Fix a bug transform_matrix_offset_center (#855)

* Fix a bug `transform_matrix_offset_center`

- calculation of offset: sign error

* Update CHANGELOG.md

* Add PR number

* minor modification

* minor modification

Author: 2wins

CHANGELOG.md

@@ -85,13 +85,14 @@ To release a new version, please update the changelog as followed:
 ### Deprecated
 
 ### Fixed
+- Correct offset calculation in `tl.prepro.transform_matrix_offset_center` (PR #855)
 
 ### Removed
 
 ### Security
 
 ### Contributors
-- @2wins: #850
+- @2wins: #850 #855
 - @DEKHTIARJonathan: #853
 
 ## [1.10.1] - 2018-09-07

tensorlayer/prepro.py

@@ -1719,8 +1719,8 @@ def transform_matrix_offset_center(matrix, x, y):
     - See ``tl.prepro.rotation``, ``tl.prepro.shear``, ``tl.prepro.zoom``.
 
     """
-    o_x = float(x) / 2 + 0.5
-    o_y = float(y) / 2 + 0.5
+    o_x = (x - 1) / 2.0
+    o_y = (y - 1) / 2.0
     offset_matrix = np.array([[1, 0, o_x], [0, 1, o_y], [0, 0, 1]])
     reset_matrix = np.array([[1, 0, -o_x], [0, 1, -o_y], [0, 0, 1]])
     transform_matrix = np.dot(np.dot(offset_matrix, matrix), reset_matrix)