Also moved building of entry compressed tarballs to last in the `make www` and `make quick_www` top level Makefile rules.
487 lines
26 KiB
HTML
487 lines
26 KiB
HTML
<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
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<title>2000/bmeyer - Best Utility</title>
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<meta name="description" content="2000 IOCCC entry bmeyer - Best Utility">
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<meta name="keywords" content="IOCCC, 2000, IOCCC 2000, IOCCC entry, bmeyer, Best Utility">
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<div class="header">
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<img src="../../png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
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<h1>The International Obfuscated C Code Contest</h1>
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<h2>2000/bmeyer - Best Utility</h2>
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<a class="Left" href="../bellard/index.html">← bellard</a>
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<a class="Left" href="../index.html">↑ 2000 ↑</a>
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<a class="Left" href="../briddlebane/index.html">briddlebane →</a>
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<a href="../../bugs.html">Bugs & (mis)features</a>
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<a href="../../faq.html#fix_an_entry">Fixing IOCCC entries</a>
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<a href="../../faq.html#fix_author">Fixing author info</a>
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<a href="../../judges.html">The IOCCC Judges</a>
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<h2 id="author"><a name="author"></a>Author:</h2>
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<ul>
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<li>Name: <a href="../../authors.html#Bernd_Meyer">Bernd Meyer</a><br>
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Location: <a href="../../location.html#AU">AU</a> <br><br></li>
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<h2 id="to-build">To build:</h2>
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<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">make</span></span></code></pre></div>
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<h2 id="to-use">To use:</h2>
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<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./bmeyer</span></span></code></pre></div>
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<h2 id="try">Try:</h2>
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<p>In a video window (white text on black background):</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./try.sh</span></span></code></pre></div>
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<p>The script will suggest you compare the output of the program with
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images, prompting you to press any key to continue so that you may do
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so.</p>
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<h2 id="judges-remarks">Judges’ remarks:</h2>
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<p>Don’t even think of supplying different compression factors (the
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dashed argument) for compression and decompression! You’ve been
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warned.</p>
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<p>The author recommended on linux/x86, glibc 2.0, libc4/5:</p>
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<pre class="make"><code>bmeyer: bmeyer.c
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CC -DY="__setfpucw(0x127f)" -O6 $? -o $@ -lm</code></pre>
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<p>However many compilers use <code>-O3</code> as the maximum level for
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<code>-O</code> optimization.</p>
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<p>And on linux/x86, glibc 2.1:</p>
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<pre class="make"><code>bmeyer: bmeyer.c
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gcc -DY='int x=0x127f; __asm__ ("fldcw %0" : : "m" (*&x))' $? -o $@ -lm</code></pre>
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<p>Compile with the maximum possible optimization on your system. If
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your system has 80-bit internal representation of floating point values
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(x86, some Motorola processors) and you want the compressed format to be
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portable across platforms, you need to set your FPU to use 64 bit mode.
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Read the author’s remarks below for more details.</p>
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<p>If you cannot work out how to get into 64 bit mode, you can try using
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<code>-ffloat-store</code> if you compile with GCC (this may have a
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performance penalty). The resulting binary will have a good chance to
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produce portable files, but due to the resulting double-rounding (first
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internally set to 80 bit, then to 64 bit on writing the value to
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memory), there is a small but very real chance any given file produced
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might not be portable.</p>
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<p>The <a href="Makefile">Makefile</a> defaults to <code>-O3</code> and
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no <code>asm()</code> call.</p>
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<h2 id="authors-remarks">Author’s remarks:</h2>
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<p>This program is called <code>GLICBAWLS</code></p>
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<h4 id="function">Function</h4>
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<p><code>GLICBAWLS</code> stands for “Grey Level Image Compression By
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Adaptive Weighted Least Squares”.</p>
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<p>And that’s exactly what this program does — feed it a .PGM file
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(either raw [i.e. P5] or ASCII [i.e. P2]) on stdin, and it will output a
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compressed version to stdout. Feed it a compressed file on stdin, and it
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will decompress it to stdout — as either raw or ASCII .PGM, depending on
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what format it was compressed from. For example:</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> <span class="op"><</span> michael.pgm <span class="op">></span> michael.glic</span></code></pre></div>
