scan

Scanning and File Formates

        Before we get going on scanning, there is one term that needs to be explained. Printers print using very small dots. When the dots are spaced close enough together, they overlap producing the appearance of black. As you start to see gray areas in print, the dots are being spaced futher and further apart. The spacing in between these dots is called halftone. Halftone is measured in lines per inch (lpi). Keep in mind this is very similar to resolution measured in pixels per inch (ppi) or dots per inch (dpi), dpi is the term I will use most often.

The general rule for scanning comes in two parts:
1.   Scan at the resolution you want. This is done by scanning at 1.5-2 x the halftone. The halftone for most printers for computers is about 72 lpi (144 lpi and up is considered very high quality). Grey scale images should be scanned at 100 dpi and color images should be scanned at 150 dpi. Anything over this is extra quality that will increase the files size significantly but will barely be visible in a final print from your printer. For instance a file scanned at 300 dpi is 4 x larger than one at 150 dpi, 600 dpi is even bigger and is pushing into photo quality (the human eye begins to not tell the difference at this point). This also applies to the net. Most images on the net are 72 dpi, this is fine but produces some
poor quality. For really high quality images on the net use 100 dpi for black and white images (gray scale) and 150 dpi for color. Doing anything more is useless really.
2. THIS IS VERY IMPORTANT! Scan at the size you want it. Once you scan an image in it becomes a bitmap. Scaling an image after it has been scanned will cause a loss in quality. Never scale an image larger than its actual size after it has been scanned. Scaling an image smaller does not cause as noticeable an effect but still loses some quality. What this all means is if you have an image that is say 2 inches by 2 inches and you want to make it bigger (say 6 by 6) then when you scan it make sure you tell your program what size you want the final file to be. Another note on file size should be mentioned at this point. The larger the image then the larger the file size is going to be.
                                                                                                                        This information also plays a part in your scanning and other visual files (including created graphics). This is on file formats ... should you use .bmp, .gif, .jpeg, or .tiff.

GIF89a (Graphics Interchange Format) is commonly used to display indexed-colored graphics and images in hypertext markup language (HTML)files over the World Wide Web and other online services. Gif is a compressed format that is designed to minimize file transfer time over phone lines quicker. Gif's are the most basic format you can use and the compression can cause a loss of information (or quality).

The Joint Photographic Experts Group (JPEG) format is commonly used to display photographs and other continuous-tone images in hypertext markup language (HTML) files over the World Wide Web and other online services. Unlike Gif, JPEG retains all color information in an RGB image. JPEG also uses a compression scheme the effectively reduces file size by discarding extra data not essential to the display of the image. The first thing that goes are your layers (this is mostly for graphics that you have created that have multi-layers). So, the next time you open the image you will have only one layer to start with. Opening a JPEG image automatically decompresses it. Because it discards data, the JPEG compression scheme is referred to as lossy. This means that once an image has been compressed and then decompressed, it will not be identical to the original image. A higher level of compression results in lower image quality; a lower level of compression results in better image quality. In most cases, compressing an image using the Maximum quality option produces a result that is indistinguishable from the original. Another point about lossy is that each time the image is saved it losses more information, this should be kept in mind when dealing with JPEG images.

The Tagged-Image Format (TIFF) is used to exchange between applications and computer platforms. TIFF is a flexible and stable bitmap image that is supported by virtually all paint, image-editing, and page-layout applications. Also, virtually all desktop scanners can produce tiff images. TIFF supports RGB, CMYK, and gray scale color modes. It also supports LZW compression, a lossless compression method that does not discard detail from the image. These are also the largest of the file formats because of this.

Lastly, when scanning these color modes should also be taken into account. For black and white use 256 gray scale color mode and RGB for color applications. These will help keep your file size down with out any loss in quality. I will explain the differences in these color modes later on.