SYNOPSISra_ppm [ -r ][ -a ][ -b ][ -s maxv ][ -g gamma ][ -e +/-stops ] [ input [ output ] ]
DESCRIPTIONRa_ppm converts between RADIANCE and Poskanzer Portable Pixmap formats. The -g option specifies the exponent used in gamma correction; the default value is 2.2. An exponent of 1 turns gamma correction off. The -e option specifies an exposure compensation in f-stops (powers of two). Only integer stops are allowed, for efficiency. The -r option invokes a reverse conversion, from a Pixmap to a RADIANCE picture. If the output file is missing, the standard output is used. If the input file is missing as well, the standard input is used.
The -a option produces a standard ASCII Pixmap representation instead of the default binary file. The file is much larger and the conversion is much slower, which is why this format is not normally used. The -b option forces greyscale output. The -s option controls the output scale, which is 255 by default. If this value is set above 255, then two bytes will be output for each component in binary mode. This may not be understood by some PPM readers, which do not understand files with maximum values greater than 255. The maximum allowed setting for this parameter is 65535.
With the -r option, the type of the Pixmap input file is determined automatically. Ra_ppm will read either greyscale or color Pixmaps, with any precision up to a maximum scale of 65535.