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<p>and then</p>
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<div class="sourceCode" id="cb7"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> <span class="op"><</span> michael.glic <span class="op">></span> michael.decoded.pgm</span></code></pre></div>
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<p>As it turns out, <code>glicbawls</code> compresses greyscale images
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(or, more precisely, “natural” greyscale images) better than any other
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program I am aware of (with the exception of the not-yet-published
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program developed for my PhD thesis). Of course, this might take some
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time, so a progress indicator is provided. To make things more
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interesting, instead of a boring bar or percentage display, an ASCII
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rendition of the image being compressed/decompressed is output to
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<code>stderr</code> (assuming white text on black background). If you
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have trouble recognizing the image, try squinting at it ;-).</p>
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<p>If you still have trouble, simply use a terminal or xterm with more
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than 80 columns, and tell <code>glicbawls</code> about it by giving the
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number of columns as an argument. For example</p>
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<div class="sourceCode" id="cb8"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> 260 <span class="op"><</span>lavabus.pgm <span class="op">></span>lavabus.glic</span></code></pre></div>
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<p>will allow <code>glicbawls</code> to use up to 260 columns, which
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makes things much more recognizable.</p>
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<p>Still running out of space? Need to compress those images just a tad
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more? And maybe you can handle some loss due to the compression? If so,
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you can use <code>glicbawls</code>’ “near lossless” compression mode. In
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that mode, the decoded pixel values are allowed to differ from the
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originals, but there is an upper limit on the magnitude of the
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difference. For example, if a maximum error of 2 is allowed, an original
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pixel value of 37 could be decoded as 35, 36, 37, 38 or 39. In order to
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use this mode, simply give the maximum allowed error as an argument,
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with a dash (<code>-</code>) before it. For example:</p>
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<div class="sourceCode" id="cb9"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> <span class="at">-2</span> <span class="op"><</span> graph.pgm <span class="op">></span> graph.2.glic</span></code></pre></div>
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<p>will compress in a way that allows decompression with a maximum error
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of two. You can combine both types of arguments, i.e.:</p>
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<div class="sourceCode" id="cb10"><pre
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class="sourceCode sh"><code class="sourceCode bash"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> <span class="at">-3</span> 180 <span class="op"><</span> graph.pgm <span class="op">></span>graph.3.glic</span></code></pre></div>
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<p>will compress with a maximum error of 3, using a 180 column terminal
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for the progress display, and:</p>
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<div class="sourceCode" id="cb11"><pre
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class="sourceCode sh"><code class="sourceCode bash"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./glicbawls</span> <span class="at">-3</span> 180 <span class="op"><</span> graph.3.glic <span class="op">></span> graph.3maxerr.pgm</span></code></pre></div>
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<p>will decompress the result on such a terminal.</p>
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<p>But wait, there is more — you can also feed <code>glicbawls</code>
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.PPM files (i.e. colour images), raw or ASCII, your choice.
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<code>glicbawls</code> will compress and decompress them just fine.
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However, it only does “reasonably well” on them; It beats PNG easily,
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but there are certainly programs out there that can compress colour
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images considerably better. And don’t even <em>think</em> about using it
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for so-called “artistic” images (i.e. anything that wasn’t scanned from
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a photo or camera)….</p>
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<h3 id="warning">WARNING</h3>
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<p><code>glicball</code> is rather CPU and FPU intensive. Don’t run it
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on anything slow. Things start to be usable around 300+MHz, with 600+
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being highly desirable.</p>
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<h3 id="compile-instructions">Compile Instructions</h3>
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<p>Unlike most compression program, <code>glicbawls</code> makes heavy
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use of floating point maths — and not all FPUs are created equal. Most
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importantly, the x86 and 68k FPUs use 80 bit floating point formats
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internally, while most other processors use IEEE 64 bit formats by
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default. If not taken care of, this means that a <code>glicbawls</code>
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file compressed on one CPU cannot generally be decompressed on
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another.</p>
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<p>Fortunately, just about all current FPUs can be put into IEEE 64 bit
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mode (or at least something close enough for <code>glicbawls</code>).
