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</div><div class="twikiRevInfo"><span class="twikiGrayText"><span class="twikiToolbarElem">Cinelerra.VideoChromaKey</span><span class="twikiToolbarElem">r1.2 - 16 Jan 2004 - 22:19 - <a class="twikiLink" href="../Main/AlexFerrer">AlexFerrer</a></span><span class="twikiToolbarElem"><a href="#TopicEnd">topic&nbsp;end</a></span></span></div></div>
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<h3><a name="VideoChromaKey"> </a> <a class="twikiLink" href="VideoChromaKey">VideoChromaKey</a> </h3>
This effect replaces the selected color or intensity with black if there is no <span class="twikiNewLink" style="background : #FFFFCE;"><font color="#0000FF">AlphaChannel</font><a href="http://www.ftconsult.com/twiki/bin/edit/Cinelerra/AlphaChannel?topicparent=Cinelerra.VideoChromaKey"><sup>?</sup></a></span> and replaces it with <a class="twikiLink" href="Transparency">transparency</a> if there is an <span class="twikiNewLink" style="background : #FFFFCE;"><font color="#0000FF">AlphaChannel</font><a href="http://www.ftconsult.com/twiki/bin/edit/Cinelerra/AlphaChannel?topicparent=Cinelerra.VideoChromaKey"><sup>?</sup></a></span>. The selection of <a class="twikiLink" href="ColorModel">ColorModel</a> is important.
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<a class="twikiLink" href="VideoChromaKey">VideoChromaKey</a> uses either the value or the <span class="twikiNewLink" style="background : #FFFFCE;"><font color="#0000FF">hue</font><a href="http://www.ftconsult.com/twiki/bin/edit/Cinelerra/Hue?topicparent=Cinelerra.VideoChromaKey"><sup>?</sup></a></span> to determine what is erased. If this parameter is within a certain threshold it's erased. It's not a simple on/off switch, however. As the selected parameter approaches the edge of the threshold, it gradually gets erased if the slope is low or is completely erased if the slope is high.
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The slope tries to soften the edges of the <a class="twikiLink" href="VideoChromaKey">VideoChromaKey</a> but it doesn't work well for compressed sources. A popular softening technique is to use a maximum slope and chain a <span class="twikiNewLink" style="background : #FFFFCE;"><font color="#0000FF">VideoBlur</font><a href="http://www.ftconsult.com/twiki/bin/edit/Cinelerra/VideoBlur?topicparent=Cinelerra.VideoChromaKey"><sup>?</sup></a></span> effect below the <a class="twikiLink" href="VideoChromaKey">VideoChromaKey</a> effect to blur just the alpha.
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-- <a class="twikiLink" href="../Main/AlexFerrer">AlexFerrer</a> - 19 Dec 2003
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