5-4 Image Enhancement

Image enhancement is conversion of the original imagery to a better understandable level in spectral quality for feature extraction or image interpretation.

The following techniques are typical in image enhancement.

Gray scale conversion (see Figure 5.8.)
Contrast stretch (linear), fold conversion, saw conversion etc. are involved.

Histogram conversion (see Figure 5.9.)
The histogram of the original image is converted to other types of histogram as specified by users. Histogram equalization with flat histogram or linear cumulative histogram and histogram normalization with normalized histogram are popular.

Color composition
Color composition is the assignment of three primary colors; red (R), green (G) and blue (B) to three selected bands from multispectral bands usually available for satellite remote sensing image data.

Table 5.1 shows several color compositions and their characteristics.

Color composition is demonstrated in the front pages of this book.

Color Conversion
Though R, G and B are primary colors with more convenience for computer processing, three color elements of hue (H), intensity (I) and saturation (S) are more easily understood by human visual sense. Color conversion between RGB and HIS color system will be useful to obtain better color quality or variety for interpretation. Multi-sensor fusion, for example, with optical multi-spectral bands and another sensor’s black and white image such as SAR (synthetic aperture radar) or high resolution panchromatic band is implemented by replacing I (intensity) by B/W band after color conversion from RGB (only with optical multi-bands) to HIS.