adding a CountMethod to default pixel selction and including tests

rio_tiler/mosaic/methods/defaults.py

@@ -188,3 +188,32 @@ def feed(self, array: Optional[numpy.ma.MaskedArray]):
             mask = numpy.where(pidex, array.mask, self.mosaic.mask)
             self.mosaic = numpy.ma.where(pidex, array, self.mosaic)
             self.mosaic.mask = mask
+
+
+@dataclass
+class CountMethod(MosaicMethodBase):
+    """Stack the arrays and return the valid pixel count."""
+
+    stack: List[numpy.ma.MaskedArray] = field(default_factory=list, init=False)
+
+    @property
+    def data(self) -> Optional[numpy.ma.MaskedArray]:
+        """Return valid data count of the data stack."""
+        if self.stack:
+            data = numpy.ma.count(numpy.ma.stack(self.stack, axis=0), axis=0).astype(numpy.uint16)
+
+            # only need the counts from one band
+            if len(data.shape) > 2:
+                data = data[0]
+
+            # mask is always empty
+            mask = numpy.zeros(data.shape, dtype=bool)
+            array = numpy.ma.MaskedArray(data, mask)
+
+            return array
+
+        return None
+
+    def feed(self, array: Optional[numpy.ma.MaskedArray]):
+        """Add array to the stack."""
+        self.stack.append(array)

tests/test_mosaic.py

@@ -285,6 +285,16 @@ class aClass(object):
     assert t.dtype == "uint16"
     assert m.dtype == "uint8"
 
+    # Test count pixel selection
+    (t, m), _ = mosaic.mosaic_reader(
+        assets, _read_tile, x, y, z, pixel_selection=defaults.CountMethod()
+    )
+    assert t.shape == (1, 256, 256)
+    assert m.shape == (256, 256)
+    assert m.all()
+    assert t.dtype == "uint16"
+    assert m.dtype == "uint8"
+
 
 def mock_rasterio_open(asset):
     """Mock rasterio Open."""