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| author | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-08-19 03:12:28 +0000 |
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| committer | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-08-19 03:12:28 +0000 |
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diff --git a/editor/ppmshadow.doc b/editor/ppmshadow.doc new file mode 100644 index 00000000..1539c708 --- /dev/null +++ b/editor/ppmshadow.doc @@ -0,0 +1,627 @@ +<html> +<head> +<title>pnmshadow: How it Works</title> + +</head> + +<body> + +<center> +<h1><img src="figures/how_title.jpg" width=417 height=116 alt="pnmshadow: How it Works"></h1> +</center> + +<hr> +<p> + +This document describes the process, including PBMplus commands +and the intermediate images they create, by +which <b><a href="./">pnmshadow</a></b> +adds black shadows to source images. +A <a href="how-t.html">companion document</a> +describes how translucent shadows are created when the +<b>-t</b> option is specified. + +<h3>The Starting Point</h3> + +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 <b>pnmshadow</b>. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadin.gif" width=536 height=141 alt="Input image"> +</table> +</center> + +<h3>The Blank Background</h3> + +We start by determining the size of the input image with +<b>pnmfile</b> 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: + +<p> +<pre> + pnmcut 0 0 1 1 <em>ifile</em> | pnmscale -xsize <em>xsize</em> -ysize <em>ysize</em> ><em>fname</em>-5.ppm +</pre> +<p> + +yielding the image: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt5.gif" width=536 height=141 alt="Blank background image"> +</table> +</center> + +<h3>The Positive Mask</h3> + +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 +<tt>-difference</tt> option on <b>pnmarith</b> to avoid clipping at +zero or the maximum pixel value), then inverting the result and +thresholding it to a monochrome bitmap. + +<p> +<pre> + pnmarith -difference <em>ifile</em> <em>fname</em>-5.ppm | pnminvert | ppmtopgm | pgmtopbm -thresh -value 1.0 ><em>fname</em>-1.ppm +</pre> +<p> + +This produces the following mask image. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt1.gif" width=536 height=141 alt="Positive mask image"> +</table> +</center> + +<h3>The Blurred Image</h3> + +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 <b>-t</b> option is not specified on +<b>pnmshadow</b> 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>-b</b> option +(default 11), written into the temporary file <tt><em>fname</em>-2.ppm</tt> +in ASCII PGM format, and then the blurred image is created with +the command: + +<p> +<pre> + pnmconvol <em>fname</em>-2.ppm <em>fname</em>-1.ppm ><em>fname</em>-3.ppm +</pre> +<p> + +With the default blur setting of 11 pixels, the blurred image +below is generated. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt3.jpg" width=536 height=141 alt="Blurred shadow"> +</table> +</center> + +<h3>Shadow on Background Color</h3> + +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. + +<p> +<pre> + pnmarith -multiply <em>fname</em>-3.ppm <em>fname</em>-5.ppm ><em>fname</em>-10.ppm +</pre> +<p> + +This yields the following shadow, with the background of the +original image. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt10.jpg" width=536 height=141 alt="Shadow with background color"> +</table> +</center> + +<h3>Offset Shadow Clip</h3> + +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>-b</b> option +which default to half the blur (<b>-b</b>) setting. The +<em>xsize</em> and <em>ysize</em> arguments in the following +command are the size of the input image in pixels less the shadow +displacement in the respective axis. + +<p> +<pre> + pnmcut 0 0 <em>xsize</em> <em>ysize</em> <em>fname</em>-10.ppm ><em>fname</em>-4.ppm +</pre> +<p> + +The shadow clip is the identical to the shadow on background color, but +smaller by the offset in each direction. