated out of enlarging for exhibiting at the Science Museum, I tried multiple re-sizing using Registax and came up with files composed of 2, 4 or even 8 megapixels. When blowing these files up to 11×14 we are cramming more and more pixels (up to 8 megabytes) into a constant space and the only thing that could give is for the pixels to get smaller. The same re-sizing can be done with Adobe Photoshop, Microsoft Photo Editor, Kodak Easy Share and numerous other digital processing packages currently available.
The image definition held up admirably at 11×14 (see included picture). The concept of manipulating the pixel number and size, in the resultant image, independent of the original number and size of pixels in the raw frames seemed exciting and worth noting,. As usual, after researching, I found the concept well documented by Reeves in his DLSR Astrophotograpy and Lodriguss in his Guide To Digital Astrophotography. Reeves calls it pixel interpolation.
At any rate, the web cam photographers in the club should be able to produce some dynamite 11x14s for Astronomy Days 2008.
