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.

Can This Be Overcome?

So what can be done to keep a larger print from looking blocky? The best solution is to start with a larger photo to be scanned. If you don't have a larger copy of the picture, consider having it enlarged photographically before you scan it.

I recently explained all this to someone who replied, "Well, that may be the case with 'bitmaps' — but all my pictures are 'JPGs.'"

Well, JPGs are, in fact, bitmaps, as are BMPs, GIFs, TIFs and many, many other picture formats too numerous to list here.

Why Are There So Many Different Formats?

Well, in the early days of computer imaging, different programs were being written by different people — and the results were several different ways to accomplish, more or less, the same thing. So Is One of These Formats Better Than All the Others?

You may get varying opinions from computer technicians and graphic artists, but here's what most of us need to know: the JPG format (a.k.a. JPEG and JPE) has become the most popular for reproducing color photographs on Windows-based PCs. The GIF format is used most often for drawings seen on the Internet, and it is also used for animated graphics. However, GIFs are limited to 256 colors, while JPGs can display millions of colors.

But an important feature regarding saving your picture as a JPG, about which many users are unaware, is the fact that you can choose from different "image quality" to "file size" ratios; high-quality/large-size, medium-quality/medium-size, and low-quality/small-size.

However, no-frills image-editing programs (such as Windows Paint) default to "medium quality" without offering the other choices. Graphics software by Adobe, Corel, Jasc PaintShop Pro and many others, do ask which quality/size ratio you want, when saving a picture.

So Which Ratio Is best to Use?

It pays to experiment — and you'll probably discover that very little difference can be seen between the high quality and medium quality images. In any case, keep in mind that smaller file sizes can be uploaded and downloaded faster, and that they take up less disk space. (However, in these times of high-speed cable connections and huge-capacity hard drives, this may not be quite the issue it was just a few years ago.)

One other thing you need to be aware of, though, is that once a JPG has been saved as a lower quality image, it can NOT be restored to a higher quality. So always save it with a different filename. This will preserve the original image, in case you later change you mind about which quality to use when saving.