NAME

       ppmshadow - add simulated shadows to a portable pixmap image

SYNOPSIS

       ppmshadow [-b blur_size] [-k] [-t] [-x xoffset] [-y yoffset] [-u] [pnmfile]

DESCRIPTION

       ppmshadow adds a simulated shadow to an image, giving the appearance that the contents of the image float
       above  the  page,  casting  a  diffuse shadow on the background.  Shadows can either be black, as cast by
       opaque objects, or translucent, where the shadow takes on the colour of the object which casts  it.   You
       can specify the extent of the shadow and its displacement from the image with command line options.

OPTIONS

       -b blur_size

              Sets the distance of the light source from the image.  Larger values move the light source closer,
              casting  a  more  diffuse  shadow,  while smaller settings move the light further away, yielding a
              sharper shadow.  blur_size defaults to 11 pixels.

       -k     Keep the intermediate temporary image files.  When debugging,  these  intermediate  files  provide
              many clues as to the source of an error.  See FILES below for a list of the contents of each file.

       -t     Consider  the  non-background  material  in  the  image translucent -- it casts shadows of its own
              colour rather than a black shadow, which is default.  This often results in  fuzzy,  difficult-to-
              read images but in some circumstances may look better.

       -u     Print command syntax and a summary of options.

       -x xoffset
              Specifies  the  displacement  of  the  light  source to the left of the image.  Larger settings of
              xoffset displace the shadow to the right, as would be cast by a light further to the left.  If not
              specified, the horizontal offset is half of blur_size (above), to the left.

       -y yoffset
              Specifies the displacement of the light source above  the  top  of  the  image.   Larger  settings
              displace  the  shadow  downward,  corresponding  to  moving the light further above the top of the
              image.  If you don't specify -y, the vertical offset defaults to the same as the horizontal offset
              (above), upward.

FILES

       Input is an anymap named by the pnmfile command line argument; if you don't specify pnmfile, the input is
       the Standard Input file.

       Output is a always a PPM file, written to Standard Output.

       pnmfile creates a number of temporary files as it executes.  It creates them in the /tmp directory,  with
       names of the form:

       _PPMshadowpid-N.ppm

       where  pid  is  the  process  number  of  the ppmshadow process and N is a number identifying the file as
       described below.  In normal operation, ppmshadow deletes temporary files as soon as it is done with  them
       and  leaves  no  debris around after it completes.  To preserve the intermediate files for debugging, use
       the -k command line option.

       N in the filename means:

       1      Positive binary mask

       2      Convolution kernel for blurring shadow

       3      Blurred shadow image

       4      Clipped shadow image, offset as requested

       5      Blank image with background of source image

       6      Offset shadow

       7      Inverse mask file

       8      Original image times inverse mask

       9      Generated shadow times positive mask

       10     Shadow times background colour

LIMITATIONS

       The source image must contain sufficient space on the edges in the direction in which the shadow is  cast
       to  contain  the shadow -- if it doesn't some of the internal steps may fail.  You can usually expand the
       border of a too-tightly-cropped image with pnmmargin before processing it with ppmshadow.

       Black pixels and pixels with the same color as the image background don't cast a shadow.  If this  causes
       unintentional  "holes"  in  the shadow, fill the offending areas with a color which differs from black or
       the background by RGB values of 1, which will be imperceptible to the viewer.  Since  the  comparison  is
       exact, the modified areas will now cast shadows.

       The  background color of the source image (which is preserved in the output) is deemed to be the color of
       the pixel at the top left of the input image.  If that pixel isn't part of the background, simply  add  a
       one-pixel border at the top of the image, generate the shadow image, then delete the border from it.

       If  something  goes  wrong  along  the way, the error messages from the various Netpbm programs ppmshadow
       calls will, in general, provide little or no clue as to where  ppmshadow  went  astray.   In  this  case,
       Specify  the  -k  option  and  examine the intermediate results in the temporary files (which this option
       causes to be preserved).  If you manually run the commands that ppmshadow runs on these  files,  you  can
       figure  out where the problem is.  In problem cases where you want to manually tweak the image generation
       process along the way, you can keep the intermediate files with the -k option, modify them  appropriately
       with  an  image  editor,  then  recombine  them  with  the  steps used by the code in ppmshadow.  See the
       ppmshadow.doc document for additional details and examples of the intermediate files.

       Shadows are by default black, as cast by opaque material in the image occluding white light.  Use the  -t
       option  to  simulate  translucent material, where the shadow takes on the colour of the object that casts
       it.  If the contrast between  the  image  and  background  is  insufficient,  the  -t  option  may  yield
       unattractive results which resemble simple blurring of the original image.

       Because  Netpbm  used  to  have  a maximum maxval of 255, which meant that the largest convolution kernel
       pnmconvol could use was 11 by 11, ppmshadow includes a horrid, CPU-time-burning kludge which, if  a  blur
       of  greater  than  11  is  requested,  performs  an  initial convolution with an 11×11 kernel, then calls
       pnmsmooth (which is actually a script that calls pnmconvol with a  3×3  kernel)  as  many  times  as  the
       requested  blur exceeds 11.  It's ugly, but it gets the job done on those rare occasions where you need a
       blur greater than 11.

       If you wish to generate an image at high resolution, then scale it to publication size with  pnmscale  in
       order  to  eliminate  jagged  edges  by resampling, it's best to generate the shadow in the original high
       resolution image, prior to scaling it down in size.  If you scale first and then add the  shadow,  you'll
       get  an  unsightly  jagged  stripe  between  the edge of material and its shadow, due to resampled pixels
       intermediate between the image and background obscuring the shadow.

EXIT STATUS

       ppmshadow returns status 0 if processing was completed without errors, and a nonzero Unix error  code  if
       an  error  prevented  generation  of  output.   Some  errors  may  result in the script aborting, usually
       displaying error messages from various Netpbm components it uses, without returning a nonzero error code.
       When this happens, the output file will be empty, so be sure to test this if you  need  to  know  if  the
       program succeeded.

SEE ALSO

       pnm(5), pnmmargin(1), pnmconvol(1), pnmscale(1), pnmsmooth(1), ppm(5)

AUTHOR

       John Walker <http://www.fourmilab.ch> August 8, 1997

COPYRIGHT

       This software is in the public domain.  Permission to use, copy, modify, and distribute this software and
       its  documentation  for  any  purpose  and  without  fee  is  hereby  granted,  without any conditions or
       restrictions.

                                                  12 March 2000                                     ppmshadow(1)