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<title>A-buffer</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">A-buffer</span></span>

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In <a href="Computer_graphics" title="Computer graphics">computer graphics</a>, <b>A-buffer</b>, also known as <b>anti-aliased</b>, <b>area-averaged</b> or <b>accumulation buffer</b>, is a general hidden surface mechanism suited to medium scale virtual memory computers. It resolves visibility among an arbitrary collection of opaque, transparent, and intersecting objects. Using an easy to compute Fourier window (box filter), it increases the effective image resolution many times over the Z-buffer, with a moderate increase in cost.<sup id="cite_ref-CARP84_1-0" class="reference"><a href="#cite_note-CARP84-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

</p><p>The A-buffer method is a descendant of the well known Z-buffer, which provides good quality results in moderate time.

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<p>In 3-D image synthesis system, the balance between the quality and the cost of computation has always been needed. Using a full object-precision visible-surface algorithm at each pixel is expensive. A-buffer method provides moderate quality results in moderate cost of computation.

</p><p>A-buffer helps in using visibility techniques and supports all conceivable geometric modeling primitives: polygons, patches, quadrics, fractals, and so forth. It also helps to handle transparency and intersecting surfaces (and transparent intersecting surfaces).

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<div class="mw-heading mw-heading2"><h2 id="Strategy">Strategy</h2></div>

<p>Carpenter's A-buffer algorithm<sup id="cite_ref-CARP84_1-1" class="reference"><a href="#cite_note-CARP84-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> addresses this problem by approximating <a href="Edwin_Catmull" title="Edwin Catmull">Catmull's</a> per-pixel object-precision area sampling<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> with per-pixel image-precision operation performed on a sub-pixel grid. Polygons are first processed in scan-line order by clipping them to each square pixel they cover. This results in list of clipped polygon fragments corresponding to each square pixel. Each fragment has 4 by 8 bit mask of parts of the pixel it covers.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p>The bit-mask for a fragment is computed by <a href="Bitwise_XOR" class="mw-redirect" title="Bitwise XOR">xoring</a> together masks representing each of the fragment's edges. When all polygons intersecting a pixel have been processed, the area-weighted average of the colors of the pixel's visible surfaces is obtained by selecting fragments in depth-sorted order and using their bit masks to clip those of farther fragments.

</p><p>The bit masks can be manipulated efficiently with <a href="Boolean_operations_on_polygons" title="Boolean operations on polygons">Boolean operations</a>. For example, two fragment bit masks can be added together to determine the overlap between them. The A-buffer algorithm saves only a small amount of additional information with each fragment. For example. It includes the fragment's z extent, but no information about which part of the fragment is associated with these z values. Thus, the algorithm must make assumptions about the sub-pixel geometry in cases in which fragment bit masks overlap in z.

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFCarpenter1984" class="citation journal cs1"><a href="Loren_Carpenter" title="Loren Carpenter">Carpenter, Loren</a> (July 1984). "The A-buffer, an Antialiased Hidden Surface Method". <i>Computer Graphics</i>. <b>18</b> (3): <span class="nowrap">103–</span>108. <a href="CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.210.5497">10.1.1.210.5497</a></span>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F964965.808585">10.1145/964965.808585</a>.</cite></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFCatmull1978" class="citation journal cs1"><a href="Edwin_Catmull" title="Edwin Catmull">Catmull, Edwin</a> (August 1978). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20160304185852/http://design.osu.edu/carlson/history/PDFs/catmull-zbuffer.pdf">"A Hidden-Surface Algorithm with Anti-Aliasing"</a> <span class="cs1-format">(PDF)</span>. <i>Computer Graphics</i>. <b>12</b> (3): <span class="nowrap">6–</span>11. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F965139.807360">10.1145/965139.807360</a>. Archived from <a rel="nofollow" class="external text" href="http://design.osu.edu/carlson/history/PDFs/catmull-zbuffer.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2016-03-04<span class="reference-accessdate">. Retrieved <span class="nowrap">2015-06-19</span></span>.</cite></span>

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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFFoleyFeinervan_DamHughes1995" class="citation book cs1"><a href="James_D._Foley" title="James D. Foley">Foley, James D.</a>; Feiner, Steven K.; <a href="Andries_van_Dam" title="Andries van Dam">van Dam, Andries</a>; <a href="John_F._Hughes" title="John F. Hughes">Hughes, John F.</a> (1995). <a href="Computer_Graphics%3A_Principles_and_Practice" title="Computer Graphics: Principles and Practice"><i>Computer Graphics: Principles and Practice in C</i></a> (2nd ed.). Addison-Wesley. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-201-84840-3</bdi>.</cite></span>

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This article is issued from <a class="external text" title="Last edited on 2025-04-30" href="https://en.wikipedia.org/wiki/?title=A-buffer&oldid=1288119153">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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