Support pillow resize (#426)

* Support pillow resize

* add more types

mmcv/image/geometric.py

@@ -4,6 +4,13 @@ import numbers
 import cv2
 import numpy as np
 
+from .io import imread_backend
+
+try:
+    from PIL import Image
+except ImportError:
+    Image = None
+
 
 def _scale_size(size, scale):
     """Rescale a size by a ratio.
@@ -19,7 +26,7 @@ def _scale_size(size, scale):
     return int(w * float(scale) + 0.5), int(h * float(scale) + 0.5)
 
 
-interp_codes = {
+cv2_interp_codes = {
     'nearest': cv2.INTER_NEAREST,
     'bilinear': cv2.INTER_LINEAR,
     'bicubic': cv2.INTER_CUBIC,
@@ -27,12 +34,23 @@ interp_codes = {
     'lanczos': cv2.INTER_LANCZOS4
 }
 
+if Image is not None:
+    pillow_interp_codes = {
+        'nearest': Image.NEAREST,
+        'bilinear': Image.BILINEAR,
+        'bicubic': Image.BICUBIC,
+        'box': Image.BOX,
+        'lanczos': Image.LANCZOS,
+        'hamming': Image.HAMMING
+    }
+
 
 def imresize(img,
              size,
              return_scale=False,
              interpolation='bilinear',
-             out=None):
+             out=None,
+             backend=None):
     """Resize image to a given size.
 
     Args:
@@ -40,16 +58,32 @@ def imresize(img,
         size (tuple[int]): Target size (w, h).
         return_scale (bool): Whether to return `w_scale` and `h_scale`.
         interpolation (str): Interpolation method, accepted values are
-            "nearest", "bilinear", "bicubic", "area", "lanczos".
+            "nearest", "bilinear", "bicubic", "area", "lanczos" for 'cv2'
+            backend, "nearest", "bilinear" for 'pillow' backend.
         out (ndarray): The output destination.
+        backend (str | None): The image resize backend type. Options are `cv2`,
+            `pillow`, `None`. If backend is None, the global imread_backend
+            specified by ``mmcv.use_backend()`` will be used. Default: None.
 
     Returns:
         tuple | ndarray: (`resized_img`, `w_scale`, `h_scale`) or
             `resized_img`.
     """
     h, w = img.shape[:2]
-    resized_img = cv2.resize(
-        img, size, dst=out, interpolation=interp_codes[interpolation])
+    if backend is None:
+        backend = imread_backend
+    if backend not in ['cv2', 'pillow']:
+        raise ValueError(f'backend: {backend} is not supported for resize.'
+                         f"Supported backends are 'cv2', 'pillow'")
+
+    if backend == 'pillow':
+        assert img.dtype == np.uint8, 'Pillow backend only support uint8 type'
+        pil_image = Image.fromarray(img)
+        pil_image = pil_image.resize(size, pillow_interp_codes[interpolation])
+        resized_img = np.array(pil_image)
+    else:
+        resized_img = cv2.resize(
+            img, size, dst=out, interpolation=cv2_interp_codes[interpolation])
     if not return_scale:
         return resized_img
     else:
@@ -58,7 +92,11 @@ def imresize(img,
         return resized_img, w_scale, h_scale
 
 
-def imresize_like(img, dst_img, return_scale=False, interpolation='bilinear'):
+def imresize_like(img,
+                  dst_img,
+                  return_scale=False,
+                  interpolation='bilinear',
+                  backend=None):
     """Resize image to the same size of a given image.
 
     Args:
@@ -66,13 +104,14 @@ def imresize_like(img, dst_img, return_scale=False, interpolation='bilinear'):
         dst_img (ndarray): The target image.
         return_scale (bool): Whether to return `w_scale` and `h_scale`.
         interpolation (str): Same as :func:`resize`.
+        backend (str | None): Same as :func:`resize`.
 