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt4.jpg" width=531 height=136 alt="Offset shadow clip"> +</table> +</center> + +<h3>Offset Shadow</h3> + +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. + +<p> +<pre> + pnmpaste -replace <em>fname</em>-4.ppm <em>xoffset</em> <em>yoffset</em> <em>fname</em>-5.ppm ><em>fname</em>-6.ppm +</pre> +<p> + +This gives the following result: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt6.jpg" width=536 height=141 alt="Offset shadow"> +</table> +</center> + +<h3>Inverse Mask</h3> + +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 <b>pnminvert</b>: + +<p> +<pre> + pnminvert <em>fname</em>-1.ppm ><em>fname</em>-7.ppm +</pre> +<p> + +yielding: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt7.gif" width=536 height=141 alt="Inverse mask"> +</table> +</center> + +<h3>Masked Input Image</h3> + +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: + +<p> +<pre> + pnmarith -multiply <em>ifile</em> <em>fname</em>-7.ppm ><em>fname</em>-8.ppm +</pre> +<p> + +<em>et voilą:</em> + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt8.gif" width=536 height=141 alt="Masked Input Image"> +</table> +</center> + +<h3>Shadow with Source Masked</h3> + +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. + +<p> +<pre> + pnmarith -multiply <em>fname</em>-6.ppm <em>fname</em>-1.ppm ><em>fname</em>-9.ppm +</pre> +<p> + +This is accomplished by multiplying the shadow by the positive +mask image, which sets all non-background pixels in the source +image to black: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt9.jpg" width=536 height=141 alt="Shadow with Source Masked"> +</table> +</center> + +<h3>The Final Product</h3> + +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): + +<p> +<pre> + pnmarith -add <em>fname</em>-8.ppm <em>fname</em>-9.ppm +</pre> +<p> + +The resulting image, with shadow, is as follows: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadout.jpg" width=536 height=141 alt="Output: Image with shadow"> +</table> +</center> + +<h3>Smooth Operator</h3> + +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 <b>pnmshadow</b> as the starting +point and scaling to half size with <b>pnmscale</b>, we arrive at +the following smoothed image, with shadow, ready to adorn a +Web page: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadout2.jpg" width=268 height=71 alt="Half scale image with shadow"> +</table> +</center> + +<h4><a href="how-t.html">How it works with translucent shadows</a></h4> +<h4><a href="./"><b>pnmshadow</b> main page</a></h4> + +<p> +<hr> +<p> +<address> +by <a href="/">John Walker</a><br> +August 8th, 1997 +</address> + +</body> +</html> +<html> +<head> +<title>pnmshadow: How it Works in Translucent Mode</title> + +</head> + +<body> + +<center> +<h1><img src="figures/how_title-t.jpg" width=421 height=159 alt="pnmshadow: How it Works in Translucent Mode"></h1> +</center> + +<hr> + +<p> + +This document describes the process, including PBMplus commands +and the intermediate images they create, by +which <b><a href="./">pnmshadow</a></b> +adds translucent shadows when the <b>-t</b> command line +option is specified. A <a href="how.html">companion document</a> +describes how the default black shadows are generated. + +<h3>The Starting Point</h3> + +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 <b>pnmshadow</b>. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadin.gif" width=536 height=141 alt="Input image"> +</table> +</center> + +<h3>The Blank Background</h3> + +We start by determining the size of the input image with +<b>pnmfile</b> 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: + +<p> +<pre> + pnmcut 0 0 1 1 <em>ifile</em> | pnmscale -xsize <em>xsize</em> -ysize <em>ysize</em> ><em>fname</em>-5.ppm +</pre> +<p> + +yielding the image: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt5.gif" width=536 height=141 alt="Blank background image"> +</table> +</center> + +<h3>The Positive Mask</h3> + +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 +<tt>-difference</tt> option on <b>pnmarith</b> to avoid clipping at +zero or the maximum pixel value), then inverting the result and +thresholding it to a monochrome bitmap. + +<p> +<pre> + pnmarith -difference <em>ifile</em> <em>fname</em>-5.ppm | pnminvert | ppmtopgm | pgmtopbm -thresh -value 1.0 ><em>fname</em>-1.ppm +</pre> +<p> + +This produces the following mask image. