[Bug] warp_perspective does not return the input if used with identity matrix

🐛 Bug

The output of kornia.geometry.warp_perspective does not equal the input if the identity matrix is used.

To Reproduce

Steps to reproduce the behavior:

import torch
from kornia.geometry import warp_perspective

torch.manual_seed(0)

dsize = (32, 16)
src = torch.rand(1, 3, *dsize)
M = torch.eye(3).unsqueeze(0)

dst = warp_perspective(src, M, dsize)

mae = torch.mean(torch.abs(dst - src))
print(mae.item())

0.14952071011066437

Expected behavior

0.0

Environment

kornia==0.4.1

[anibali]

I agree with the deprecation. It is further supported by the fact that no other library seems to have align_corners or equivalent for image-transform-by-matrix functions:

• NVIDIA Dali: https://docs.nvidia.com/deeplearning/dali/user-guide/docs/supported_ops.html#nvidia.dali.fn.warp_affine
• PIL: https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.transform
• OpenCV: https://docs.opencv.org/4.5.3/da/d54/group__imgproc__transform.html#ga0203d9ee5fcd28d40dbc4a1ea4451983

In my opinion the addition of an align_corners option is confusing, of limited practical use, and increases the likelihood of doing bad things like transforming images and point annotations in subtly different ways despite using the same transformation matrix.

[shijianjian]

I agree with @pmeier that we might add it in the docs to let the users more aware of the differences of the parameter choice. This pic is a nice demonstration.