Normalizing images between 0 and 1 with ToTensor() doesn't work

vision
1

Hi everyone,

I’m trying to train a segmentation network and I would like to normalize images between 0 and 1. However the ToTensor transform I use outputs very low pixel values. Here’s a code sample

img_transforms = transforms.Compose([transforms.Grayscale(num_output_channels=1),
                                     transforms.Resize((100,100), interpolation=2),
                                     transforms.ToTensor()])
img = Image.open("myimg.png")
img = np.array(img).astype(float)
img*= (255.0/img.max())
img = img.astype(np.uint8)
print(img.max())
img = Image.fromarray(img)
img = img_transforms(img)
print(img.max())

And the given output:

255
tensor(0.1137)

I don’t understand why I don’t get 1 as a max value since the ToTensor function is supposed to output values between 0 and 1.

Could anyone shed some light on what may be occurring?

Thanks

2

The ToTensor transformation will normalize the uint8 input image by dividing by 255 as seen here.
Based on your code, I guess that the Resize operation might lower the max. values of 255 to a smaller one (although I wouldn’t expect to see such a decrease). Could you try to use PIL.Image.NEAREST as the interpolation method and recheck the code?

3

Thank you @ptrblck, the resize transform was not the issue here but I figured out it was actually due to the fact that my images are mainly blue and that the Grayscale transform multiply blue values by 0.114. I solved the problem by manually setting the max value to 1. I still don’t know why the ToTensor() function didn’t normalize the values between 0 and 1 after the Grayscal transform though.

Here is my solution:

img = Image.open("myimg.png")
img = img_transforms(img)
img*= (1.0/img.max())