std.PlaneStats(clip clipa[, clip clipb, int plane=0, string prop='PlaneStats'])

This function calculates the min, max and average normalized value of all the pixels in the specified plane and stores the values in the frame properties named propMin, propMax and propAverage.

If clipb is supplied, the absolute normalized difference between the two clips will be stored in propDiff as well.

The normalization means that the average and the diff will always be floats between 0 and 1, no matter what the input format is.