How to apply mask softness only on one side of the mask?

Hi, thank you for your outstanding work. I am adding an outpainting function in my open-source project and have a question to ask: How can I modify the soften_mask function to make it only apply "soft" to one side of the mask?

g-diffuser-bot/modules/g_diffuser_lib.py

Line 392 in a003394

def soften_mask(np_rgba_image, softness, space, use_fft=True):

This is the current effect, if I only want to outpaint one side, the bottom mask will also be soft.

image	mask