Retrieve MPP from WSIs

src/eva/vision/data/wsi/backends/openslide.py

@@ -32,8 +32,26 @@ def level_downsamples(self) -> Sequence[float]:
     @override
     def mpp(self) -> float:
         # TODO: add overwrite_mpp class attribute to allow setting a default value
-        x_mpp = float(self._wsi.properties["openslide.mpp-x"])
-        y_mpp = float(self._wsi.properties["openslide.mpp-y"])
+        if self._wsi.properties.get(openslide.PROPERTY_NAME_MPP_X) and self._wsi.properties.get(
+            openslide.PROPERTY_NAME_MPP_Y
+        ):
+            x_mpp = float(self._wsi.properties[openslide.PROPERTY_NAME_MPP_X])
+            y_mpp = float(self._wsi.properties[openslide.PROPERTY_NAME_MPP_Y])
+        elif (
+            self._wsi.properties.get("tiff.XResolution")
+            and self._wsi.properties.get("tiff.YResolution")
+            and self._wsi.properties.get("tiff.ResolutionUnit")
+        ):
+            unit = self._wsi.properties.get("tiff.ResolutionUnit")
+            if unit not in _conversion_factor_to_micrometer:
+                raise ValueError(f"Unit {unit} not supported.")
+
+            conversion_factor = float(_conversion_factor_to_micrometer.get(unit))  # type: ignore
+            x_mpp = conversion_factor / float(self._wsi.properties["tiff.XResolution"])
+            y_mpp = conversion_factor / float(self._wsi.properties["tiff.YResolution"])
+        else:
+            raise ValueError("`mpp` cannot be obtained for this slide.")
+
         return (x_mpp + y_mpp) / 2.0
 
     @override
@@ -58,3 +76,15 @@ def read_region(
             data[data[:, :, 3] == 0] = 255
 
         return data[:, :, :3]
+
+
+_conversion_factor_to_micrometer = {
+    "meter": 10**6,
+    "decimeter": 10**5,
+    "centimeter": 10**4,
+    "millimeter": 10**3,
+    "micrometer": 1,
+    "nanometer": 10**-3,
+    "picometer": 10**-6,
+    "femtometer": 10**-9,
+}