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perry |
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ppmnorm |
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!!!ppmnorm |
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NAME |
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SYNOPSIS |
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DESCRIPTION |
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OPTIONS |
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SEE ALSO |
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AUTHOR |
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---- |
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!!NAME |
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ppmnorm - normalize the contrast in a portable pixmap |
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!!SYNOPSIS |
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__ppmnorm__ [[__-bpercent__ ''N'' | __-bvalue__ |
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''N''] [[__-wpercent__ ''N'' | __-wvalue__ |
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''N''] [[''ppmfile''] |
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!!DESCRIPTION |
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Reads a portable pixmap as input. Normalizes the contrast by |
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forcing the lightest pixels to white, the darkest pixels to |
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black, and linearly rescaling the ones in between; and |
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produces a portable pixmap as output. |
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Works by computing the realtive grey-level of each pixel |
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''a'la'' ppmtopgm, and uses those values to scale the RGB |
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levels. Note that this is different from using pgmnorm on |
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the individual red, green, and blue greymaps (as produced by |
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ppmtorgb3) and recombining them. |
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!!OPTIONS |
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By default, the darkest 2 percent of all pixels are mapped |
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to black, and the lightest 1 percent are mapped to white. |
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You can override these percentages by using the |
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__-bpercent__ and __-wpercent__ flags, or you can |
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specify the exact pixel values to be mapped by using the |
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__-bvalue__ and __-wvalue__ flags. Appropriate numbers |
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for the flags can be gotten from the ''ppmhist'' tool. If |
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you just want to enhance the contrast, then choose values at |
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elbows in the histogram; e.g. if value 29 represents 3% of |
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the image but value 30 represents 20%, choose 30 for |
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''bvalue''. If you want to lighten the image, then set |
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''bvalue'' to 0 and just fiddle with ''wvalue''; |
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similarly, to darken the image, set ''wvalue'' to maxval |
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and play with ''bvalue''. |
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All flags can be abbreviated to their shortest unique |
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prefix. |
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!!SEE ALSO |
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pgmnorm(1), ppmhist(1), ppm(5) |
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!!AUTHOR |
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Wilson H. Bent. Jr. (whb@usc.edu), heavily based on the |
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pgmnorm filter by Jef Poskanzer. |
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---- |
