User Manual - pancroma

24. Histogram Matching________________________________________________________________PANCROMA TM creates a pan sharpened image by combining three lowerresolution multispectral (RGB) bands with the higher resolution panchromaticband. It does this by transforming the RGB bands to the corresponding hue,saturation and intensity bands. The (computed) intensity band is discarded andThe H and S bands are doubled and interpolated. The panchromatic band isthen substituted in its place and a reverse transformation back to RGB colorspace is performed (HSI method). Although this often yields good results, somepan sharpened images may not look very pleasing and may need extrapreprocessing to create an acceptable image.One of the reasons that the pan sharpened image may not look the same as thecorresponding RGB color composite image is that the panchromatic bandfundamentally differs from the 'I' or computed intensity band. The Intensity bandcan be computed from the low resolution band files, along with the Hue andSaturation bands. Reverse transforming the Intensity band would yield theoriginal RGB image. In pan sharpening, the Intensity band is discarded and isreplaced with the panchromatic band.Problems can arise because the panchromatic image is acquired using its owndedicated sensor sensitive to a sometimes translated (toward the IR wavelengthsfor Landsat) spectrum of radiated light, and is in no way related mathematically tothe Intensity band that it replaces. The result of the replacement, necessary forpan sharpening to work, can be a pan sharpened image that has morepronounced blue color tones in green vegetated areas. A technique calledhistogram matching can be used to mitigate the problem by adjusting thepanchromatic image histogram to more closely match that of the computedIntensity histogram.In order to histogram match one image to another, a discrete transformationfunction is computed that when applied to the panchromatic band, adjusts eachpixel to a more appropriate grayscale level (more appropriate meaning a closermatch to the computed Intensity band.) Better color tones often result when thetransformed (i.e. histogram matched) panchromatic band is fused in a pansharpening algorithm. PANCROMA TM provides two histogram methods: nonlinearand linear. The non linear transform usually produces better results,although this is not always true.Non linear histogram matching is enabled as the default condition when pansharpening images with PANCROMA TM . You can select between the twomethods, and disable histogram matching entirely using the radio buttons ‘NonLinear Histogram Adjust’; ‘Linear Histogram Adjust’ and ‘No Histogram Adjust’ in56