ppmshadow - add simulated shadows to a portable pixmap image
ppmshadow [-b blur_size] [-k] [-t] [-x xoffset] [-y yoffset] [-u] [pnmfile]
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.
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.
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 an anymap named by the pnmfile command line argument; if you don't specify pnm-
file, 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:
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
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 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 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 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 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.
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(5), pnmmargin(1), pnmconvol(1), pnmscale(1), pnmsmooth(1), ppm(5)
John Walker <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.
12 March 2000 ppmshadow(1)