参考文档的参数说明:https://ffmpeg.org/ffmpeg-filters.html#overlay-1
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).
x,y代表了水印位置的坐标位置。默认为0。如果表达式无效,会被设置为一个很大的数字(意味着水印会在可视区域以外而不会显示了)