transforms: randomly grayscaling an image

test/test_transforms.py

@@ -714,6 +714,143 @@ def test_random_rotation(self):
         angle = t.get_params(t.degrees)
         assert angle > -10 and angle < 10
 
+    def test_to_grayscale(self):
+        """Unit tests for grayscale transform"""
+
+        x_shape = [2, 2, 3]
+        x_data = [0, 5, 13, 54, 135, 226, 37, 8, 234, 90, 255, 1]
+        x_np = np.array(x_data, dtype=np.uint8).reshape(x_shape)
+        x_pil = Image.fromarray(x_np, mode='RGB')
+        x_pil_2 = x_pil.convert('L')
+        gray_np = np.array(x_pil_2)
+
+        # Test Set: Grayscale an image with desired number of output channels
+        # Case 1: RGB -> 1 channel grayscale
+        trans1 = transforms.Grayscale(num_output_channels=1)
+        gray_pil_1 = trans1(x_pil)
+        gray_np_1 = np.array(gray_pil_1)
+        assert gray_pil_1.mode == 'L', 'mode should be L'
+        assert gray_np_1.shape == tuple(x_shape[0:2]), 'should be 1 channel'
+        np.testing.assert_equal(gray_np, gray_np_1)
+
+        # Case 2: RGB -> 3 channel grayscale
+        trans2 = transforms.Grayscale(num_output_channels=3)
+        gray_pil_2 = trans2(x_pil)
+        gray_np_2 = np.array(gray_pil_2)
+        assert gray_pil_2.mode == 'RGB', 'mode should be RGB'
+        assert gray_np_2.shape == tuple(x_shape), 'should be 3 channel'
+        np.testing.assert_equal(gray_np_2[:, :, 0], gray_np_2[:, :, 1])
+        np.testing.assert_equal(gray_np_2[:, :, 1], gray_np_2[:, :, 2])
+        np.testing.assert_equal(gray_np, gray_np_2[:, :, 0])
+
+        # Case 3: 1 channel grayscale -> 1 channel grayscale
+        trans3 = transforms.Grayscale(num_output_channels=1)
+        gray_pil_3 = trans3(x_pil_2)
+        gray_np_3 = np.array(gray_pil_3)
+        assert gray_pil_3.mode == 'L', 'mode should be L'
+        assert gray_np_3.shape == tuple(x_shape[0:2]), 'should be 1 channel'
+        np.testing.assert_equal(gray_np, gray_np_3)
+
+        # Case 4: 1 channel grayscale -> 3 channel grayscale
+        trans4 = transforms.Grayscale(num_output_channels=3)
+        gray_pil_4 = trans4(x_pil_2)
+        gray_np_4 = np.array(gray_pil_4)
+        assert gray_pil_4.mode == 'RGB', 'mode should be RGB'
+        assert gray_np_4.shape == tuple(x_shape), 'should be 3 channel'
+        np.testing.assert_equal(gray_np_4[:, :, 0], gray_np_4[:, :, 1])
+        np.testing.assert_equal(gray_np_4[:, :, 1], gray_np_4[:, :, 2])
+        np.testing.assert_equal(gray_np, gray_np_4[:, :, 0])
+
+    @unittest.skipIf(stats is None, 'scipy.stats not available')
+    def test_random_grayscale(self):
+        """Unit tests for random grayscale transform"""
+
+        # Test Set 1: RGB -> 3 channel grayscale
+        random_state = random.getstate()
+        random.seed(42)
+        x_shape = [2, 2, 3]
+        x_np = np.random.randint(0, 256, x_shape, np.uint8)
+        x_pil = Image.fromarray(x_np, mode='RGB')
+        x_pil_2 = x_pil.convert('L')
+        gray_np = np.array(x_pil_2)
+
+        num_samples = 250
+        num_gray = 0
+        for _ in range(num_samples):
+            gray_pil_2 = transforms.RandomGrayscale(p=0.5)(x_pil)
+            gray_np_2 = np.array(gray_pil_2)
+            if np.array_equal(gray_np_2[:, :, 0], gray_np_2[:, :, 1]) and \
