
@C1@c@b@_ External Filters

     Pipe the current image thru an external filter and replace the current
     image with the filtered one.

@b  Definitions
     A filter is defined as an external program that reads at least one
     of the BIT supported formats from argv[1] and output the (possibly)
     transformed image to stdout. All external filter definitions must 
     start with the at (@) sign and any line that does not is silently
     ignored.

     A filter definition should have the following format
@f    @title; name; options; in1;[in2;[in3;]]
     where title is the text to use to label the activation button and name
     and options are the command and option to use in the popen(3S)
     system call; in{[1-3]} is the accepted formats for the program. 
     If the option field contains a single question mark "?", a string 
     will be prompted before the external program is run. If title is empty,
     the program name is used to label the button.  The following is a valid
     filter definition

@f   @rotate45 ; pnmrotate; 45 ; ppm; pgm; pbm;

     Also See /usr/local/lib/bit/BIT_filters for some examples

@b File Locations
     The external filter definition should be in a file named BIT_filters. 
     The program will search the system directory, /usr/local/lib/bit,
     current directory, and ~/.bitrc/ (or environment variable BITDIR)
     in that order for BIT_filters. 

@b Special Note
     The input format should be in order of decreasing generality,
     i.e., ppm; pgm; pbm. 


