Radon transform: Redefine projection center for sinograms.

This definition is chosen because it is simple to express in the
documentation. No changes in accuracy are to be expected, but comparing
sinograms and reconstructions before and after this commit will give
different results in the cases where ``circle=False`` for ``radon`` or
``iradon``.

skimage/transform/radon_transform.py

@@ -82,12 +82,15 @@ def radon(image, theta=None, circle=False):
     else:
         diagonal = np.sqrt(2) * max(image.shape)
         pad = [int(np.ceil(diagonal - s)) for s in image.shape]
-        pad_width = [(p // 2, p - p // 2) for p in pad]
+        new_center = [(s + p) // 2 for s, p in zip(image.shape, pad)]
+        old_center = [s // 2 for s in image.shape]
+        pad_before = [nc - oc for oc, nc in zip(old_center, new_center)]
+        pad_width = [(pb, p - pb) for pb, p in zip(pad_before, pad)]
         padded_image = util.pad(image, pad_width, mode='constant',
                                 constant_values=0)
         out = np.zeros((max(padded_image.shape), len(theta)))
-        dh = pad[0] // 2 + image.shape[0] // 2
-        dw = pad[1] // 2 + image.shape[1] // 2
+        dh = padded_image.shape[0] // 2
+        dw = padded_image.shape[1] // 2
 
     shift0 = np.array([[1, 0, -dw],
                        [0, 1, -dh],
@@ -112,7 +115,10 @@ def radon(image, theta=None, circle=False):
 def _sinogram_circle_to_square(sinogram):
     diagonal = int(np.ceil(np.sqrt(2) * sinogram.shape[0]))
     pad = diagonal - sinogram.shape[0]
-    pad_width = ((pad // 2, pad - pad // 2), (0, 0))
+    old_center = sinogram.shape[0] // 2
+    new_center = diagonal // 2
+    pad_before = new_center - old_center
+    pad_width = ((pad_before, pad - pad_before), (0, 0))
     return util.pad(sinogram, pad_width, mode='constant', constant_values=0)
 
 
@@ -216,9 +222,7 @@ def iradon(radon_image, theta=None, output_size=None,
     radon_filtered = radon_filtered[:radon_image.shape[0], :]
     reconstructed = np.zeros((output_size, output_size))
     # Determine the center of the projections (= center of sinogram)
-    circle_size = int(np.floor(radon_image.shape[0] / np.sqrt(2)))
-    square_size = radon_image.shape[0]
-    mid_index = (square_size - circle_size) // 2 + circle_size // 2
+    mid_index = radon_image.shape[0] // 2
 
     x = output_size
     y = output_size