+               np.array_equal(gray_np_2[:, :, 1], gray_np_2[:, :, 2]) and \
+               np.array_equal(gray_np, gray_np_2[:, :, 0]):
+                num_gray = num_gray + 1
+
+        p_value = stats.binom_test(num_gray, num_samples, p=0.5)
+        random.setstate(random_state)
+        assert p_value > 0.0001
+
+        # Test Set 2: grayscale -> 1 channel grayscale
+        random_state = random.getstate()
+        random.seed(42)
+        x_shape = [2, 2, 3]
+        x_np = np.random.randint(0, 256, x_shape, np.uint8)
+        x_pil = Image.fromarray(x_np, mode='RGB')
+        x_pil_2 = x_pil.convert('L')
+        gray_np = np.array(x_pil_2)
+
+        num_samples = 250
+        num_gray = 0
+        for _ in range(num_samples):
+            gray_pil_3 = transforms.RandomGrayscale(p=0.5)(x_pil_2)
+            gray_np_3 = np.array(gray_pil_3)
+            if np.array_equal(gray_np, gray_np_3):
+                num_gray = num_gray + 1
+
+        p_value = stats.binom_test(num_gray, num_samples, p=1.0)  # Note: grayscale is always unchanged
+        random.setstate(random_state)
+        assert p_value > 0.0001
+
+        # Test set 3: Explicit tests
+        x_shape = [2, 2, 3]
+        x_data = [0, 5, 13, 54, 135, 226, 37, 8, 234, 90, 255, 1]
+        x_np = np.array(x_data, dtype=np.uint8).reshape(x_shape)
+        x_pil = Image.fromarray(x_np, mode='RGB')
+        x_pil_2 = x_pil.convert('L')
+        gray_np = np.array(x_pil_2)
+
+        # Case 3a: RGB -> 3 channel grayscale (grayscaled)
+        trans2 = transforms.RandomGrayscale(p=1.0)
+        gray_pil_2 = trans2(x_pil)
+        gray_np_2 = np.array(gray_pil_2)
+        assert gray_pil_2.mode == 'RGB', 'mode should be RGB'
+        assert gray_np_2.shape == tuple(x_shape), 'should be 3 channel'
+        np.testing.assert_equal(gray_np_2[:, :, 0], gray_np_2[:, :, 1])
+        np.testing.assert_equal(gray_np_2[:, :, 1], gray_np_2[:, :, 2])
+        np.testing.assert_equal(gray_np, gray_np_2[:, :, 0])
+
+        # Case 3b: RGB -> 3 channel grayscale (unchanged)
+        trans2 = transforms.RandomGrayscale(p=0.0)
+        gray_pil_2 = trans2(x_pil)
+        gray_np_2 = np.array(gray_pil_2)
+        assert gray_pil_2.mode == 'RGB', 'mode should be RGB'
+        assert gray_np_2.shape == tuple(x_shape), 'should be 3 channel'
+        np.testing.assert_equal(x_np, gray_np_2)
+
+        # Case 3c: 1 channel grayscale -> 1 channel grayscale (grayscaled)
+        trans3 = transforms.RandomGrayscale(p=1.0)
+        gray_pil_3 = trans3(x_pil_2)
+        gray_np_3 = np.array(gray_pil_3)
+        assert gray_pil_3.mode == 'L', 'mode should be L'
+        assert gray_np_3.shape == tuple(x_shape[0:2]), 'should be 1 channel'
+        np.testing.assert_equal(gray_np, gray_np_3)
+
+        # Case 3d: 1 channel grayscale -> 1 channel grayscale (unchanged)
+        trans3 = transforms.RandomGrayscale(p=0.0)
+        gray_pil_3 = trans3(x_pil_2)
+        gray_np_3 = np.array(gray_pil_3)
+        assert gray_pil_3.mode == 'L', 'mode should be L'
+        assert gray_np_3.shape == tuple(x_shape[0:2]), 'should be 1 channel'
+        np.testing.assert_equal(gray_np, gray_np_3)
+
 
 if __name__ == '__main__':
     unittest.main()

