[Rtk-users] Tiff lookup table question

[Simon Rit]

Hi Chao,
Good question. I can't remember exactly but looking at the test data,
the image ExternalData/testing/Data/Input/Digisens/ima0010.tif has 0
values at the top border which is probably why I did this since border
is next to air.
Don't hesitate to build your own tiff LUT if you'd prefer maximum
attenuation for 0 values. If you want it in RTK, maybe we can check
for a specific tag in the TIFF file and do a specific treatment for
your scanner.
Good luck!
Simon

On Wed, Dec 24, 2014 at 12:22 PM, Chao Wu <wuchao04 at gmail.com> wrote:
> 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
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