blackframe[=amount:threshold]
＊原文：
blackframe[=amount:threshold]
Detect frames that are (almost) completely black. Can be useful
to detect chapter transitions or commercials. Output lines con-
sist of the frame number of the detected frame, the percentage
of blackness, the frame type and the frame number of the last
encountered keyframe.
<amount>
Percentage of the pixels that have to be below the
threshold (default: 98).
<threshold>
Threshold below which a pixel value is considered black
(default: 32).
