-/*
- I use less accurate approximation here by simply left-shifting the input
- value and filling the low order bits with zeroes. This method improves PNG
- compression but this scheme cannot reproduce white exactly, since it does
- not generate an all-ones maximum value; the net effect is to darken the
- image slightly.
-
- The better method should be "left bit replication":
-
- 4 3 2 1 0
- ---------
- 1 1 0 1 1
-
- 7 6 5 4 3 2 1 0
- ----------------
- 1 1 0 1 1 1 1 0
- |=======| |===|
- | leftmost bits repeated to fill open bits
- |
- original bits
-*/