Torch.from_numpy not support negative strides

1

When use torch.from_numpy from the Ndarray with negative strides, there is a runtime error
’RuntimeError: some of the strides of a given numpy array are negative.'
For example,

import numpy as np
x=np.random.random(size=(32,32,7))
torch.from_numpy(np.flip(x,axis=0))

RuntimeError: some of the strides of a given numpy array are negative. This is currently not supported, but will be added in future releases.

Same error with np.rot90()

2

how about

torch.from_numpy(np.flip(x,axis=0).copy())
3

Thanks for your recommendation, I have solve this problem.

4

also works for me, thanks! though i don;t know why…

5

ndarray.copy() will alocate new memory for numpy array which make it normal, I mean the stride is not negative any more.

6

Excuse me, I’m puzzled about the word ‘normal’, what do you mean by ‘normal
numpy array’?

7

it’s like something contiguous in tensor. the data is stored orderly.

9

hello，

I just don’t understand what is “negative stride” means, could you please interpret it for me? Thank you very much!!

10

This means that your numpy array has undergone such operation:
image = image[..., ::-1]
I guess this has something to do with how numpy array are stored in memory, and unfortunately PyTorch doesn’t currently support numpy array that has been reversed using negative stride.

A simple fix is to do

image = image[..., ::-1] - np.zeros_like(image)
11

Ok, I understand it. There is a function in numpy like contiguous in pytorch. You can try it. Thanks.




信工 刘蓬博

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12

If you don’t want to flip the image, if for example you have already trained a network with un-flipped images, then you can save and load the image before passing it for inference.

13

I think this is a more elegant solution to the problem leveraging the PyTorch API.

torch.flip(torch.from_numpy(x), dims=(0,))

14

Thanks! It does works. :partying_face:

15

One can also pin the memory again by

x = np.flip(x,axis=0)
torch.from_numpy(np.ascontiguousarray(x))