60 
to the input channels. The selected bands are then loaded into memory. For display, the 
three input bands with a total of 24 bits are compressed into one 8-bit composite image. 
Both, the original image with three bands and the composite image are stored in memory 
(Figure 5). This ensures that image processing can be performed on the original image 
data whereas the composite image is only needed for display purposes. To display one 
spectral band only, all three colors can be assigned to the selected band. This mode 
allows the full 8-bit range to be used for monochrome display (256 gray levels). 
Green Image 
8 bits 
Blue Image 
8 bits 
Composite Image 
8 bits 
(— 
\ 
Display 
V 
) 
Figure 5: Composite image generation. 
Application Programs 
All application programs follow strictly the Dirigo user interface design and can be 
executed by using the appropriate pull-down menu or dialog box. The menus include 
(1) point operations such as linear and Gaussian contrast stretch and histogram 
equalization; (2) spatial filtering such as moving average, edge enhancement and 
extraction, and median and mode filtering; (3) geometric correction such as image-to- 
map rectification and image-to-image registration using first-degree polynomials and 
standard resampling techniques; and (4) classification techniques such as parallelepiped, 
minimum distance and maximum likelihood (Figure 6). Execution times for a 512 x 512 
image with three bands range from seconds to approximately five minutes for a 
maximum likelihood classification with six classes. In all instances, a color bar 
provides feedback to the user about the progress of the selected application program. 
CONCLUSION 
The course on digital image processing in remote sensing with the design of an 
interactive image processing system on the Macintosh-II turned out to be more 
successful than anybody involved in the course had hoped for. The students were