UBI: about ubinize tool

[Nancy]

Hi,
    As I know, ubinize can generate UBI images. I don't understand why
its .cfg file need parameter "image=ubifs.img", why can't it collect
the content
from the defined vol_id automatically and output to file "ubi.img"?
    How to make the input image file? mkfs.ubifs can only generate
ubifs image. If it was another filesystem eg. vfat, there's no tool.
That's why I need a tool like ubinize. Its job should be collect all
the PEBs( not include ech) that the volume used and put them to output
files
.