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Unfortunately, there is no standard way of doing so. For this reason,
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the build file contains <em>three</em> compile lines; one for machines
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that already use 64 bit mode by default (most RISC processors), one for
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linux/x86 machines using libc4, libc5 or glibc2.0, and one for linux/x86
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machines using glibc2.1 (glibc2.1 no longer exports the
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<code>__setfpucw()</code> function, necessitating the use of gcc inline
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assembly. *Grumble*).</p>
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<p>So, if you are building on a machine that does not, by default, use
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IEEE 64 bit floating point doubles, you’ll have to define <code>Y</code>
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to be a command that switches it into that mode. If you fail to do so,
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your <code>glicbawls</code> executable will still be able to decompress
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its own compressed files, but you may be unable to exchange files with
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<code>glicbawls</code> on other machines.</p>
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<h3 id="obfuscation">Obfuscation</h3>
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<p>Most of the obfuscation of this program was driven by the desperate
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struggle to fit all of <code>glicbawls</code>’ features into code that
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fits the IOCCC’s size limits. All identifiers’ names are
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single-character, which was only possible by ruthless exploitation of
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scope. There is a function <code>M()</code> that handles nearly all
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input from <code>stdin</code>, handles all output to
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<code>stdout</code>, and also recursively calculates one of the
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mathematical expressions needed for error modelling. There is a function
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that calculates <code>(A^(-1)*b)*x</code> with <code>A</code> being a
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matrix and <code>b</code> and <code>x</code> being vectors, and does so
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in a very efficient way — but one would be hard pressed to find it.
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There is an arithmetic coder that is even less recognizable. A lot of
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loop variables don’t get initialized, but instead the code is arranged
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in such a way that they just happen to have the right value when control
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flow reaches the loops. Lots of functions use the same two global
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scratch variables. Variables change their meaning all the time, and the
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differences between compressing and decompressing are handled in rather
|
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subtle ways all over the program. The string that holds the ASCII values
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for displaying the progress image doubles as a description on how to
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find a pixel’s causal neighbours. And then there are a few
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<code>#define</code>s that really are just in there to save a few
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characters, but as an added “benefit” make the source ever more
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unreadable.</p>
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<p>In short — running it through the preprocessor and the pretty-printer
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will give you something that looks slightly less like line noise and
|
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slightly more like a C program, but unless you are a true wizard, it is
|
||
unlikely to gain you any insights into what is actually going on….<a
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href="#fn1" class="footnote-ref" id="fnref1"
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role="doc-noteref"><sup>1</sup></a></p>
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<h3 id="bugs-assumptions-and-todo">Bugs, Assumptions and TODO</h3>
|
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<p><code>glicbawls</code> assumes that characters are 8 bit wide, and
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that the character set used is ASCII. If one of these isn’t true, all
|
||
hell will break loose.</p>
|
||
<p>The .PGM headers are not checked very thoroughly —
|
||
<code>glicbawls</code> will happily try to encode lots of files that it
|
||
shouldn’t touch. The old adage of “garbage in, garbage out” holds in
|
||
that case. Furthermore, even if <code>glicbawls</code> detects an
|
||
incorrect input, it fails to provide a meaningful error message, and
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||
instead simply aborts.</p>
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||
<p>When decompressing a near-lossless encoded image, the maximum allowed
|
||
error is required as a command line parameter. This should really be
|
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saved in the compressed file!</p>
|
||
<p><code>glicbawls</code> relies on the decimal representation of all
|
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integers having less than 255 digits. While ANSI C does not guarantee
|
||
this, it seems rather unlikely that anyone in the foreseeable future
|
||
would use images with <code>10^255</code> rows, columns or different
|
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levels of grey. Also, it is assumed that any integer used can be cast to
|
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a <code>double</code> and back without loss of precision. This limits
|
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the number of rows, columns and grey levels to 9 quintillion, which
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should also be sufficient for the next few years.</p>
|
||
<p>There is a theoretical chance that you might get a division by zero
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error. You have a MUCH better chance of getting hit by lightning the
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very second you realize that you just won a few million in a lottery.
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But if it happens to you, don’t say I didn’t warn you! Instead, go out
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and buy a lightning rod and a lottery ticket ;-)</p>
|
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<p>At one point, a pointer to <code>int</code> is used as the target of
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a <code>"%1[#]"</code> conversion in a <code>scanf(3)</code>, which
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makes gcc rather unhappy, but shouldn’t really cause anyone any trouble.
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The same goes for a couple of assignments which are used as truth
|
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values.</p>
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<h3 id="algorithm">Algorithm</h3>
|
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<p>An exhaustive explanation of the algorithm used would go beyond the
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scope of an info file, and would also be quite difficult in pure ASCII,
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so the following is just a rough outline, and contains some
|
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compression-speak…</p>
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<p>The image is handled in <a
|
||
href="https://en.wikipedia.org/wiki/Scan_line">scanline</a> ordering.
