DIGITAL IMAGE PROCESSING SUMMARY
Jet Propulsion Laboratories

   A digital image is a picture converted to numerical form so that 
   it can be stored and used in a computer. The image is divided 
   into a matrix of small regions called picture elements, or 
   pixels. The rows and columns of pixels are called "lines" and 
   "samples", respectively. 

   Each pixel has a numerical value, or DN (data number) value, 
   quantifying the darkness or brightness of the image at that spot. 
   In total, each pixel has an address (line number, sample number) 
   and a DN value, which is all that the computer needs for 
   processing. The DN value of each pixel usually represents a shade 
   of darkness or brightness between black and white (gray levels). 
   How many gray levels there are in an image depends on the number 
   of bits used to represent each pixel intensity in the computer. 
   The number of gray levels will be equal to 2 to the nth power, 
   where n is the number of bits per pixel's DN value. If eight bits 
   are used to represent a pixel's DN value (gray level), the system 
   will be capable of using 2 to the 8th power or 256, gray levels 
   in an image, where DN 0 is pure black, and DN 255 is pure white. 
   If each DN used only four bits of storage, the image would 
   contain only 2 to the 4th power or 16 gray levels; if there were 
   only one bit per DN, the image would contain only black and white 
   pixels (bit values of 0 or 1).

   Gray Level and False Color Images

   Most images are composed of 8-bit DN values representing 
   monochrome brightness levels in the scene. To obtain color 
   images, separate images are taken through color filters (red, 
   green, blue) and are then combined by ground processing systems 
   to produce a true color image. Only a very small fraction of 
   images are available in color versions. Most display and analysis 
   is done on monochrome images.

   A display with 256 (2 to the 8th power) gray levels is required 
   to present the information contained in a standard image. 
   However, the human eye can only distinguish about 32 gray levels. 
   Thus 5-bit DN values would satisfy most display requirements. 
   Unfortunately, computers are oriented to the storage of and 
   manipulation of items which are a power of two, and 5-bit pixels 
   would be very clumsy. The display devices which the IMDISP 
   program supports are limited to 16 gray levels (PGA), four gray 
   levels (EGA), or two gray levels (CGA). The 16 gray levels of the 
   PGA are adequate to support image analysis; however, four or two 
   gray levels are practically useless for viewing images. 

   Fortunately, the EGA display will support 16 different colors; 
   and a color palette can be selected which uses a graduated scale 
   of colors to represent gray levels, producing a "false color" 
   image (false because the displayed color does not represent the 
   actual color of the scene). 

   A false color image (also called a pseudo color image) is created 
   from a black and white image by assigning a color (rather than a 
   gray level) to each DN value in the image. For instance, a DN 
   value of 128 could be reassigned to yellow if the user so 
   desired. Ranges of DN's (e.g., 100-125) may also be assigned one 
   color. Pseudo colors get assigned to DN values in a pseudo color 
   table, which the display program then uses to determine how to 
   color the image on the monitor. 

   Display programs often have preset pseudo color tables with 
   commonly used DN-color combinations, and the user can simply call 
   for one of these when generating a pseudo color image. The option 
   exists, of course, for users to generate their own pseudo color 
   tables. Pseudo colors are often used to highlight features of an 
   unusual nature in an image.

   Since the Enhanced Graphic Adapter (EGA) for the IBM PC provides 
   only four gray shades (black, dark gray, light gray, and white), 
   most image viewing is done using a pseudo color table which 
   interprets gray levels as color values ranging from black through 
   reds, greens, and blues up to white.

   This is only an introduction to digital image processing.  Many 
   of these activities were written to familiarize the investigator 
   with remote sensing and image processing techniques.  As these 
   activities are completed more knowledge is attained and more 
   skills are perfected.