Scanning/Plotting Settings and Terms

There are three factors that matter most when finding efficient scanning parameters:

Pixel Resolution (dpi)
Compression in file formats
Color Depth

This example may give you an idea of the general parameters: an 8" x 10" document scanned at 300 dpi in 24-bit color generates a 21 MB image (which would take someone surfing the web with a fast modem, a few hours to look at.)

(8" x 300 pixels/inch) x (10" x 300 pixels/inch) x (24 bits/pixel) = 21.6 MB

Most computer displays are only 72 dpi with 8-bit color. The same image scanned at 72 dpi will download over a modem in seconds, and will look the same on a monitor as the huge one discussed above.

(8" x 72 pixels/inch) x (10" x 72 pixels/inch) x (8 bits/pixel) = 0.414 MB

Note that very large image files can be produced with the large format scanner: scanning an 18" x 24" document at 300 dpi in 24-bit color generates 18 * 300 * 24 * 300 * 3 = 116.64 MB uncompressed.

Scanning in lower color resolution (8-bit, gray scale, or monochrome) at lower spatial resolutions (150 dpi or 200 dpi) can significantly reduce file size.

How much is too little? As with all computer operations, it makes sense to do some small test runs with varying parameters to find out what resolutions you really need; and to plan ahead for sufficient disk space to manage the resultant files. If the computer runs out of disk space during scanning, you may encounter an error; make sure sufficient disk space exists before you start. You may wish to directly map the nettmp network disk drive before starting, and save your file to that directory. The details of attaching the nettmp directory are covered in the page about using central disk resources. Keep in mind that nettmp can fill up, so please delete your images from here after you have successfully transferred them.

Glossary

DPI - dots(pixels) per inch in an image

max dpi

Monitors

Standard Color Printer

300

Large Format Scanner

400

Laser Printer

600

Professional Color Printer

1000+

Fine Art publishing/books

3000+

File Size - is the product of DPI-squared multiplied by tonal resolution
(2"x2" at 72 dpi = 144 x 144 = 20736 = 20K bytes for 8bit image (Web)
(5" x 5" at 200 dpi = 25 x 40000 = 1000000 pixels:
= 1 MB for 8-bit color, or 3 MB for 24-bit color)
(8" x 10" at 300 dpi = 80 x 90000 = 72 00000 = 72 MB)

OCR - Optical Character Recognition software which turns scanned images (raster) from image-processor into text (ASCII code) for word-processor

Pixel - one dot of color, on the computer screen or on a printer, etc

may be equal to one bit if black and white (monochrome, or 2-colors)
equal to one byte (8 bits) for Indexed Color (256 colors) (for the Web, eg)
equal to 2 bytes for 16-bit color (Thousands of Colors)
equal to 3 bytes (24 bits) for True Color, or RGB (Millions of Colors
equal to 4 bytes (32 bits) for " " + 'alpha' (transparency) channel

Resolution - a measure of the smallest spatial unit in a scan (how big is a pixel?) or how many colors are recorded (tonal resolution) 100 dpi, 8bits/pixel is relatively 'low' res; 400 dpi, 24-bit color, is hi-res For web work, remember that monitor resolution is generally only 72 dpi

Raster - an image, or any data, made up of a rectangular array of pixels

Vector - Image stored as lines, rather than as dots (raster) Can be generated by 'raster->vector' conversion, but usually preferable to digitize using digitizer

RGB - or True-Color Format for storing Red, Green, Blue amounts for any color (3x8bits = 24 bit color, sometimes stored as 32-bit color) used for monitor output

Indexed Color - Format for storing a small number of colors in a palette (Index) 8-bit color, = 256 different values (0 255)

CLUT - Color Lookup Table (or Palette) - a table used to choose which of 256 colors are used in 8-bit, Indexed color Every computer has a built-in (System) CLUT; else the image has to provide it

CMYK - Format for storing amounts of Cyan, Magenta, Yellow and Black ink for color printing

File formats

GIF - file format treated as TEXT, 256 colors, (8 bit), CLUT is required), for the Web

JPG - JPEG file format treated as Binary, millions of colors, lossy-compressed, for the Web

TIF - TIFF file format, Binary, cross-platform, not for the Web

BMP - bitmap file format, binary, generally considered windows-only but manipulable in some mac programs not for web

PNG - non-lossy compressed file format specifically designed for Web publishing PNG-8 supports 1-bit alpha (transparency) with 256 CLUT - functionally equivalent to non-animated GIF but often with a smaller file size PNG-24 supports 8-bit alpha (transparency) without CLUT

RTL - 'Raster Transfer Language' file format optimized for color plotters

PICT - Apple-only file format

PCX - Windows-only file format