Found it at http://www.question2answer.org/modules.php?module=viewer
calc_quality($content, $format) should return a numerical value indicating your viewer's ability to render the supplied $content in $format, as retrieved from Q2A's database. You should return 1.0 to indicate perfect ability, and 0.0 for complete inability. For each piece of content, the viewer module returning the highest value will be used.
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