Nanozoomer annotation coordinate system to openslide pixels

Ahmed Bassiouni ahmedbassi at
Mon Nov 10 02:27:32 EST 2014

Hi everyone,

I am currently trying to use openslide to automatically open up nanozoomer
ndpi files and extract regions automatically. NDP annotation files (ndpa)
are used to mark ROIs. I know that annotations are not part of openslide as
of yet, but I wonder if anyone has an idea about my simple query.

The NDP annotations seems to use their own coordinate system which I can
not convert to the openslide pixel based system to extract the pictures.
(some of the point coordinates are actually negative!! weird! see example

    <ndpviewstate id="1">
        <title>area 3</title>
        <annotation type="freehand" displayname="Rectangular Area"

I wonder if anyone dabbled in something like this and have an idea what
sort of scaling +/- offsetting that they use for marking ROIs like this.

There is only one github module that may be of help in this conversion, but
I failed to run the conversion code on linux and the project seems

Best wishes,
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