Fix Affine BB aug using only 2 corner points

This fixes a bug where the bounding box augmentation
in Affine uses only two corner cordinates (top-left,
bottom-right) instead of all four, leading to malformed
output bounding boxes during rotation and shear.

This bug was likely introduced by #451 (2d3fdb7,
single_step_augment).

imgaug/augmenters/geometric.py

@@ -1007,9 +1007,23 @@ def _augment_batch(self, batch, random_state, parents, hooks):
                     if (not _is_identity_matrix(matrix)
                             and not cbaoi.empty
                             and not (0 in cbaoi.shape[0:2])):
-                        coords = cbaoi.to_xy_array()
-                        coords_aug = tf.matrix_transform(coords, matrix.params)
-                        cbaoi = cbaoi.fill_from_xy_array_(coords_aug)
+                        # TODO this is hacky
+                        if augm_name == "bounding_boxes":
+                            # Ensure that 4 points are used for bbs.
+                            # to_keypoints_on_images() does return 4 points,
+                            # to_xy_array() does not.
+                            kpsoi = cbaoi.to_keypoints_on_image()
+                            coords = kpsoi.to_xy_array()
+                            coords_aug = tf.matrix_transform(coords,
+                                                             matrix.params)
+                            kpsoi = kpsoi.fill_from_xy_array_(coords_aug)
+                            cbaoi = cbaoi.invert_to_keypoints_on_image_(
+                                kpsoi)
+                        else:
+                            coords = cbaoi.to_xy_array()
+                            coords_aug = tf.matrix_transform(coords,
+                                                             matrix.params)
+                            cbaoi = cbaoi.fill_from_xy_array_(coords_aug)
 
                     cbaoi.shape = output_shape
                     augm_value[i] = cbaoi

test/augmenters/test_geometric.py

@@ -1851,6 +1851,27 @@ def test_alignment_between_images_and_smaller_heatmaps_for_fixed_rot(self):
             assert hm_aug.arr_0to1.shape == (28, 24, 1)
             assert (same / img_aug_mask.size) >= 0.9
 
+    def test_bounding_boxes_have_expected_shape_after_augmentation(self):
+        image = np.zeros((100, 100), dtype=np.uint8)
+        image[20:80, 20:80] = 255
+        bb = ia.BoundingBox(x1=20, y1=20, x2=80, y2=80)
+        bbsoi = ia.BoundingBoxesOnImage([bb], shape=image.shape)
+        for rotate in [10, 20, 40, 80, 120]:
+            with self.subTest(rotate=rotate):
+                aug = iaa.Affine(rotate=rotate, order=0)
+
+                image_aug, bbsoi_aug = aug(image=image, bounding_boxes=bbsoi)
+
+                xx = np.nonzero(np.max(image_aug > 100, axis=0))[0]
+                yy = np.nonzero(np.max(image_aug > 100, axis=1))[0]
+                bb_exp_x1 = xx[0]
+                bb_exp_x2 = xx[-1]
+                bb_exp_y1 = yy[0]
+                bb_exp_y2 = yy[-1]
+                bb_expected = ia.BoundingBox(x1=bb_exp_x1, y1=bb_exp_y1,
+                                             x2=bb_exp_x2, y2=bb_exp_y2)
+                assert bbsoi_aug.bounding_boxes[0].iou(bb_expected) > 0.95
+
 
 class TestAffine_cval(unittest.TestCase):
     @property