[Rtk-users] Tiff lookup table question

[Chao Wu]

Hi everyone, Merry Christmas!

I have some minor questions about the tiff lookup table for converting
tiff values to attenuation in rtkTiffLookupTableImageFilter.h. I found
the table a little bit strange. Taking 8 bit unsigned integer tiff
pixels as an example.
1) The reference value will be log(257),
2) pixel value p=0 is no attenuation, and
3) for 1<=p<=255 the attenuation is reference - log(p+1).

Therefore the table looks like:
p    attenuation
0    0, or log(257)-log(257)
1    log(257)-log(2)
2    log(257)-log(3)
3    log(257)-log(4)
...
254    log(257)-log(255)
255    log(257)-log(256)

My questions are:
Why is p=0 treated differently? Is this an industrial standard?
For pixel values from 1 to 255, why is the attenuation
log(257)-log(p+1), not log(256)-log(p)?

Thanks and best regards,
Chao