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Scanning, Saving and Using Digital Images
(much of this also applies to photos from digital cameras)
Brief Form
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• Pixels--basics of digital images, terminology, "what's '640x480'?"

• Color, Grey Scale, etc.--storage required depends on this

• Choose the resolution--more is not better: choose by purpose

  • Images already in digital form--optimize 'em with software

• Formats for Saving--best choice depends on purpose and image type

• Insert your image in Word--the entire image is stored in your file

• Insert your image in a web page--only the definition is stored in your file

• Moiré patterns--avoid the blotches from newspaper and magazine images

• Introduction--The choice of scanning resolution and color storage mode determine the space needed to store your digital image. And the choice of a file format (when you save the file) further controls the final file size and even the quality of the resulting image.

• Pixels--Digital images are composed of "pixels" (picture-elements) arranged in rows and columns. [Examples]The size of an image is expressed as number of rows by number of columns (i.e. 640 x 480 means 640 rows with 480 columns). There is no physical size (such as inches) equivalent to these dimensions, because the physical size depends on the "output device"-- i.e. will you see it displayed on a monitor screen, printed on paper, or what?

• Color, Grey Scale, etc.--Each pixel is stored as digital (binary) numbers, according to whether it is full color, gray scale, or line-art. [details]

  • Full color images are coded as RGB (Red-Green-Blue). Each pixel is represented as three bytes, each having values from zero through 255, storing the brightness of the three "additive primary colors", red, green, and blue. ( Printers use inks that "subtract" rather than add wavelengths to produce color. The computer converts its RGB data to the correct amount of ink in each of the "subtractive primary colors" (Cyan, Magenta, Yellow) and blacK (CMYK) to deposit to produce each pixel.)
  • Grey scale images are coded with one byte for each pixel, representing its brightness only.  Line-art images are coded with eight pixels in each byte, representing black or white only.

• Choose the resolution--Scan for a particular output device: screen or printer, etc. Choose only as much resolution as you need--more is not better. (Exception: if this is your last chance at scanning a particular photo and you think you may want to do greater things with it at a later time.)  Sloppy rule of thumb:  [details] 72 dpi for web pages, e-mail images, etc. and 200 dpi (or 300 dpi) for most printers. Here's how to do more careful calculation.

  • Images already in digital form (digital camera image, image sent to you, etc): Always "optimize": use photo-editing software to reduce the pixel dimensions of your image to only what is needed before you insert an image into a webpage in final form--download times depend drastically on image filesize.

• Formats for Saving--Save the image in the format that best fits your intended use. Balance the need for accurate reproduction against the need to reduce file size for a certain application. Consider saving it as more than one format--maybe .tif format for archive purposes and another (maybe .jpg or .gif [details]) to send by e-mail or place on a web page. Probably, avoid formats that are limited to only one computer "platform", like .bmp for Windows only but not MACs, etc.

• You can insert your image in Word [details]--You can insert an image into a Word document, adjust its size, control how the text wraps around it, etc. A Word document with several images can make a really big file, sometimes too large to fit on a floppy disk. If you start by inserting an image that is smaller, (instead of making it look smaller by dragging the corners[details]) the final Word document size will be smaller.

•  You can insert your image in a web page--Some special considerations apply if you are inserting your image into a web page.[details]

  •  the .htm file you save does not actually contain the image file, but only a pointer to the image file. So in transferring your webpage, you must be sure that all the image files are copied or sent along with the html file.
  •  You can specify the size to display the image. If you do, specify the actual pixel dimensions of the image itself. That will let the browser leave space for the image as the page first loads,so it will not have to be re-drawn after the image file has downloaded and its size determined. You could specify a display size different from the actual file size, but
    •  If you display it much larger than its actual size it will start to look blurry.
    •  If you display it much smaller than its actual size it will look OK but it still has to download the entire file, so it will take just as long to download the image as it would at the original size. You can save download time by going back to the original file, re-sampling it to the smaller size, and inserting that image file in your webpage.
• Moiré patterns are an unwanted and geometrically regular pattern that sometimes appears over an image scanned from a picture printed using a half-tone screen (newspaper and magazine photos). Some scanner TWAIN interfaces have a setting you can choose to "De-Screen" during the scan. Otherwise, several tricks for removing the Moiré patterns are possible using image processing software.[details]