Fix failing regionprop_table for multichannel properties (#6861)

* Add test for multichannel weighted centroid
* Return table is a dict, not a dataframe
* Check shape with np.shape to support array-likes
* test shape for rp0 as well as rp1 and rpm

Co-authored-by: Lars Grüter <lagru+github@mailbox.org>

skimage/measure/_regionprops.py

@@ -854,15 +854,10 @@ def _props_to_dict(regions, properties=('label', 'bbox'), separator='-'):
                 column_buffer[i] = regions[i][prop]
             out[orig_prop] = np.copy(column_buffer)
         else:
-            if isinstance(rp, np.ndarray):
-                shape = rp.shape
-            else:
-                shape = (len(rp),)
-
             # precompute property column names and locations
             modified_props = []
             locs = []
-            for ind in np.ndindex(shape):
+            for ind in np.ndindex(np.shape(rp)):
                 modified_props.append(
                     separator.join(map(str, (orig_prop,) + ind))
                 )
@@ -872,7 +867,9 @@ def _props_to_dict(regions, properties=('label', 'bbox'), separator='-'):
             n_columns = len(locs)
             column_data = np.empty((n, n_columns), dtype=dtype)
             for k in range(n):
-                rp = regions[k][prop]
+                # we coerce to a numpy array to ensure structures like
+                # tuple-of-arrays expand correctly into columns
+                rp = np.asarray(regions[k][prop])
                 for i, loc in enumerate(locs):
                     column_data[k, i] = rp[loc]
 

skimage/measure/tests/test_regionprops.py

@@ -33,6 +33,9 @@
 INTENSITY_SAMPLE = SAMPLE.copy()
 INTENSITY_SAMPLE[1, 9:11] = 2
 INTENSITY_FLOAT_SAMPLE = INTENSITY_SAMPLE.copy().astype(np.float64) / 10.0
+INTENSITY_FLOAT_SAMPLE_MULTICHANNEL = (
+        INTENSITY_FLOAT_SAMPLE[..., np.newaxis] * [1, 2, 3]
+        )
 
 SAMPLE_MULTIPLE = np.eye(10, dtype=np.int32)
 SAMPLE_MULTIPLE[3:5, 7:8] = 2
@@ -714,6 +717,27 @@ def test_solidity():
     assert_almost_equal(solidity, target_solidity)
 
 
+def test_multichannel_centroid_weighted_table():
+    """Test for https://github.com/scikit-image/scikit-image/issues/6860."""
+    intensity_image = INTENSITY_FLOAT_SAMPLE_MULTICHANNEL
+    rp0 = regionprops(SAMPLE, intensity_image=intensity_image[..., 0])[0]
+    rp1 = regionprops(SAMPLE, intensity_image=intensity_image[..., 0:1])[0]
+    rpm = regionprops(SAMPLE, intensity_image=intensity_image)[0]
+    np.testing.assert_almost_equal(rp0.centroid_weighted,
+                                   np.squeeze(rp1.centroid_weighted))
+    np.testing.assert_almost_equal(rp0.centroid_weighted,
+                                   np.array(rpm.centroid_weighted)[:, 0])
+    assert np.shape(rp0.centroid_weighted) == (SAMPLE.ndim,)
+    assert np.shape(rp1.centroid_weighted) == (SAMPLE.ndim, 1)
+    assert np.shape(rpm.centroid_weighted) == (SAMPLE.ndim,
+                                               intensity_image.shape[-1])
+
+    table = regionprops_table(SAMPLE, intensity_image=intensity_image,
+                              properties=('centroid_weighted',))
+    # check the number of returned columns is correct
+    assert len(table) == np.size(rpm.centroid_weighted)
+
+
 def test_moments_weighted_central():
     wmu = regionprops(SAMPLE, intensity_image=INTENSITY_SAMPLE
                       )[0].moments_weighted_central