     Returns:
         tuple or ndarray: (`resized_img`, `w_scale`, `h_scale`) or
             `resized_img`.
     """
     h, w = dst_img.shape[:2]
-    return imresize(img, (w, h), return_scale, interpolation)
+    return imresize(img, (w, h), return_scale, interpolation, backend=backend)
 
 
 def rescale_size(old_size, scale, return_scale=False):
@@ -112,7 +151,11 @@ def rescale_size(old_size, scale, return_scale=False):
         return new_size
 
 
-def imrescale(img, scale, return_scale=False, interpolation='bilinear'):
+def imrescale(img,
+              scale,
+              return_scale=False,
+              interpolation='bilinear',
+              backend=None):
     """Resize image while keeping the aspect ratio.
 
     Args:
@@ -124,13 +167,15 @@ def imrescale(img, scale, return_scale=False, interpolation='bilinear'):
         return_scale (bool): Whether to return the scaling factor besides the
             rescaled image.
         interpolation (str): Same as :func:`resize`.
+        backend (str | None): Same as :func:`resize`.
 
     Returns:
         ndarray: The rescaled image.
     """
     h, w = img.shape[:2]
     new_size, scale_factor = rescale_size((w, h), scale, return_scale=True)
-    rescaled_img = imresize(img, new_size, interpolation=interpolation)
+    rescaled_img = imresize(
+        img, new_size, interpolation=interpolation, backend=backend)
     if return_scale:
         return rescaled_img, scale_factor
     else:

mmcv/image/io.py

@@ -131,10 +131,10 @@ def imread(img_or_path, flag='color', channel_order='bgr', backend=None):
             candidates are `color`, `grayscale` and `unchanged`.
             Note that the `turbojpeg` backened does not support `unchanged`.
         channel_order (str): Order of channel, candidates are `bgr` and `rgb`.
-        backend (str|None): The image decoding backend type. Options are `cv2`,
-            `pillow`, `turbojpeg`, `None`. If backend is None, the global
-            imread_backend specified by ``mmcv.use_backend()`` will be used.
-            Default: None.
+        backend (str | None): The image decoding backend type. Options are
+            `cv2`, `pillow`, `turbojpeg`, `None`. If backend is None, the
+            global imread_backend specified by ``mmcv.use_backend()`` will be
+            used. Default: None.
 
     Returns:
         ndarray: Loaded image array.
@@ -181,10 +181,10 @@ def imfrombytes(content, flag='color', channel_order='bgr', backend=None):
     Args:
         content (bytes): Image bytes got from files or other streams.
         flag (str): Same as :func:`imread`.
-        backend (str|None): The image decoding backend type. Options are `cv2`,
-            `pillow`, `turbojpeg`, `None`. If backend is None, the global
-            imread_backend specified by ``mmcv.use_backend()`` will be used.
-            Default: None.
+        backend (str | None): The image decoding backend type. Options are
+            `cv2`, `pillow`, `turbojpeg`, `None`. If backend is None, the
+            global imread_backend specified by ``mmcv.use_backend()`` will be
+            used. Default: None.
 
     Returns:
         ndarray: Loaded image array.

tests/test_image/test_geometric.py

@@ -35,6 +35,18 @@ class TestGeometric:
                 self.img, (1000, 600), interpolation=mode)
             assert resized_img.shape == (600, 1000, 3)
 
+        # test pillow resize
+        for mode in [
+                'nearest', 'bilinear', 'bicubic', 'box', 'lanczos', 'hamming'
+        ]:
+            resized_img = mmcv.imresize(
+                self.img, (1000, 600), interpolation=mode, backend='pillow')
+            assert resized_img.shape == (600, 1000, 3)
+
+        # resize backend must be 'cv2' or 'pillow'
+        with pytest.raises(ValueError):
+            mmcv.imresize(self.img, (1000, 600), backend='not support')
+
     def test_imresize_like(self):
         a = np.zeros((100, 200, 3))
         resized_img = mmcv.imresize_like(self.img, a)