torchvision/transforms/functional.py

@@ -545,7 +545,35 @@ def rotate(img, angle, resample=False, expand=False, center=None):
             Origin is the upper left corner.
             Default is the center of the image.
     """
+
     if not _is_pil_image(img):
         raise TypeError('img should be PIL Image. Got {}'.format(type(img)))
 
     return img.rotate(angle, resample, expand, center)
+
+
+def to_grayscale(img, num_output_channels=1):
+    """Convert image to grayscale version of image.
+
+    Args:
+        img (PIL Image): Image to be converted to grayscale.
+
+    Returns:
+        PIL Image:  Grayscale version of the image.
+                    if num_output_channels == 1 : returned image is single channel
+                    if num_output_channels == 3 : returned image is 3 channel with r == g == b
+    """
+    if not _is_pil_image(img):
+        raise TypeError('img should be PIL Image. Got {}'.format(type(img)))
+
+    if num_output_channels == 1:
+        img = img.convert('L')
+    elif num_output_channels == 3:
+        img = img.convert('L')
+        np_img = np.array(img, dtype=np.uint8)
+        np_img = np.dstack([np_img, np_img, np_img])
+        img = Image.fromarray(np_img, 'RGB')
+    else:
+        raise ValueError('num_output_channels should be either 1 or 3')
+
+    return img

torchvision/transforms/transforms.py

@@ -17,7 +17,8 @@
 
 __all__ = ["Compose", "ToTensor", "ToPILImage", "Normalize", "Resize", "Scale", "CenterCrop", "Pad",
            "Lambda", "RandomCrop", "RandomHorizontalFlip", "RandomVerticalFlip", "RandomResizedCrop",
-           "RandomSizedCrop", "FiveCrop", "TenCrop", "LinearTransformation", "ColorJitter", "RandomRotation"]
+           "RandomSizedCrop", "FiveCrop", "TenCrop", "LinearTransformation", "ColorJitter", "RandomRotation",
+           "Grayscale", "RandomGrayscale"]
 
 
 class Compose(object):
@@ -619,7 +620,6 @@ def get_params(degrees):
 
     def __call__(self, img):
         """
-        Args:
             img (PIL Image): Image to be rotated.
 
         Returns:
@@ -629,3 +629,61 @@ def __call__(self, img):
         angle = self.get_params(self.degrees)
 
         return F.rotate(img, angle, self.resample, self.expand, self.center)
+
+
+class Grayscale(object):
+    """Convert image to grayscale.
+    Args:
+        num_output_channels (int): (1 or 3) number of channels desired for output image
+
+    Returns:
+        PIL Image: grayscale version of the input
+                   if num_output_channels == 1 : returned image is single channel
+                   if num_output_channels == 3 : returned image is 3 channel with r == g == b
+
+    """
+
+    def __init__(self, num_output_channels=1):
+        self.num_output_channels = num_output_channels
+
+    def __call__(self, img):
+        """
+        Args:
+            img (PIL Image): Image to be converted to grayscale.
+
+        Returns:
+            PIL Image: Randomly grayscaled image.
+        """
+        return F.to_grayscale(img, num_output_channels=self.num_output_channels)
+
+
+class RandomGrayscale(object):
+    """Randomly convert image to grayscale with a probability of p (default 0.1).
+    Args:
+        p (float): probability that image should be converted to grayscale.
+
+    Returns:
+        PIL Image: grayscale version of the input image with probability p
+                   and unchanged with probability (1-p)
+                   - if input image is 1 channel:
+                        grayscale version is 1 channel
+                   - if input image is 3 channel:
+                        grayscale version is 3 channel with r == g == b
+
+    """
+
+    def __init__(self, p=0.1):
+        self.p = p
+
+    def __call__(self, img):
+        """
+        Args:
+            img (PIL Image): Image to be converted to grayscale.
+
+        Returns:
+            PIL Image: Randomly grayscaled image.
+        """
+        num_output_channels = 1 if img.mode == 'L' else 3
+        if random.random() < self.p:
+            return F.to_grayscale(img, num_output_channels=num_output_channels)
+        return img