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt1.gif" width=536 height=141 alt="Positive mask image"> +</table> +</center> + +<h3>The Blurred Image</h3> + +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>-b</b> option +(default 11), written into the temporary file <tt><em>fname</em>-2.ppm</tt> +in ASCII PGM format, and then the blurred image is created with +the command: + +<p> +<pre> + pnmconvol <em>fname</em>-2.ppm <em>ifile</em> ><em>fname</em>-10.ppm +</pre> +<p> + +With the default blur setting of 11 pixels, the blurred image +below is generated. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt10-t.jpg" width=536 height=141 alt="Blurred shadow"> +</table> +</center> + +<h3>Offset Shadow Clip</h3> + +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>-b</b> option +which default to half the blur (<b>-b</b>) setting. The +<em>xsize</em> and <em>ysize</em> arguments in the following +command are the size of the input image in pixels less the shadow +displacement in the respective axis. + +<p> +<pre> + pnmcut 0 0 <em>xsize</em> <em>ysize</em> <em>fname</em>-10.ppm ><em>fname</em>-4.ppm +</pre> +<p> + +The shadow clip is the identical to the shadow on background color, but +smaller by the offset in each direction. + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt4-t.jpg" width=531 height=136 alt="Offset shadow clip"> +</table> +</center> + +<h3>Offset Shadow</h3> + +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. + +<p> +<pre> + pnmpaste -replace <em>fname</em>-4.ppm <em>xoffset</em> <em>yoffset</em> <em>fname</em>-5.ppm ><em>fname</em>-6.ppm +</pre> +<p> + +This gives the following result: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt6-t.jpg" width=536 height=141 alt="Offset shadow"> +</table> +</center> + +<h3>Inverse Mask</h3> + +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 <b>pnminvert</b>: + +<p> +<pre> + pnminvert <em>fname</em>-1.ppm ><em>fname</em>-7.ppm +</pre> +<p> + +yielding: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt7.gif" width=536 height=141 alt="Inverse mask"> +</table> +</center> + +<h3>Masked Input Image</h3> + +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: + +<p> +<pre> + pnmarith -multiply <em>ifile</em> <em>fname</em>-7.ppm ><em>fname</em>-8.ppm +</pre> +<p> + +<em>et voilą:</em> + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt8.gif" width=536 height=141 alt="Masked Input Image"> +</table> +</center> + +<h3>Shadow with Source Masked</h3> + +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. + +<p> +<pre> + pnmarith -multiply <em>fname</em>-6.ppm <em>fname</em>-1.ppm ><em>fname</em>-9.ppm +</pre> +<p> + +This is accomplished by multiplying the shadow by the positive +mask image, which sets all non-background pixels in the source +image to black: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadt9-t.jpg" width=536 height=141 alt="Shadow with Source Masked"> +</table> +</center> + +<h3>The Final Product</h3> + +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): + +<p> +<pre> + pnmarith -add <em>fname</em>-8.ppm <em>fname</em>-9.ppm +</pre> +<p> + +The resulting image, with shadow, is as follows: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadout-t.jpg" width=536 height=141 alt="Output: Image with translucent shadow"> +</table> +</center> + +<h3>Smooth Operator</h3> + +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 <b>pnmshadow</b> as the starting +point and scaling to half size with <b>pnmscale</b>, we arrive at +the following smoothed image, with shadow, ready to adorn a +Web page: + +<p> +<center> +<table border=5> +<tr><td> +<img src="figures/shadout2-t.jpg" width=268 height=71 alt="Half scale image with translucent shadow"> +</table> +</center> + +<h4><a href="how.html">How it works with black shadows</a></h4> +<h4><a href="./"><b>pnmshadow</b> main page</a></h4> + +<p> +<hr> +<p> +<address> +by <a href="/">John Walker</a><br> +August 8th, 1997 +</address> + +</body> +</html> |