Density slicing is also examined. We move now to two of the most common image processing routines for improving scene quality. These routines fall into the descriptive category of Image Enhancement or Transformation. We used the first image enhancer, contrast stretching , on most of the TM images we have looked at so far in the previous 11 pages to enhance their pictorial quality . Different stretching options are described next, followed by a brief look at density slicing We will then evaluate the other routine, filtering , shortly.
Contrast stretching involves altering the distribution and range of DN values.
This is done by changing one or more variables in the photographic process, such as , the type of recording film, paper contrast, developer conditions, etc. Linear expansion of DN's into this full scale is a common option. Other stretching functions are available for special purposes. Sinusoidal Linear with Saturation. Histogram Equalization is a stretch that favorably expands some parts of the DN range at the expense of others by dividing the histogram into classes containing equal numbers of pixels.