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For each pixel, an expected value is calculated by creating a least
|
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squares predictor of order 12 based on all of the previously
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encoded/decoded pixels. However, the contribution of each pixel is
|
||
weighted according to its <a
|
||
href="https://en.wikipedia.org/wiki/Taxicab_geometry">Manhattan-distance</a>
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from the current pixel, <code>D</code>. That weight used is
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<code>pow(0.8,D)</code>. <a href="#fn2" class="footnote-ref" id="fnref2"
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role="doc-noteref"><sup>2</sup></a> <a href="#fn3" class="footnote-ref"
|
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id="fnref3" role="doc-noteref"><sup>3</sup></a> <a href="#fn4"
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class="footnote-ref" id="fnref4" role="doc-noteref"><sup>4</sup></a></p>
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<p>In a similar way, the prediction <em>errors</em> for all previous
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pixels are combined — a weighted average (with weight
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<code>pow(0.7,D)</code>) of the squares of all previous prediction
|
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errors is taken, and the square root of the (appropriately scaled)
|
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result is called sigma.</p>
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<p>The predicted value and sigma are then used as the parameters of a <a
|
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href="https://en.wikipedia.org/wiki/Student%27s_t-distribution">Student
|
||
or t-distribution</a> (which is similar to a <a
|
||
href="https://en.wikipedia.org/wiki/Normal_distribution">normal
|
||
distribution</a>, but has more weight in the tails), providing a <a
|
||
href="https://en.wikipedia.org/wiki/Probability_distribution">probability
|
||
distribution</a> for the current pixel.</p>
|
||
<p>Next, the possible interval of values for the current pixel is
|
||
halved, and through integration of the probability distribution, a
|
||
probability is calculated for the pixel value being in the lower half.
|
||
During encoding, the actual value is examined, and the result encoded
|
||
using an arithmetic coder. During decoding, the arithmetic decoder
|
||
provides the information which half the actual value is in. In both
|
||
cases, the interval borders are adjusted accordingly, the arithmetic
|
||
coder/decoder updated, and the interval halving repeated until the
|
||
precise value has been en/decoded<a href="#fn5" class="footnote-ref"
|
||
id="fnref5" role="doc-noteref"><sup>5</sup></a>.</p>
|
||
<h3 id="photo-credit">Photo Credit:</h3>
|
||
<p><a href="lavabus.pgm">Lavabus</a> was taken by the judge, Landon Curt
|
||
Noll. <a href="michael.pgm">Michael</a> was provided by the author. <a
|
||
href="lenna.glic">Lenna</a> is a 1972 Playboy centerfold (doesn’t show
|
||
very much).</p>
|
||
<ul>
|
||
<li><p><a href="michael.pgm">michael.pgm</a></p>
|
||
<p>The author’s Godchild and nephew.</p></li>
|
||
<li><p><a href="lavabus.pgm">lavabus.pgm</a></p>
|
||
<p>A bus that was trapped by lava from <a
|
||
href="https://en.wikipedia.org/wiki/Kīlauea">Kilauea volcano</a> in
|
||
Hawaii, US. Nobody was in the bus at the time, BTW. Photo date:
|
||
1981.</p></li>
|
||
<li><p><a href="lenna.glic">lenna.glic</a></p>
|
||
<p>A November 1972 Playboy centerfold was scanned in long ago and has
|
||
been used for image compression research since then. Playboy originally
|
||
threatened to prosecute unauthorized use, but eventually granted that,
|
||
having been in use for image compression testing for so long, it was now
|
||
a useful benchmark. (Note: This picture doesn’t show very
|
||
much.)</p></li>
|
||
</ul>
|
||
<h2 id="copyright-and-cc-by-sa-4.0-license">Copyright and CC BY-SA 4.0
|
||
License:</h2>
|
||
<p>This file is Copyright (c) 2023 by Landon Curt Noll. All Rights
|
||
Reserved. You are free to share and adapt this file under the terms of
|
||
this license:</p>
|
||
<pre><code>Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)</code></pre>
|
||
<p>For more information, see:
|
||
https://creativecommons.org/licenses/by-sa/4.0/</p>
|
||
<section id="footnotes" class="footnotes footnotes-end-of-document"
|
||
role="doc-endnotes">
|
||
<hr />
|
||
<ol>
|
||
<li id="fn1"><p>Yes, this could be seen as a challenge….<a
|
||
href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
|
||
