This file is a concatenation of two HTML files that John Walker wrote for the
original 'pnmshadow' program. It is not all relevant to the current
'ppmshadow' program.
pnmshadow: How it Works
This document describes the process, including PBMplus commands
and the intermediate images they create, by
which pnmshadow
adds black shadows to source images.
A companion document
describes how translucent shadows are created when the
-t option is specified.
The Starting Point
Let's start with the following source image, 536 pixels wide and 141
pixels high. We convert the image from whatever form in which
it was originally created (GIF, JPEG, etc.) to a PPM file before
processing it with pnmshadow.
The Blank Background
We start by determining the size of the input image with
pnmfile and then constructing an image with the same
size as the input image consisting entirely of the background
color, which is defined as the color of the pixel at the upper
left corner of the source image. This is performed by the
command:
pnmcut 0 0 1 1 ifile | pnmscale -xsize xsize -ysize ysize >fname-5.ppm
yielding the image:
The Positive Mask
A positive mask image is created in which all pixels of the background
color are set to white and all other pixels are black. This is accomplished
by subtracting the blank background image from the input (using the
-difference option on pnmarith to avoid clipping at
zero or the maximum pixel value), then inverting the result and
thresholding it to a monochrome bitmap.
pnmarith -difference ifile fname-5.ppm | pnminvert | ppmtopgm | pgmtopbm -thresh -value 1.0 >fname-1.ppm
This produces the following mask image.
The Blurred Image
Since we wish to simulate a shadow from a nearby extended
light source rather than a sharp shadow as cast by the
Sun, we need to prepare a blurred version of the original
image. If the -t option is not specified on
pnmshadow the shadow cast by an object of any color
is always black, so the positive mask serves as the source image
when preparing the shadow. A convolution kernel which averages
the number of pixels specified by the -b option
(default 11), written into the temporary file fname-2.ppm
in ASCII PGM format, and then the blurred image is created with
the command:
pnmconvol fname-2.ppm fname-1.ppm >fname-3.ppm
With the default blur setting of 11 pixels, the blurred image
below is generated.
Shadow on Background Color
Having generated the blurred shadow from the monochrome mask image,
it will consist of pixels ranging from white to black on a white
background. In order to preserve the background color in the
original image, we multiply the shadow by the blank background color
image created previously. White pixels take on the background color
and pixels belonging to the shadow are scaled to be relative to the
background.
pnmarith -multiply fname-3.ppm fname-5.ppm >fname-10.ppm
This yields the following shadow, with the background of the
original image.
Offset Shadow Clip
Shadows, even bogus ones like we're generating, usually look best when
cast by a light source diagonally displaced from the centre of the
shadow-casting object. To achieve this effect, we first cut a
rectangle from the blurred shadow image reduced in size by the
the number of pixels specified by the -b option
which default to half the blur (-b) setting. The
xsize and ysize arguments in the following
command are the size of the input image in pixels less the shadow
displacement in the respective axis.
pnmcut 0 0 xsize ysize fname-10.ppm >fname-4.ppm
The shadow clip is the identical to the shadow on background color, but
smaller by the offset in each direction.
Offset Shadow
Now we're ready to assemble the shadow offset by the specified number
of pixels. We do this by pasting the image cut in the previous step
into the blank background, yielding an image the same size as the
source image with the blurred shadow displaced to the right and
down.
pnmpaste -replace fname-4.ppm xoffset yoffset fname-5.ppm >fname-6.ppm
This gives the following result:
Inverse Mask
In order to stitch everything together, we need an inverse of the
mask prepared earlier--one where black pixels represent the background
and all other material is white. This is easily accomplished by
running the positive mask through pnminvert:
pnminvert fname-1.ppm >fname-7.ppm
yielding:
Masked Input Image
Now we use the inverse mask prepared in the previous step to create
an image containing all non-background pixels from the source image,
with background pixels set to black. We simply multiply the
inverse mask by the source image:
pnmarith -multiply ifile fname-7.ppm >fname-8.ppm
et voilą:
Shadow with Source Masked
Our last intermediate step before joining the image with its
shadow is preparing a shadow image with all non-background pixels
in the source image set to black. This ensures that when we add
the image and the shadow, the shadow will not override any pixel
in the source image.
pnmarith -multiply fname-6.ppm fname-1.ppm >fname-9.ppm
This is accomplished by multiplying the shadow by the positive
mask image, which sets all non-background pixels in the source
image to black:
The Final Product
At long last, we're ready to put together the pieces and deliver
the result to our ever-patient user. This amounts simply to
adding the masked input image (consisting solely of non-background
pixels from the original image) to the shadow with source masked
(in which all source pixels are black):
pnmarith -add fname-8.ppm fname-9.ppm
The resulting image, with shadow, is as follows:
Smooth Operator
Since many computer graphics programs create sharp edges on
text, it's often best to create an image at a greater resolution
than that used for presentation, then scale it to the final
resolution with a tool which resamples the image, thus
minimising jagged edges by averaging adjacent
pixels. Using the output of pnmshadow as the starting
point and scaling to half size with pnmscale, we arrive at
the following smoothed image, with shadow, ready to adorn a
Web page:
by John Walker
August 8th, 1997
pnmshadow: How it Works in Translucent Mode
This document describes the process, including PBMplus commands
and the intermediate images they create, by
which pnmshadow
adds translucent shadows when the -t command line
option is specified. A companion document
describes how the default black shadows are generated.
