draw: Add alpha ability to set_color

skimage/draw/_draw.pyx

@@ -21,20 +21,24 @@ def _coords_inside_image(rr, cc, shape, val=None):
     shape : tuple
         Image shape which is used to determine the maximum extent of output
         pixel coordinates.
-    val : ndarray of float, optional
-        Values of pixels at coordinates [rr, cc].
+    val : (N, D) ndarray of float, optional
+        Values of pixels at coordinates ``[rr, cc]``.
 
     Returns
     -------
-    rr, cc : (N,) array of int
+    rr, cc : (M,) array of int
         Row and column indices of valid pixels (i.e. those inside `shape`).
-    val : (N,) array of float, optional
+    val : (M, D) array of float, optional
         Values at `rr, cc`. Returned only if `val` is given as input.
     """
     mask = (rr >= 0) & (rr < shape[0]) & (cc >= 0) & (cc < shape[1])
-    if val is not None:
-        return rr[mask], cc[mask], val[mask]
-    return rr[mask], cc[mask]
+    if val is None:
+        return rr[mask], cc[mask]
+    else:
+        if np.isscalar(val):
+           return rr[mask], cc[mask], val
+        else:
+           return rr[mask], cc[mask], val[mask]
 
 
 def line(Py_ssize_t y0, Py_ssize_t x0, Py_ssize_t y1, Py_ssize_t x1):

skimage/draw/draw.py

@@ -125,7 +125,7 @@ def circle(r, c, radius, shape=None):
     return ellipse(r, c, radius, radius, shape)
 
 
-def set_color(img, coords, color):
+def set_color(img, coords, color, alpha=None):
     """Set pixel color in the image at the given coordinates.
 
     Coordinates that exceed the shape of the image will be ignored.
@@ -134,10 +134,13 @@ def set_color(img, coords, color):
     ----------
     img : (M, N, D) ndarray
         Image
-    coords : ((P,) ndarray, (P,) ndarray)
-        Coordinates of pixels to be colored.
+    coords : tuple of ((P,) ndarray, (P,) ndarray)
+        Row and column coordinates of pixels to be colored.
     color : (D,) ndarray
         Color to be assigned to coordinates in the image.
+    alpha : scalar or (N,) ndarray
+        Alpha values used to blend color with image.  0 is transparent,
+        1 is opaque.
 
     Returns
     -------
@@ -163,7 +166,38 @@ def set_color(img, coords, color):
            [0, 0, 0, 0, 0, 0, 0, 0, 0, 1]], dtype=uint8)
 
     """
-
     rr, cc = coords
-    rr, cc = _coords_inside_image(rr, cc, img.shape)
-    img[rr, cc] = color
+
+    if alpha is None:
+        rr, cc = _coords_inside_image(rr, cc, img.shape)
+    else:
+        rr, cc, alpha = _coords_inside_image(rr, cc, img.shape, val=alpha)
+
+    if not np.isscalar(color):
+        color = np.array(color)
+        if color.shape[0] != img.shape[-1]:
+            raise ValueError('Color shape ({}) must match last '
+                             'image dimension ({}).'.format(color.shape[0],
+                                                            img.shape[-1]))
+
+    if alpha is None:
+        img[rr, cc] = color
+    else:
+        if np.isscalar(alpha) or np.isscalar(color):
+            color = color * alpha
+        else:
+            color = color * alpha[:, None]
+
+
+        # Strategy: try operation directly.  If scalar / correctly shaped
+        # vector, this will work.  If not (e.g. vector alpha but color image),
+        # try broadcasting.
+        try:
+            vals = img[rr, cc] * (1 - alpha)
+        except ValueError:
+            vals = img[rr, cc] * (1 - alpha[:, None])
+
+        try:
+            img[rr, cc] = vals + color
+        except ValueError:
+            img[rr, cc] = vals + color[:, None]

skimage/draw/tests/test_draw.py

@@ -1,4 +1,4 @@
-from numpy.testing import assert_array_equal, assert_equal
+from numpy.testing import assert_array_equal, assert_equal, assert_raises
 import numpy as np
 from skimage._shared.testing import test_parallel
 
@@ -20,6 +20,24 @@ def test_set_color():
     assert_array_equal(img, img_)
 
 
+def test_set_color_with_alpha():
+    img = np.zeros((10, 10))
+
+    rr, cc, alpha = line_aa(0, 0, 0, 30)
+    set_color(img, (rr, cc), 1, alpha=alpha)
+
+    # Wrong dimensionality color
+    assert_raises(ValueError, set_color, img, (rr, cc), (255, 0, 0), alpha=alpha)
+
+    img = np.zeros((10, 10, 3))
+
+    rr, cc, alpha = line_aa(0, 0, 0, 30)
+    set_color(img, (rr, cc), 1, alpha=alpha)
+
+    rr, cc, alpha = line_aa(0, 0, 0, 30)
+    set_color(img, (rr, cc), (1, 0, 0), alpha=alpha)
+
+
 @test_parallel()
 def test_line_horizontal():
     img = np.zeros((10, 10))
@@ -72,7 +90,7 @@ def test_line_aa_horizontal():
     img = np.zeros((10, 10))
 
     rr, cc, val = line_aa(0, 0, 0, 9)
-    img[rr, cc] = val
+    set_color(img, (rr, cc), 1, alpha=val)
 
     img_ = np.zeros((10, 10))
     img_[0, :] = 1
@@ -329,12 +347,14 @@ def test_circle_perimeter_aa_shape():
     img = np.zeros((15, 20), 'uint8')
     rr, cc, val = circle_perimeter_aa(7, 10, 9, shape=(15, 20))
     img[rr, cc] = val * 255
+
     shift = 5
     img_ = np.zeros((15 + 2 * shift, 20), 'uint8')
     rr, cc, val = circle_perimeter_aa(7 + shift, 10, 9, shape=None)
     img_[rr, cc] = val * 255
     assert_array_equal(img, img_[shift:-shift, :])
 
+
 def test_ellipse_trivial():
     img = np.zeros((2, 2), 'uint8')
     rr, cc = ellipse(0.5, 0.5, 0.5, 0.5)