Minor bug with Aperio Images with fractional objectives

[Benjamin Gilbert]

On Tue, Mar 22, 2016 at 03:35:12AM +0000, Derek Magee via openslide-users wrote:
> >Yep, OpenSlide parses AppMag as integer, and doesn't add the generic
> >property if parsing fails.  How did you produce the affected slides? 
> >Were they actually scanned with non-integer objectives?
> 
> They are the result of image analysis where there is a downsample as the
> first step => 20X image becomes 2.5X if downsampled by a power of 2 factor
> 3 times.  Power of 2 downsampling is a fairly sensible thing to do, but
> you quickly get to non-integer objective.

Downsampling will affect the MPP, but shouldn't change the objective power,
since that's a property of the optical path.  (See, for example,
MIRAX/CMU-1-Saved-1_16.)

> I'd say even if you objected to non-integer objectives (I don't see why),
> the Aperio strategy of Rounding at least gives some clue.  I personally
> use it to work out the ratio between original and downsampled versions
> (although I could just use mpp-x, or mpp-y equally as well).  It's not a
> major problem for me now I know it happens, but it may trip up others.

`openslide.objective-power` is documented to be optional.  We do typically
add workarounds for issues likely to be seen in the wild, but I can't tell
whether that applies in this case.  Thus my question: are the affected
slides created by vendor code, or by a custom image pipeline of some sort?

Thanks,
--Benjamin Gilbert