The following is a Plus Edition article written by and copyright by Dick Eastman.I have written several times about the method of creating and preserving digital images. I have also written about scanning old photographs as well as scanning various printed or hand-written documents. Today I thought I would discuss the various file formats available and briefly describe the advantages and disadvantages of several of the more popular formats.
Today's technology allows for a selection of image file formats, including JPG, GIF, TIFF, BMP, PSD, RAW, PNG, EPS, PDF, and others in a seemingly endless alphabet soup of abbreviations and acronyms.
You can find several good reasons and bad reasons for selecting any of these file formats. However, from a genealogist's point of view, there are two significant issues to deal with: image size and image compression.
Image size has been an issue since the first scanned images were stored on a computer, back in the vacuum tube days. In this case, the physical size of the picture is not the issue, but the size of the file you create is very important. That is, the problem revolves around the number of bytes required to store a faithful reproduction of the original image.
Not many years ago, disk drives were expensive. Luckily, that problem is disappearing as the price per byte of storage has plummeted in the past two decades. Prices for two-terabyte disk drives have now dropped to the $80 to $120 range, a price undreamed of only a few years ago. It is now cost-effective to store hundreds of thousands of very large digital image files. Prices for disk storage are still dropping nearly every week.
However, file size remains an issue when transferring those files to another computer or when inserting images into a web page. Not everyone uses high-speed, multi-megabyte-per-second Internet connections. Even those who do use such high-speed connections find that including very large digital images in a web page results in slow performance. Finally, sending a hundred or so old family photographs to a cousin can be a painstaking effort if the files are very large.
Image file size, expressed as the number of bytes, increases with the number of pixels composing an image and the color depth of the pixels. The greater the number of rows and columns, the greater the image resolution and the larger the file. Also, each pixel of an image increases in size when its color depth increases: an 8-bit pixel (1 byte) stores 256 colors, and a 24-bit pixel (3 bytes) stores 16 million colors. Most color images these days are stored as 16-bit or, even better, as 24-bit colors. However, if the original picture is large (perhaps 8-by-10 inches or larger) and is scanned as a high-resolution image, the resultant digital image can be huge.
The issue of file size was a problem back in the days of expensive disk drives, when typical computer connection speeds were 300 baud or so. Storing hundreds of images on the limited storage capacity disk drives of the day was a problem, as was the inability to send large images across very slow network connections. To solve these problems, image compression was invented.
File compression refers to the application of computer algorithms to analyze images and to find pixels to delete, thereby reducing the file size. For instance, if the picture had three red pixels in a row, the compression algorithms might eliminate one, or even two, of those pixels. The human eye probably won't notice the difference, and the savings in file size is significant when thousands of pixels can be combined and duplicates eliminated. The elimination of duplicate pixels is only one part of the sophisticated compression techniques used.
Of course, any time you delete pixels, you are also reducing the quality of the original image. However, modern compression algorithms are very good at reducing file sizes without inducing significant loss of image quality. The rest of this article is going to address the word "significant."
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