NWA-PCUG 12/1999 NewsLetter Article 18861 N. Hwy 112, Springdale, AR 72762-9303 Image Formats Submitted by Pat McAlister, NWA-PCUG, P1TTYPAT@aol.com, taken from one of her "lists" X-Message: #5 Date: Sat, 27 Nov 1999 11:48:18 -0800 From: "John Slaughter" , Spokane, WA Subject: Image formats > At 06:51 PM 11/26/99 -0800, Anelle Kloski wrote: > >I started out scanning my old photos and saving them to .jpg format, >until I read that every time a .jpg file is changed (whether size or dpi >or whatever) it loses quality. Anelle, I think some clarification is in order. Bitmap (.BMP) images are the Microsoft format for storing graphics. They have the highest resolution. They also have the highest cost in terms of disk space required. This is because each and every pixel is mapped. The color depth is achieved by storing the color of each pixel in 8-bit, 16- bit or 24-bit format. 16-bit is 256 colors and 24-bit is 16,000,000 colors possible for each pixel. 24-bits is 3 bytes. As every pixel is mapped, if you open the file in a graphics program and then save it without changing anything, no detail is lost. If you make changes to it without altering its size, there will still be no degradation to the image. However, if you change its size, there will be loss. If you shrink it, the graphics program has to remove some pixels and then has to make some changes to the pixels that are now side by side so that the color change between them looks correct. If you enlarge the image, pixels must be added and, again, the graphics program must make changes. .GIF and .JPG images are NOT bitmapped. They are compressed versions of the original using mathematical and other formulas to retain as much of the original information as possible with the least amount of loss of quality. However, there IS loss. If you open these files in a graphics program and then save it, there will be further loss. This is because it had loss brought about by the original compression algorithyms and then decompressing. Now, it has to compress an image that has had some loss of integrity. Over time, the image degradation can be considerable. However, if you simply open the image file and then close it, as a browser would, there is no further degradation because no changes are made. The best procedure to follow for your photos is to scan them as close to the resolution that you desire as possible the first time. For purposes of displaying on the web it is recommended that you use between 72-DPI and 100-DPI for a standard 3x5 or 4x6 photo. You can increase the resolution if you are scanning a smaller area of the photo. If you want to be making changes to a photo, always change the file name or directory when you make changes. Keep your original scans in a single directory and never overwrite them when you save a changed image. Note that most browsers only handle .GIF and .JPG formats. A .BMP image will not be displayed on the web.