w, out_w
The width of the output video. It defaults to iw. This expression is evaluated only once during the filter configuration, or when the ‘w’ or ‘out_w’ command is sent.
h, out_h
The height of the output video. It defaults to ih. This expression is evaluated only once during the filter configuration, or when the ‘h’ or ‘out_h’ command is sent.
x
The horizontal position, in the input video, of the left edge of the output video. It defaults to (in_w-out_w)/2. This expression is evaluated per-frame.
y
The vertical position, in the input video, of the top edge of the output video. It defaults to (in_h-out_h)/2. This expression is evaluated per-frame.
crop=w=100:h=100:x=12:y=34
scale缩放，宽高
width, w
height, h
Set the output video dimension expression. Default value is the input dimension.
If the width or w value is 0, the input width is used for the output. If the height or h value is 0, the input height is used for the output.
If one and only one of the values is -n with n >= 1, the scale filter will use a value that maintains the aspect ratio of the input image, calculated from the other specified dimension. After that it will, however, make sure that the calculated dimension is divisible by n and adjust the value if necessary.
If both values are -n with n >= 1, the behavior will be identical to both values being set to 0 as previously detailed.
See below for the list of accepted constants for use in the dimension expression.
scale=w=200:h=100
scale=w=iw/2:h=ih/2
overlay视频叠加，xy
x
y
Set the expression for the x and y coordinates of the overlaid video on the main video.
Default value is "0" for both expressions.
In case the expression is invalid, it is set to a huge value
(meaning that the overlay will not be displayed within the output visible area)