ppmshadow


SYNOPSIS
       ppmshadow [-b blur_size] [-k] [-t] [-x xoffset] [-y yoffset] [-u] [pnm-
       file]



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, corre-
              sponding 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.

       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 intermedi-
       ate 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 pro-
       cessing 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 var-
       ious 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
       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  11x11 kernel, then calls pnmsmooth (which is actually a script that
       calls pnmconvol with a 3x3 kernel) as many times as the requested  blur
       exceeds  11.   It's  ugly, but it gets the job done on those rare occa-
       sions 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 reso-
       lution 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 out-
       put.  Some errors may result in the script aborting, usually displaying
       error  messages from various Netpbm components it uses, without return-
       ing 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 suc-
       ceeded.


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,  mod-
       ify, and distribute this software and its documentation for any purpose
       and without fee is hereby granted, without any conditions  or  restric-
       tions.



                                 12 March 2000                    ppmshadow(1)
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