固定比例加黑边：without scaling, pad an image with fixed ratioffmpeg -i “seven.jpg” -filter:v “pad=iw1.5:ih:(ow-iw)/2” out.jpg左右两边各加(output_weight-input_weight)/2 output_weight=input_weight1.5
固定比例加黑边：without scaling, pad an image with fixed ratio
