From b83ece8b0583093ea1692af4bcf84f0fb8f53066 Mon Sep 17 00:00:00 2001
From: Stefan van der Walt <stefan@sun.ac.za>
Date: Fri, 16 Dec 2011 23:47:32 -0800
Subject: [PATCH] ENH: Allow choice between 4 and 8 neighbor mode.  Improve
 documentation.

---
 skimage/morphology/ccomp.pyx           | 55 +++++++++++++++++++++-----
 skimage/morphology/tests/test_ccomp.py | 10 +++++
 2 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/skimage/morphology/ccomp.pyx b/skimage/morphology/ccomp.pyx
index 54585f3a..b1fdc828 100644
--- a/skimage/morphology/ccomp.pyx
+++ b/skimage/morphology/ccomp.pyx
@@ -66,16 +66,27 @@ cdef join_trees(np.int_t *work, np.int_t n, np.int_t m):
 
 # Connected components search as described in Fiorio et al.
 
-def label(np.ndarray[DTYPE_t, ndim=2] input):
+def label(np.ndarray[DTYPE_t, ndim=2] input,
+          int neighbors=8):
     """Label connected regions of an integer array.
 
-    Connectivity is defined as two (8-connected) neighboring entries
-    having equal value.
+    Two pixels are connected when they are neighbors and have the same value.
+    They can be neighbors either in a 4- or 8-connected sense::
+
+      4-connectivity      8-connectivity
+
+           [ ]           [ ]  [ ]  [ ]
+            |               \  |  /
+      [ ]--[ ]--[ ]      [ ]--[ ]--[ ]
+            |               /  |  \ 
+           [ ]           [ ]  [ ]  [ ]
 
     Parameters
     ----------
     input : ndarray of dtype int
         Image to label.
+    neighbors : {4, 8}, int
+        Whether to use 4- or 8-connectivity.
 
     Returns
     -------
@@ -83,6 +94,24 @@ def label(np.ndarray[DTYPE_t, ndim=2] input):
         Labeled array, where all connected regions are assigned the
         same integer value.
 
+    Examples
+    --------
+    >>> x = np.eye(3).astype(int)
+    >>> print x
+    [[1 0 0]
+     [0 1 0]
+     [0 0 1]]
+
+    >>> print m.label(x, neighbors=4)
+    [[0 1 1]
+     [2 3 1]
+     [2 2 4]]
+
+    >>> print m.label(x, neighbors=8)
+    [[0 1 1]
+     [1 0 1]
+     [1 1 0]]
+
     """
     cdef np.int_t rows = input.shape[0]
     cdef np.int_t cols = input.shape[1]
@@ -97,6 +126,9 @@ def label(np.ndarray[DTYPE_t, ndim=2] input):
 
     cdef np.int_t i, j
 
+    if neighbors != 4 and neighbors != 8:
+        raise ValueError('Neighbors must be either 4 or 8.')
+
     # Initialize the first row
     for j in range(1, cols):
         if data[0, j] == data[0, j-1]:
@@ -107,19 +139,22 @@ def label(np.ndarray[DTYPE_t, ndim=2] input):
         if data[i, 0] == data[i-1, 0]:
             join_trees(work_p, i*cols, (i-1)*cols)
 
-        if data[i, 0] == data[i-1, 1]:
-            join_trees(work_p, i*cols, (i-1)*cols + 1)
+        if neighbors == 8:
+            if data[i, 0] == data[i-1, 1]:
+                join_trees(work_p, i*cols, (i-1)*cols + 1)
 
         for j in range(1, cols):
-            if data[i, j] == data[i-1, j-1]:
-                join_trees(work_p, i*cols + j, (i-1)*cols + j - 1)
+            if neighbors == 8:
+                if data[i, j] == data[i-1, j-1]:
+                    join_trees(work_p, i*cols + j, (i-1)*cols + j - 1)
 
             if data[i, j] == data[i-1, j]:
                 join_trees(work_p, i*cols + j, (i-1)*cols + j)
 
-            if j < cols - 1:
-                if data[i, j] == data[i - 1, j + 1]:
-                    join_trees(work_p, i*cols + j, (i-1)*cols + j + 1)
+            if neighbors == 8:
+                if j < cols - 1:
+                    if data[i, j] == data[i - 1, j + 1]:
+                        join_trees(work_p, i*cols + j, (i-1)*cols + j + 1)
 
             if data[i, j] == data[i, j-1]:
                 join_trees(work_p, i*cols + j, i*cols + j - 1)
diff --git a/skimage/morphology/tests/test_ccomp.py b/skimage/morphology/tests/test_ccomp.py
index 5bf4cfdc..04c21feb 100644
--- a/skimage/morphology/tests/test_ccomp.py
+++ b/skimage/morphology/tests/test_ccomp.py
@@ -37,5 +37,15 @@ class TestConnectedComponents:
         assert_array_equal(label(x),
                            x)
 
+    def test_4_vs_8(self):
+        x = np.array([[0, 1],
+                      [1, 0]], dtype=int)
+        assert_array_equal(label(x, 4),
+                           [[0, 1],
+                            [2, 3]])
+        assert_array_equal(label(x, 8),
+                           [[0, 1],
+                            [1, 0]])
+
 if __name__ == "__main__":
     run_module_suite()