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