 format as the <em>clip</em>.
Failure to do so will produce an error. If for conditional reasons you do not
need to modify the current frame’s properties, you can simply pass it through.
The selector function is passed <em>n</em>, the current frame number, and <em>f</em>, which
is a frame or a list of frames if there is more than one clip specified.</p>
<p>If you do not need to modify frame properties but only read them, you should
probably be using <em>FrameEval</em> instead.</p>
<p>How to set the proper