Fix matplotlib#5520: Don't round up if already rounded

lib/matplotlib/image.py

@@ -273,7 +273,9 @@ def _make_image(self, A, in_bbox, out_bbox, clip_bbox, magnification=1.0,
         # to round up the output width to the next integer.  This also
         # means scaling the transform just slightly to account for the
         # extra subpixel.
-        if t.is_affine and round_to_pixel_border:
+        if (t.is_affine and round_to_pixel_border and
+            (out_width_base % 1.0 != 0.0 or
+             out_height_base % 1.0 != 0.0)):
             out_width = int(ceil(out_width_base) + 1)
             out_height = int(ceil(out_height_base) + 1)
             extra_width = (out_width - out_width_base) / out_width_base

lib/matplotlib/tests/test_image.py

@@ -580,6 +580,29 @@ def test_rotate_image():
     ax1.set_ylim(0, 4)
 
 
+@cleanup
+def test_image_preserve_size2():
+    n = 7
+    data = np.identity(n, float)
+
+    fig = plt.figure(figsize=(n, n), frameon=False)
+
+    ax = plt.Axes(fig, [0.0, 0.0, 1.0, 1.0])
+    ax.set_axis_off()
+    fig.add_axes(ax)
+    ax.imshow(data, interpolation='nearest', origin='lower',aspect='auto')
+    buff = io.BytesIO()
+    fig.savefig(buff, dpi=1)
+
+    buff.seek(0)
+    img = plt.imread(buff)
+
+    assert img.shape == (7, 7, 4)
+
+    assert_array_equal(np.asarray(img[:, :, 0], bool),
+                       np.identity(n, bool)[::-1])
+
+
 if __name__=='__main__':
     import nose
     nose.runmodule(argv=['-s','--with-doctest'], exit=False)