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Problems with
Enlarging Digital Pictures

I recently explained that a photograph scanned and copied into a computer is converted to a "bitmap" image consisting of tiny squares which, when viewed from a distance, give the illusion of being the same "smooth flowing" colors seen in the original photo. However, I continue to be asked why enlarging a bitmap photo makes it look "blocky."

"When my photos are enlarged at Kodak," I'm told, "the prints still looks smooth and natural."

Well, it's because of the those little squares. Colors captured on film by a camera are "continuous tone" images that continue to be smooth flowing when they are enlarged or reduced. However, computer images are broken down into tiny "bits" which must be "mapped" in a way that makes them appear to be smooth flowing at a given "dots per inch" resolution.

If a photo is scanned and subsequently printed at the same size as the original, using a high DPI on quality paper, the result can be an image that looks very much like a Kodak print. But a little arithmetic will explain why enlarging a bitmap can make it look "blocky."

Doing Some Math Might Make It Easier to Understand

Let's assume that the original photo is a sunset with vibrant shades of yellow, orange and red. Then let's zoom in on one of the tiny yellow squares, which is probably surrounded by squares in varying shades of yellow-orange. Well, if you ask your image-editing program to double the size of this picture, it will actually come out four times larger (double the height by double the width). This means that the number of tiny squares will need to be quadrupled.

Well, when all these new squares are added in, how does your computer know what color to make each one? Take the aforementioned yellow square, for instance. It needs to be supplemented by three new squares; and the most logical thing to do is make them all yellow.

Now, where you had one yellow square surrounded by others of various yellow-orange hues, you have a block of four yellow squares surrounded by blocks of yellow-orange shades, which are also four times larger than they were originally. These blocks are what make the enlargement look "blocky."

Yes, a professional digital graphic artist may be able to edit these colored blocks, one pixel at a time, to compensate for all this — but your image-editing software is not likely to be quite that smart.

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