<li id="fn2"><p>Actually, the weight is <code>pow(0.8,D)/s</code>, where
|
||
<code>s</code> is the sigma used for encoding/decoding the particular
|
||
previous pixel. Dividing by <code>s</code> is a rather arbitrary action
|
||
justified only by the improvement in results. Mathematically, I can only
|
||
justify dividing by <code>s^2</code>, or not dividing at all….<a
|
||
href="#fnref2" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
|
||
<li id="fn3"><p>Obviously, these least squares predictors are not
|
||
calculated from scratch for each pixel. Some clever reuse of earlier
|
||
results allows <code>O(1)</code> calculation, rather than the
|
||
<code>O(previous_rows*columns)</code> this description would suggest.<a
|
||
href="#fnref3" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
|
||
<li id="fn4"><p>As anyone who has ever tried local least squares knows
|
||
from bitter experience, there is a lot of numerical instability
|
||
associated with them. So <code>glicbawls</code> adds a bias towards an
|
||
averaging predictor to the equation system the least squares predictor
|
||
is calculated from. The weight that is given to this bias is constantly
|
||
adjusted throughout coding/decoding, thus justifying the ‘a’ in
|
||
<code>glicbawls</code>.<a href="#fnref4" class="footnote-back"
|
||
role="doc-backlink">↩︎</a></p></li>
|
||
<li id="fn5"><p>In the case of near-lossless coding, the interval is not
|
||
necessarily <em>halved</em>, but rather split at a convenient point (the
|
||
split points are chosen in such a way as to minimise the expected number
|
||
of bits needed for coding). Also, the iteration ends as soon as the
|
||
interval is small enough to guarantee decoding within the requested
|
||
maximum error.<a href="#fnref5" class="footnote-back"
|
||
role="doc-backlink">↩︎</a></p></li>
|
||
</ol>
|
||
</section>
|
||
|
||
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||
|
||
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||
|
||
<h1 id="inventory-for-2000bmeyer"><a name="inventory"></a>Inventory for
|
||
2000/bmeyer</h1>
|
||
<h2 id="primary-files">Primary files</h2>
|
||
<ul>
|
||
<li><a href="index.html">index.html</a> - entry home page</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/bmeyer.c">bmeyer.c</a>
|
||
- winning source</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/Makefile">Makefile</a>
|
||
- entry Makefile</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/bmeyer.orig.c">bmeyer.orig.c</a>
|
||
- original source</li>
|
||
<li><a href="lavabus.pgm">lavabus.pgm</a> - Kilauea volcano bus trapped
|
||
in lava</li>
|
||
<li><a href="michael.pgm">michael.pgm</a> - author’s Godchild and
|
||
Nephew</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/try.sh">try.sh</a>
|
||
- how to run</li>
|
||
<li><a href="lenna.glic">lenna.glic</a> - P10 scan of 1972 Playboy
|
||
centerfold</li>
|
||
<li><a href="2000_bmeyer.tar.bz2">2000_bmeyer.tar.bz2</a> - download
|
||
winning source</li>
|
||
</ul>
|
||
<h2 id="secondary-files">Secondary files</h2>
|
||
<ul>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/.entry.json">.entry.json</a>
|
||
- description of entry in JSON</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/.gitignore">.gitignore</a>
|
||
- list of files that should not be committed under git</li>
|
||
<li><a href=".path">.path</a> - directory path from top directory</li>
|
||
<li><a
|
||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2000/bmeyer/README.md">README.md</a>
|
||
- markdown source for index.html</li>
|
||
</ul>
|
||
|
||
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||
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||
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||
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|
||
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||
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|
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|
||
</div>
|
||
|
||
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|
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||
</div>
|
||
|
||
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|
||
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|
||
|
||
<div class="footer">
|
||
<hr>
|
||
<h2>Coda</h2>
|
||
<p><a href="https://validator.w3.org/nu/?doc=https%3A%2F%2Fioccc-src.github.io%2Ftemp-test-ioccc%2F2000%2Fbmeyer%2Findex.html">Nu HTML check this web page</a></p>
|
||
<p><a href="#top">Jump to top</a> <a href="#content">Jump to Content</a> <a href="#inventory">Jump to Inventory</a></p>
|
||
</div>
|
||
|
||
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||
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||
|
||
</body>
|
||
</html>
|
||
|
||
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