User Manual - pancroma

Now select ‘Pre Process’ | ‘Histogram Match’. The image histogram of theAdjust image will be matched to the Reference image. The Adjust image will bedisplayed and can be saved as usual for subsequent processing by PANCROMAor using another program if desired.Histogram EqualizationHistogram equalization improves the contrast of an image by mapping a narrowdistribution of grayscale values to one that spans the entire grayscale colorspace. (This is also known as linear contrast stretch). This operation isconducted on a single image. Select ‘File’ | ‘Open’ and open your file. Nowselect ‘Pre Process’ | ‘Histogram Equalization’. The input image will be adjustedand displayed and can be saved as usual. Histogram equalization can oftenimprove the detail of a dark or muddy image. However it will have a powerfuleffect on the color balance of the image if used for subsequent processing andmay produce unwanted effects. Histogram equalization is frequently needed forprocessing Digital Globe imagery.Histogram StretchingOften equalizing the histogram over the entire span of grayscale pixel levels hasa very disruptive effect on image color balance. This is especially true if you areaiming to produce natural color RGB or pan sharpened images. In this case, it isoften useful to stretch the histogram in a non-linear manner so that the pixelsclosest to the mean are displaced less than those further from the mean. Theoverall effect is heightened contrast without the disruptive effect on colorbalance. To histogram stretch an image, Select ‘File’ | ‘Open’ and open your file.Now select ‘Pre Process’ | ‘Histogram Stretch’. A Histogram Form will bedisplayed that lets you input the Histogram Scale Factor using a track bar. Thegreater the Scale Factor, the greater the degree of stretch. The stretched imagewill be computed as with equalization and the stretched image displayed. Worksfor grayscale and color images.Gamma CorrectionGamma correction originated in computer graphics in compensating for the nonlinearresponse of cathode ray tube (CRT) terminals to power inputs. Since theCRT response was a power function rather than a linear response, an inversepower function was used to correct it. Gamma correction controls the overallbrightness of an image. Images, which are not properly corrected, can look eitherbleached out, or too dark. Trying to reproduce colors accurately also requiressome knowledge of gamma. Varying the amount of gamma correction changesnot only the brightness, but also the ratios of red to green to bluePANCROMA TM can apply an inverse gamma function (gamma correction) toeither a single grayscale band or alternatively a color RGB image. To do so169