Add imresize to mxnet.image documentation

docs/api/python/image/image.md

@@ -17,6 +17,7 @@ images provided in
     :nosignatures:
 
     image.imdecode
+    image.imresize
     image.scale_down
     image.resize_short
     image.fixed_crop
@@ -164,6 +165,7 @@ and a list of augmenters specific for `Object detection` is provided
     :members:
 
 .. automethod:: mxnet.image.imdecode
+.. automethod:: mxnet.image.imresize
 .. automethod:: mxnet.image.scale_down
 .. automethod:: mxnet.image.resize_short
 .. automethod:: mxnet.image.fixed_crop

python/mxnet/image/image.py

@@ -38,7 +38,6 @@
 from ..base import numeric_types
 from .. import ndarray as nd
 from ..ndarray import _internal
-from ..ndarray._internal import _cvimresize as imresize
 from ..ndarray._internal import _cvcopyMakeBorder as copyMakeBorder
 from .. import io
 from .. import recordio
@@ -47,7 +46,7 @@
 def imread(filename, *args, **kwargs):
     """Read and decode an image to an NDArray.
 
-    Note: `imread` uses OpenCV (not the CV2 Python library).
+    .. note:: `imread` uses OpenCV (not the CV2 Python library).
     MXNet must have been built with USE_OPENCV=1 for `imdecode` to work.
 
     Parameters
@@ -85,10 +84,67 @@ def imread(filename, *args, **kwargs):
     return _internal._cvimread(filename, *args, **kwargs)
 
 
+def imresize(src, w, h, interp):
+    r"""Resize image with OpenCV.
+
+    .. note:: `imresize` uses OpenCV (not the CV2 Python library). MXNet must have been built
+    with USE_OPENCV=1 for `imresize` to work.
+
+    Parameters
+    ----------
+    src : NDArray
+        source image
+    w : int, required
+        Width of resized image.
+    h : int, required
+        Height of resized image.
+    interp : int, optional, default=1
+        Interpolation method (default=cv2.INTER_LINEAR).
+        Possible values:
+        0: Nearest Neighbors Interpolation.
+        1: Bilinear interpolation.
+        2: Area-based (resampling using pixel area relation). It may be a
+        preferred method for image decimation, as it gives moire-free
+        results. But when the image is zoomed, it is similar to the Nearest
+        Neighbors method. (used by default).
+        3: Bicubic interpolation over 4x4 pixel neighborhood.
+        4: Lanczos interpolation over 8x8 pixel neighborhood.
+        9: Cubic for enlarge, area for shrink, bilinear for others
+        10: Random select from interpolation method metioned above.
+        Note:
+        When shrinking an image, it will generally look best with AREA-based
+        interpolation, whereas, when enlarging an image, it will generally look best
+        with Bicubic (slow) or Bilinear (faster but still looks OK).
+        More details can be found in the documentation of OpenCV, please refer to
+        http://docs.opencv.org/master/da/d54/group__imgproc__transform.html.
+
+    out : NDArray, optional
+        The output NDArray to hold the result.
+
+    Returns
+    -------
+    out : NDArray or list of NDArrays
+        The output of this function.
+
+    Example
+    -------
+    >>> with open("flower.jpeg", 'rb') as fp:
+    ...     str_image = fp.read()
+    ...
+    >>> image = mx.img.imdecode(str_image)
+    >>> image
+    <NDArray 2321x3482x3 @cpu(0)>
+    >>> new_image = mx.img.resize(image, 240, 360)
+    >>> new_image
+    <NDArray 2321x3482x3 @cpu(0)>
+    """
+    return _internal._cvimresize(src, w, h, interp)
+
+
 def imdecode(buf, *args, **kwargs):
     """Decode an image to an NDArray.
 
-    Note: `imdecode` uses OpenCV (not the CV2 Python library).
+    .. note:: `imdecode` uses OpenCV (not the CV2 Python library).
     MXNet must have been built with USE_OPENCV=1 for `imdecode` to work.
 
     Parameters
@@ -235,7 +291,7 @@ def _get_interp_method(interp, sizes=()):
 def resize_short(src, size, interp=2):
     """Resizes shorter edge to size.
 
-    Note: `resize_short` uses OpenCV (not the CV2 Python library).
+    .. note:: `resize_short` uses OpenCV (not the CV2 Python library).
     MXNet must have been built with OpenCV for `resize_short` to work.
 
     Resizes the original image by setting the shorter edge to size

tests/python/unittest/test_image.py

@@ -84,7 +84,7 @@ def test_imdecode(self):
         try:
             import cv2
         except ImportError:
-            return
+            raise unittest.SkipTest("Unable to import cv2.")
         for img in TestImage.IMAGES:
             with open(img, 'rb') as fp:
                 str_image = fp.read()
@@ -97,11 +97,12 @@ def test_scale_down(self):
         assert mx.image.scale_down((360, 1000), (480, 500)) == (360, 375)
         assert mx.image.scale_down((300, 400), (0, 0)) == (0, 0)
 
+    @with_seed()
     def test_resize_short(self):
         try:
             import cv2
         except ImportError:
-            return
+            raise unittest.SkipTest("Unable to import cv2")
         for img in TestImage.IMAGES:
             cv_img = cv2.imread(img)
             mx_img = mx.nd.array(cv_img[:, :, (2, 1, 0)])
@@ -118,6 +119,23 @@ def test_resize_short(self):
                     mx_resized = mx.image.resize_short(mx_img, new_size, interp)
                     assert_almost_equal(mx_resized.asnumpy()[:, :, (2, 1, 0)], cv_resized, atol=3)
 
+    @with_seed()
+    def test_imresize(self):
+        try:
+            import cv2
+        except ImportError:
+            raise unittest.SkipTest("Unable to import cv2")
+        for img in TestImage.IMAGES:
+            cv_img = cv2.imread(img)
+            mx_img = mx.nd.array(cv_img[:, :, (2, 1, 0)])
+            for _ in range(3):
+                new_h = np.random.randint(1, 1000)
+                new_w = np.random.randint(1, 1000)
+                for interp in range(0, 2):
+                    cv_resized = cv2.resize(cv_img, (new_w, new_h), interpolation=interp)
+                    mx_resized = mx.image.imresize(mx_img, new_w, new_h, interp)
+                    assert_almost_equal(mx_resized.asnumpy()[:, :, (2, 1, 0)], cv_resized, atol=3)
+
     def test_color_normalize(self):
         for _ in range(10):
             mean = np.random.rand(3) * 255