gentoo-overlay/media-plugins/frei0r-plugins/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>video</herd>
  <use>
    <flag name='facedetect'>Enables building of facedetect plugin, which
      depends on <pkg>media-libs/opencv</pkg>
    </flag>
    <flag name='scale0tilt'>Enables building of scale0tilt and vectorscope
      plugins, which depend on <pkg>media-libs/gavl</pkg>
    </flag>
  </use>
</pkgmetadata>