Resolution, magnification, and tiling

Resolution choices determine both what a model can see and how much computation a workflow requires.

MPP and magnification

MPP is a physical measurement; magnification is scanner and optics terminology. Common approximations are:

Nominal magnification	Approximate MPP
10×	1.0
20×	0.5
40×	0.25

These are conventions, not a substitute for metadata. Inspect the slide before assuming a value:

print(wsi.properties)
print(wsi.fetch.pyramids())

If MPP is missing, provide the known source resolution through slide_mpp when tiling:

zs.pp.tile_tissues(wsi, 256, mpp=0.5, slide_mpp=0.25)

Do this only when the scanner or acquisition record provides a trustworthy value.

Field of view

The physical width of a square tile is:

field of view in microns = tile_px * mpp

Tile	MPP	Field of view
256 px	0.5	128 µm
512 px	0.5	256 µm
256 px	0.25	64 µm

Changing tile pixels and changing MPP are therefore not equivalent. Choose the input expected by the model whenever using a pretrained model.

Stride and overlap

With no stride_px or overlap, adjacent tiles do not overlap. Smaller strides improve spatial coverage but increase tile count and compute.

# 25% overlap
zs.pp.tile_tissues(wsi, 256, mpp=0.5, overlap=0.25)

# Equivalent explicit stride for square 256-pixel tiles
zs.pp.tile_tissues(wsi, 256, mpp=0.5, stride_px=192)

overlap and stride_px are alternatives; specify one, not both.

Pyramid levels

Use level for operations that work directly on a stored pyramid level, such as traditional tissue detection. Use MPP for algorithms whose behavior must be physically comparable across scanners.