Ppmshadow User Manual(0) Ppmshadow User Manual(0)
ppmshadow - add simulated shadows to a PPM image
ppmshadow [-b blur_size] [-k] [-t] [-x xoffset] [-y yoffset] [ppmfile]
This program is part of Netpbm(1)
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
color of the object which casts it. You can specify the crispness of the shadow and its
displacement from the image with command line options.
ppmshadow sees your image as a foreground on a background. The background color is what-
ever color the top left pixel of your image is. The background is all the pixels that are
that color and the foreground is everything else. The shadow that ppmshadow generates is
a shadow of the foreground, cast on the background.
The shadow is the same size as the foreground, plus some fringes as determined by the -b
option. It is truncated to fit in your image. The output image is the same dimensions as
the input image.
You can use pamcomp to place a foreground image over a background before running ppmshadow
on it. You can use ppmmake to make the background image (just an image of a solid color).
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 is the number of pixels of
fringe there is on the shadow, beyond where the shadow would be if there were no
The default is 11 pixels.
Note that this option controls only the fringing effect of moving the light source
closer to the object. It does not make the shadow grow or shrink as would happpen
in the real world if you moved a point light source closer to and further from an
-k Keep the intermediate temporary image files. When debugging, these intermediate
files provide many clues as to the source of an error. See below <#files> 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 color rather than a black shadow, which is default. This often results
in fuzzy, difficult-to-read images but in some circumstances may look better.
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.
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.
Input is a PPM file named by the ppmfile command line argument; if you don't specify ppm-
file, the input is Standard Input.
The output is a PPM file, written to Standard Output.
ppmshadow creates a number of temporary files as it executes. It creates a new directory
for them, /tmp/ppmshadowpid, where pid is the process ID of the ppmshadow process. If the
TMPDIR environment variable is set, ppmshadow creates the directory there instead of /tmp.
In normal operation, ppmshadow deletes each temporary file as soon as it is done with it
and leaves no debris around after it completes. To preserve the intermediate files for
debugging, use the -k command line option.
The temporary files are:
A copy of the input.
Positive binary mask
Convolution kernel for blurring shadow
Blurred, colored shadow image
Blurred shadow image before coloring
Clipped shadow image, offset as requested
Blank image with background of source image
Inverse mask file
Just the foreground. Rest is black. Original image times inverse mask.
Generated shadow times positive mask
Everything but the foreground (foreground area is black).
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 com-
mands 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 file in the Netpbm source tree for additional details and examples
of the intermediate files and debugging ppmshadow.
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
color 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 convolu-
tion kernel pnmconvol could use was 11 by 11, ppmshadow includes a horrid, CPU-time-burn-
ing kludge which, if a blur of greater than 11 is requested, performs an initial convolu-
tion with an 11 x 11 kernel, then calls pnmsmooth (which is itself a program that calls
pnmconvol with a 3 x 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 pamscale 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.
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.
pnm(1) , pnmmargin(1) , pnmconvol(1) , pamscale(1) , pnmsmooth(1) , ppm(1)
John Walker http://www.fourmilab.ch <http://www.fourmilab.ch> August 8, 1997
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.
netpbm documentation 17 April 2005 Ppmshadow User Manual(0)