I packed images into atlas to use it in unity, and I unpacked it because I needed to modify it in spine again. However, it seems like the transparency setting is gone, and all transparent parts and outlines look grayish now. The transparency works fine as image atlas in unity, but since spine doesn't seem to read atlas files so I have to unpack it to see it in spine. So I would like to know how to unpack image atlas without damaging image transparencies.

Hello neoeric! You likely packed the atlas with the premultiply alpha setting if you are experiencing dark borders around the images.
You can easily restore the images by checking the unpremultiply alpha setting when using the texture unpacker in Spine.

I was searching for alpha setting, and found out it is in 3.8. I use 3.7 so I didn't know the existence of it. Anyway, now I can unpack the atlas without problem. Thank you very much!