explain narrow band concept

[martinweismann]

2.1.1 File Formats

A typical approach to store this levelset information is to encode the signed distance field of the boundary of the object in this channel, or to limit the encoding to a narrow region around the boundary of the object.

Referencing a narrow band here is misleading without giving more context. Why is the narrow band mentioned here? One may exploit the "narrow band" concept for better compression by scaling the scalar function so that it will be clamped inside narrow band, thus most of the pixels will be 0/255 or 0/65535.

Extracted from #42

[martinweismann]

Even better:
Add three images for how to encode levelset information: signed distance, narrow band SDF, high resolution binary/black and white image.

[martinweismann]

Fixed in #52

[martinweismann]

Let's explain how this is achieved in the png (truncating values).
and that this gives much better compression.