[WIP] Add Scriptable Transform: Grayscale (#1505)

* Add Scriptable Transform: Grayscale

* add scriptable transforms: rgb_to_grayscale

* add scriptable transform: rgb_to_grayscale

* add scriptable transform: rgb_to_grayscale

* add scriptable transform: rgb_to_grayscale

* update code: rgb_to_grayscale

* add test: rgb_to_grayscale

* update parameters: rgb_to_grayscale

* add scriptable transform: rgb_to_grayscale

* update rgb_to_grayscale

* update rgb_to_grayscale

test/test_functional_tensor.py

@@ -3,6 +3,7 @@ import torch
 import torchvision.transforms as transforms
 import torchvision.transforms.functional_tensor as F_t
 import torchvision.transforms.functional as F
+import numpy as np
 import unittest
 import random
 
@@ -68,6 +69,13 @@ class Tester(unittest.TestCase):
                 max_diff = (ft_img - f_img).abs().max()
                 self.assertLess(max_diff, 5 / 255 + 1e-5)
 
+    def test_rgb_to_grayscale(self):
+        img_tensor = torch.randint(0, 255, (3, 16, 16), dtype=torch.uint8)
+        grayscale_tensor = F_t.rgb_to_grayscale(img_tensor).to(int)
+        grayscale_pil_img = torch.tensor(np.array(F.to_grayscale(F.to_pil_image(img_tensor)))).to(int)
+        max_diff = (grayscale_tensor - grayscale_pil_img).abs().max()
+        self.assertLess(max_diff, 1.0001)
+
 
 if __name__ == '__main__':
     unittest.main()

torchvision/transforms/functional_tensor.py

@@ -26,7 +26,6 @@ def hflip(img_tensor):
     Returns:
         Tensor:  Horizontally flipped image Tensor.
     """
-
     if not F._is_tensor_image(img_tensor):
         raise TypeError('tensor is not a torch image.')
 
@@ -35,12 +34,14 @@ def hflip(img_tensor):
 
 def crop(img, top, left, height, width):
     """Crop the given Image Tensor.
+
     Args:
         img (Tensor): Image to be cropped in the form [C, H, W]. (0,0) denotes the top left corner of the image.
         top (int): Vertical component of the top left corner of the crop box.
         left (int): Horizontal component of the top left corner of the crop box.
         height (int): Height of the crop box.
         width (int): Width of the crop box.
+
     Returns:
         Tensor: Cropped image.
     """
@@ -50,6 +51,24 @@ def crop(img, top, left, height, width):
     return img[..., top:top + height, left:left + width]
 
 
+def rgb_to_grayscale(img):
+    """Convert the given RGB Image Tensor to Grayscale.
+    For RGB to Grayscale conversion, ITU-R 601-2 luma transform is performed which
+    is L = R * 0.2989 + G * 0.5870 + B * 0.1140
+
+    Args:
+        img (Tensor): Image to be converted to Grayscale in the form [C, H, W].
+
+    Returns:
+        Tensor: Grayscale image.
+
+    """
+    if img.shape[0] != 3:
+        raise TypeError('Input Image does not contain 3 Channels')
+
+    return (0.2989 * img[0] + 0.5870 * img[1] + 0.1140 * img[2]).to(img.dtype)
+
+
 def adjust_brightness(img, brightness_factor):
     """Adjust brightness of an RGB image.
 
@@ -83,7 +102,7 @@ def adjust_contrast(img, contrast_factor):
     if not F._is_tensor_image(img):
         raise TypeError('tensor is not a torch image.')
 
-    mean = torch.mean(_rgb_to_grayscale(img).to(torch.float))
+    mean = torch.mean(rgb_to_grayscale(img).to(torch.float))
 
     return _blend(img, mean, contrast_factor)
 
@@ -103,14 +122,9 @@ def adjust_saturation(img, saturation_factor):
     if not F._is_tensor_image(img):
         raise TypeError('tensor is not a torch image.')
 
-    return _blend(img, _rgb_to_grayscale(img), saturation_factor)
+    return _blend(img, rgb_to_grayscale(img), saturation_factor)
 
 
 def _blend(img1, img2, ratio):
     bound = 1 if img1.dtype.is_floating_point else 255
     return (ratio * img1 + (1 - ratio) * img2).clamp(0, bound).to(img1.dtype)
-
-
-def _rgb_to_grayscale(img):
-    # ITU-R 601-2 luma transform, as used in PIL.
-    return (0.2989 * img[0] + 0.5870 * img[1] + 0.1140 * img[2]).to(img.dtype)