Blender 2.5 python uv pack islands pixel precision

I'm tryi开发者_运维百科ng to pack the uv's of a mesh such that there is a border between the UV islands of a specific pixel size. For this I'm using:

m = 4.0 / imageWidth
bpy.ops.uv.pack_islands(margin=m)

So it should create a border about 4 pixels around each island. The problem is that depending on the amount of uv islands the border size varies a lot.

How is the margin parameter used?