Question on MPP metadata

[dt-yuhui]

Hi,

I noticed that when reading the image header (slide.contents.picHead.contents), there are two fields that potentially represent the spatial resolution (MPP - Microns Per Pixel) of the highest magnification layer (Level 0).

I would appreciate it if you could clarify which of the following two fields should be used as the definitive MPP (Microns Per Pixel) value for the Level 0 image.

Observed Header Fields:

Here is the relevant output from reading the header structure:

Field Name	Type	Observed Value
hierarchy	ctypes.c_uint	8
srcWidth	ctypes.c_uint	99456
srcHeight	ctypes.c_uint	77952
scale	ctypes.c_float	0.5
ruler	ctypes.c_double	0.20721726190476192
rate	ctypes.c_uint	20

Questions:

1. Which field is the true MPP value? Is it scale (0.5) or ruler (0.207...)?
2. What do the values represent?
   • If scale (0.5) is the MPP, it corresponds perfectly to a 20X magnification scan (which typically uses 0.5 mpp).
   • If ruler (~0.207) is the MPP, it is very close to 0.25 mpp (40X), but the rate field is set to 20 (suggesting 20X).

This contradiction between scale/ruler and rate is confusing. Could you please confirm the exact unit and meaning of the scale and ruler fields, and which one we should use to accurately set the spatial resolution for Level 0?

Thank you for your help and for maintaining this useful library!

[Fuar25]

Actually, I think that should be ruler. Because that remains the same as I saw the metadata in ImageViewer software.