The Starting Point
Let's start with the following source image, 536 pixels wide and 141
pixels high. We convert the image from whatever form in which
it was originally created (GIF, JPEG, etc.) to a PPM file before
processing it with pnmshadow.
The Blank Background
We start by determining the size of the input image with
pnmfile and then constructing an image with the same
size as the input image consisting entirely of the background
color, which is defined as the color of the pixel at the upper
left corner of the source image. This is performed by the
command:
pnmcut 0 0 1 1 ifile | pnmscale -xsize xsize -ysize ysize >fname-5.ppm
yielding the image:
The Positive Mask
A positive mask image is created in which all pixels of the background
color are set to white and all other pixels are black. This is accomplished
by subtracting the blank background image from the input (using the
-difference option on pnmarith to avoid clipping at
zero or the maximum pixel value), then inverting the result and
thresholding it to a monochrome bitmap.
pnmarith -difference ifile fname-5.ppm | pnminvert | ppmtopgm | pgmtopbm -thresh -value 1.0 >fname-1.ppm
This produces the following mask image.
The Blurred Image
Since we wish to simulate a shadow from a nearby extended
light source rather than a sharp shadow as cast by the
Sun, we need to prepare a blurred version of the original
image.
A convolution kernel which averages
the number of pixels specified by the -b option
(default 11), written into the temporary file fname-2.ppm
in ASCII PGM format, and then the blurred image is created with
the command:
pnmconvol fname-2.ppm ifile >fname-10.ppm
With the default blur setting of 11 pixels, the blurred image
below is generated.
Offset Shadow Clip
Shadows, even bogus ones like we're generating, usually look best when
cast by a light source diagonally displaced from the centre of the
shadow-casting object. To achieve this effect, we first cut a
rectangle from the blurred shadow image reduced in size by the
the number of pixels specified by the -b option
which default to half the blur (-b) setting. The
xsize and ysize arguments in the following
command are the size of the input image in pixels less the shadow
displacement in the respective axis.
pnmcut 0 0 xsize ysize fname-10.ppm >fname-4.ppm
The shadow clip is the identical to the shadow on background color, but
smaller by the offset in each direction.
Offset Shadow
Now we're ready to assemble the shadow offset by the specified number
of pixels. We do this by pasting the image cut in the previous step
into the blank background, yielding an image the same size as the
source image with the blurred shadow displaced to the right and
down.
pnmpaste -replace fname-4.ppm xoffset yoffset fname-5.ppm >fname-6.ppm
This gives the following result:
Inverse Mask
In order to stitch everything together, we need an inverse of the
mask prepared earlier--one where black pixels represent the background
and all other material is white. This is easily accomplished by
running the positive mask through pnminvert:
pnminvert fname-1.ppm >fname-7.ppm
yielding:
Masked Input Image
Now we use the inverse mask prepared in the previous step to create
an image containing all non-background pixels from the source image,
with background pixels set to black. We simply multiply the
inverse mask by the source image:
pnmarith -multiply ifile fname-7.ppm >fname-8.ppm
et voilą:
Shadow with Source Masked
Our last intermediate step before joining the image with its
shadow is preparing a shadow image with all non-background pixels
in the source image set to black. This ensures that when we add
the image and the shadow, the shadow will not override any pixel
in the source image.
pnmarith -multiply fname-6.ppm fname-1.ppm >fname-9.ppm
This is accomplished by multiplying the shadow by the positive
mask image, which sets all non-background pixels in the source
image to black:
The Final Product
At long last, we're ready to put together the pieces and deliver
the image to our ever-patient user. This amounts simply to
adding the masked input image (consisting solely of non-background
pixels from the original image) to the shadow with source masked
(in which all source pixels are black):
pnmarith -add fname-8.ppm fname-9.ppm
The resulting image, with shadow, is as follows:
Smooth Operator
Since many computer graphics programs create sharp edges on
text, it's often best to create an image at a greater resolution
than that used for presentation, then scale it to the final
resolution with a tool which resamples the image, thus
minimising jagged edges by averaging adjacent
pixels. Using the output of pnmshadow as the starting
point and scaling to half size with pnmscale, we arrive at
the following smoothed image, with shadow, ready to adorn a
Web page:
by John Walker
August 8th, 1997
