NAME

       pcomb - combine RADIANCE pictures

SYNOPSIS

       pcomb [ -h ][ -w ][ -x xres ][ -y yres ][ -f file ][ -e expr ] [ [ -o ][ -s factor ][ -c r g b ] input ..
       ]

DESCRIPTION

       Pcomb  combines  equal-sized  RADIANCE pictures and sends the result to the standard output.  By default,
       the result is just a linear combination of the input pictures multiplied by -s and -c  coefficients,  but
       an arbitrary mapping can be assigned with the -e and -f options.  Negative coefficients and functions are
       allowed, and pcomb will produce color values of zero where they would be negative.

       The  variables ro, go and bo specify the red, green and blue output values, respectively.  Alternatively,
       the single variable lo can be used to specify a  brightness  value  for  black  and  white  output.   The
       predefined  functions ri(n), gi(n) and bi(n) give the red, green and blue input values for picture n.  To
       access a pixel that is nearby the current one, these functions also accept optional x and y offsets.  For
       example, ri(3,-2,1) would return the red component of the pixel from picture 3 that is left 2  and  up  1
       from  the  current  position.   Although x offsets may be as large as width of the picture, y offsets are
       limited to a small window (+/- 32 pixels) due to efficiency considerations.  However, it is  not  usually
       necessary to worry about this problem -- if the requested offset is not available, the next best pixel is
       returned instead.

       For  additional  convenience,  the function li(n) is defined as the input brightness for picture n.  This
       function also accepts x and y offsets.

       The constant nfiles  gives  the  number  of  input  files  present,  and  WE  gives  the  white  efficacy
       (lumens/brightness) for pixel values, which may be used with the -o option or the le(n) values to convert
       to  absolute photometric units (see below).  The variables x and y give the current output pixel location
       for use in spatially dependent functions, the constants xmax and ymax give the input resolution, and  the
       constants  xres  and  yres  give  the  output resolution (usually the same, but see below).  The constant
       functions re(n), ge(n), be(n), and le(n) give the exposure values for picture  n,  and  pa(n)  gives  the
       corresponding  pixel  aspect  ratio.   Finally,  for  pictures with stored view parameters, the functions
       Ox(n), Oy(n) and Oz(n) return the ray origin in world coordinates for the current pixel in picture n, and
       Dx(n), Dy(n) and Dz(n) return the normalized ray direction.  In addition, the function T(n)  returns  the
       distance  from  the origin to the aft clipping plane (or zero if there is no aft plane), and the function
       S(n) returns the solid angle of the current pixel in steradians (always zero for parallel views).  If the
       current pixel is outside the view region, T(n) will return a negative value, and S(n) will return zero.

       The -h option may be used to reduce the information header size, which can grow disproportionately  after
       multiple  runs  of pcomb and/or pcompos(1).  The -w option can be used to suppress warning messages about
       invalid calculations.  The -o option indicates that original pixel values are to be  used  for  the  next
       picture, undoing any previous exposure changes or color correction.

       The  -x  and  -y  options can be used to specify the desired output resolution, xres and yres, and can be
       expressions involving other constants such as xmax and ymax.  The constants xres and  yres  may  also  be
       specified in a file or expression.  The default output resolution is the same as the input resolution.

       The  -x and -y options must be present if there are no input files, when the definitions of ro, go and bo
       will be used to compute each output pixel.  This is useful for producing simple test pictures for various
       purposes.  (Theoretically, one could write a complete renderer using just the functional language...)

       The standard input can be specified with a hyphen ('-').  A command that produces a RADIANCE picture  can
       be given in place of a file by preceeding it with an exclamation point ('!').

EXAMPLES

       To produce a picture showing the difference between pic1 and pic2:

         pcomb -e 'ro=ri(1)-ri(2);go=gi(1)-gi(2);bo=bi(1)-bi(2)' pic1 pic2 > diff

       Or, more efficiently:

         pcomb pic1 -s -1 pic2 > diff

       To precompute the gamma correction for a picture:

         pcomb -e 'ro=ri(1)^.4;go=gi(1)^.4;bo=bi(1)^.4' inp.hdr > gam.hdr

       To perform some special filtering:

         pcomb -f myfilt.cal -x xmax/2 -y ymax/2 input.hdr > filtered.hdr

       To make a picture of a dot:

         pcomb -x 100 -y 100 -e 'ro=b;go=b;bo=b;b=if((x-50)^2+(y-50)^2-25^2,0,1)' > dot

AUTHOR

       Greg Ward

SEE ALSO

       getinfo(1), icalc(1), pcompos(1), pfilt(1), rpict(1)

RADIANCE                                             8/31/96                                            PCOMB(1)