151 lines
4.5 KiB
XML
151 lines
4.5 KiB
XML
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd"
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[
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<!ENTITY % defs SYSTEM "defs.ent"> %defs;
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]>
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<article id='lbxalg'>
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<articleinfo>
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<title>LBX X Consortium Algorithms</title>
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<releaseinfo>X Version 11, Release &fullrelvers;</releaseinfo>
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</articleinfo>
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<sect1 id='Introduction'>
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<title>Introduction</title>
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<para>
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The Low Bandwidth X extension allows for negotiating various algorithms used
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by LBX. This document describes the algorithms used in the Consortium
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implementation of LBX in the X11 Release 6.4.
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</para>
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</sect1>
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<sect1 id='Streaming_Compression'>
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<title>Streaming Compression</title>
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<para>
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LBX negotiates the use of a stream compressor. The consortium implementation defines a stream
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compressor named XC-ZLIB, which is based on the Zlib version 1.0 compression library by
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Gailly & Adler.
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</para>
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<para>
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The XC-ZLIB compressor is presented with a simple byte stream - the X and LBX message
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boundaries are not apparent. The data is broken up into fixed sized blocks. Each block is compressed
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using zlib, then a two byte header is prepended, and then the entire packet is transmitted.
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The header has the following information:
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</para>
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<para><programlisting>
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out[0] (length & 0xfff) >> 8 | ((compflag) ? 0x80 : 0);
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out[1] = length & 0xff;
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</programlisting></para>
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<para>
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If the compflag is false, then the contents of the block are not compressed.
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</para>
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</sect1>
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<sect1 id='Bitmap_Compression'>
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<title>Bitmap Compression</title>
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<para>
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LBX also negotiates for bitmap compression. The consortium
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implementation defines a bitmap compressor named XC-FaxG42D,
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which uses the CCITT Group 4 2D compression algorithm.
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</para>
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</sect1>
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<sect1 id='Pixmap_Compression'>
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<title>Pixmap Compression</title>
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<para>
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LBX allows for the negotiation of pixmap compression. The
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consortium implementation does not define a pixmap compression
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algorithm. (A run-length encoding algorithm was proposed, but
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experimentation proved it was less efficient than allowing the
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stream compressor to compress the image.
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</para>
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</sect1>
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<sect1 id='Colormap_Algorithm'>
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<title>Colormap Algorithm</title>
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<para>
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LBX negotiates for use of a colormap algorithm, used for color
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matching when the proxy allocates pixels in a grabbed colormap.
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The consortium implementation defines an algorithm named
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XC-CMAP. This algorithm consists of three parts, resolving to a
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hardware color, finding the closest existing color, and what free
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cell to allocate.
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</para>
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<para>
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The XC-CMAP algorithm resolves a color to a hardware color in the
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following manner:
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</para>
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<para><programlisting>
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#define RESCALE(x, nbits) (x >> (16 - nbits)) * 65535 / ((1 << nbits) - 1)
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#define GRAY(r, g, b) (30L * r + 59L * g + 11L * b) / 100
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sigbits = pVisual->bitsPerRGB;
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switch (pVisual->class) {
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case PseudoColor:
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case DirectColor:
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case StaticColor:
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/* rescale to rgb bits */
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*red = RESCALE(*red, sigbits);
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*green = RESCALE(*green, sigbits);
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*blue = RESCALE(*blue, sigbits);
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break;
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case GrayScale:
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/* rescale to gray then rgb bits */
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*blue = *green = *red = RESCALE(GRAY(*red, *green, *blue), sigbits);
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break;
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case StaticGray:
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/* rescale to gray then [0..limg] then [0..65535] then rgb bits */
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*blue = *green = *red = RESCALE(RESCALE(GRAY(*red, *green, *blue),
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pVisual>numPixelBits), sigbits);
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break;
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case TrueColor:
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/* rescale to [0..limN] then [0..65535] then rgb bits */
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*red = RESCALE(RESCALE(*red, pVisual->numRedBits), sigbits);
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*green = RESCALE(RESCALE(*green, pVisual->numGreenBits), sigbits);
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*blue = RESCALE(RESCALE(*blue, pVisual->numBlueBits), sigbits);
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break;
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}
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</programlisting></para>
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<para>
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The XC-CMAP algorithm matches a color to an existing pixel in
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static visuals by finding the pixel with the lowest color match
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error, computed as follows:
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</para>
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<para><programlisting>
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error = errRed * errRed + errGreen * errGreen + errBlue * errBlue
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</programlisting></para>
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<para>
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The XC-CMAP algorithm selects a free pixel to allocate by selecting
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the free pixel with the lowest index from the free pixels known to
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the proxy. For direct visuals, it uses the lowest free or matching
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pixel subfield known to the proxy for each color.
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</para>
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</sect1>
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<sect1 id='Extensions'>
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<title>Extensions</title>
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<para>
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LBX allows for extensions to LBX to enable additional compression
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or short-circuiting. The consortium implementation does not define
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any extensions to LBX.
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</para>
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</sect1>
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</article>
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