This un-enhanced visible image is available in 8-bit (0-255) display counts. This is the starting point for the three visible channel enhancements that will follow. An 8-bit count value is written above the gray bar on the bottom of all images.
|Un-enhanced VIS (ch.-1)|
Another means of enhancing VIS imagery is by histogram stretching. In this method a histogram of the image counts is made to determine the enhancement to be applied. The histogram is then divided into three sections: brightness counts which have a value below a dark-end threshold (some value greater than the minimum count); brightness counts which have a value above a bright-end threshold (some value less than the maximum count); and brightness counts lying between the dark and bright thresholds. Brightness counts which lie below the dark threshold are assigned a new count of 0. Similarly, counts which lie above the bright threshold are assigned a value of 255. The remaining values between the dark and bright threshold are assigned new counts between 0 and 255, according to a linear stretch. In this case the un-enhanced counts between 50 and 161 (compared to 37 and 220 in contrast stretching) are stretched to cover the 0-255 range displayed, using a single ramp of gray shades.
Histogram stretching eliminates low and high counts which are infrequently used, unlike contrast stretching. The remaining counts are stretched slightly more, thereby further enhancing some of the image features.
Finally, one can apply a fixed-range enhancement table to the base image. In this example, un-enhanced counts between a fixed minimum of 10 and a fixed maximum of 212 are stretched to cover the full gray scale range of 0 to 255. Values outside the original range are fixed at 0 and 255 for the lower and higher values, respectively. This enhancement may not work well for visible images where significant numbers of values fall outside the fixed minimum and maximum counts.