RGB to grayscale, easy

vision
1

Hi everyone, I was wondering if anyone could explain to me why my code below did not work, I know that RGB conversion to grayscale is (R + G +B/3) so I used PyTorch to extract each channel, then add three of them and divide by 3, but the end result was a distorted image. I viewed my image output using Jupyter notebook. I was successful ultimate importing torch vision and using “transforms functional to gray scale” but still was wondering why my average of the RGB channels didn’t work, I’m assuming I didn’t actually obtain the individual 3 channels seperately or properly average them…

panda = np.array(Image.open(‘panda.jpg’).resize( (224,224)))
panda_tensor = torch.from_numpy(panda)

panda_tensor.size()
print ( panda_tensor.size() )

Display panda

plt.imshow(panda)

chan_r = panda_tensor[:,:,0].numpy()
chan_g = panda_tensor[:,:,1].numpy()
chan_b = panda_tensor[:,:,2].numpy()

result = (chan_r + chan_g + chan_b/3)

plt.imshow(result)

acc716d7f2b0386bb20a08bc6d595e78

torch vision grayscale

torchvision.transforms.functional.to_grayscale(Image.open(‘panda.jpg’).resize((224,224)), num_output_channels=1)

2

The division must be outside the parantheses: result = (chan_r + chan_g + chan_b)/3

Furthermore, sometimes different weights are used for converting RGB to gray-scale, like this for example: 0.2989 * R + 0.5870 * G + 0.1140 * B

3 Likes
3

This was the original image:

a0a489b6c93aeef87a1454ae7ebcd7d1

4

Ok, I just the paranthesis the way you said AND divided by 3.0 outside, but it gives me this image:
Whats is going on?
1b793e951593b1063b269013e6109eff-png

5

Could you check the dtype of your tensor?
Usually images are encoded as uint8 values, which can overflow in such an addition.
Try to cast it to float before the transformation is applied.

1 Like
6

I know this is an old post but… aside from the possible distortion mentioned, by default plt.imshow will not assume that a rank 2 tensor is a grayscale image and will use the default colormap. If you want to see it as a grayscale image, you have to set cmap=‘gray’ in imshow.