6.15     Calling VIPS functions
  You can call any VIPS operation which has the following
properties:
    
    - There must be at least 1 output argument. If there’s
    a single output argument, that becomes the value
    of the function. If there is more than one output,
    then the function returns a list with the outputs as
    members.
    
- The output arguments must all be one of:
        
        - IM_TYPE_DOUBLE,
        
- IM_TYPE_INT,
        
- IM_TYPE_COMPLEX,
        
- IM_TYPE_STRING,
        
- IM_TYPE_IMAGE,
        
- IM_TYPE_DOUBLEVEC,
        
- IM_TYPE_DMASK,
        
- IM_TYPE_IMASK
 
- The input arguments must all be one of the types
    above, or IM_TYPE_DISPLAY. If an argument is an
    input display, nip2 passes in its current display
    structure, it does not take a display from your
    program.
    
    When nip2 starts up, it loads any VIPS plug ins it can find
  on its data search path. You can call functions from plug ins
  in just the same way. For information on writing plug ins,
  see the VIPS Manual.