Nanozoomer annotation coordinate system to openslide pixels

Benjamin Gilbert bgilbert at
Tue Nov 11 01:27:02 EST 2014

On 11/10/2014 10:40 PM, Ahmed Bassiouni wrote:
> There is another tag, which I then investigated,
> but it does not look like the correct value to look at

It also doesn't exist in every slide.

> In this way, we need the complete width+length of the whole slide to
> calculate coordinates. This has to be somewhere in the ndpi file itself
> ( I hope).

Older slides don't have a lot of metadata (see CMU-1.ndpi, etc.) and 
none of it seems relevant.  Perhaps the vendor's software hardcodes the 
size of the slide?

> BTW There are also tiff.ResolutionUnit (in my slides it says
> centimeter), tiff.XResolution and tiff.YResolution. Can these tags help?

See my previous email.  Those OpenSlide properties refer to level 0 of 
the main image, and in fact are used to calculate the openslide.mpp-[xy] 
properties.  NDPI files do contain X/YResolution tags for the macro 
image (you can see this with tiffdump) but when using them to calculate 
slide dimensions, the rounding error can be significant.

--Benjamin Gilbert

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