3 Grey scale decorrelation, edge enhancement

Grey scale enhancement

One of the most basic enhancement techniques is the contrast stretch. If the screen is set to display 256 grey scale colours, band3 in leic-tm92 will appear dark, as most values are clustered around the mean value of 29, with the overall values ranging from approximately 21 to 55, taking up only 34 different values or 14% of the full value range (0-255) that can be utilised.

Using stretch will distribute the pixel values according to the algorithm used so that the whole value range is used. In doing so the algorithm leaves the pixel value unchanged, but is assigned a display value in a lookup table (LUT). As an example, the pixel value at (276, -230) is 23 in the image; the LUT value is 23 without stretch and 47 with stretch applied.

Decorrelation stretch

The effect of decorrelation stretch is clearly visible in the following images. The effect of this stretch is greater colour differentiation, clearly seen in the histograms, where the pixel values are grouped and distinctively separated from each other.

Histogram – Without decorrelation stretch

Histogram – With decorrelation stretch

Image – Without decorrelation stretch

Image – With decorrelation stretch

Edge enhancement

The “Crisp” function passes an edge-detecting or “high-pass” filter across the image. A high-pass is a sharp change in the grey scale value at a particular pixel point, and may be interpreted as features of the built environment, roads or field boundaries. In the combined image below the right part shows more distinguishable features than the left part, which has not been enhanced. As can be seen in the upper right corner of each picture, differences in neighbouring pixel values make an impact on the crisp picture, whereas the area is more or less consistently blurry green in the original image.

(image to be posted)

References

Mather, P. (1999) Computer Processing of Remotely-Sensed Images, An Itroduction, 2nd ed.

Lillesand, T.M., and Keifer, R. W. (1994) Remote Sensing and Image Interpretation, 3rd ed

Jones, C. (1997) Geographical Information Systems and Computer Cartography

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