Warping even sized matrices in the nearest mode causes 0 to apear in the last row/column

[magnux]

🐛 Bug

Warping even sized matrices in the nearest mode causes 0 to apear in the last row/column.
The odd sized matrices are handled correctly, so it seems some rounding or border case issue.

To Reproduce

import torch
import kornia

torch.manual_seed(0)
M = torch.eye(3).unsqueeze(0)

dsize = (3, 3)
src = torch.ones(1, 3, *dsize)
warper = kornia.HomographyWarper(*dsize, mode='nearest')
warper.precompute_warp_grid(M)

print(warper(src))

dsize = (4, 4)
src = torch.ones(1, 3, *dsize)
warper = kornia.HomographyWarper(*dsize, mode='nearest')
warper.precompute_warp_grid(M)

print(warper(src))

Results:

tensor([[[[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]],

         [[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]],

         [[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]]]])

tensor([[[[1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [0., 0., 0., 0.]],

         [[1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [0., 0., 0., 0.]],

         [[1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [1., 1., 1., 0.],
          [0., 0., 0., 0.]]]])

Expected behavior

Results:

tensor([[[[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]],

         [[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]],

         [[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]]]])

tensor([[[[1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.]],

         [[1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.]],

         [[1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.],
          [1., 1., 1., 1.]]]])

Environment

kornia 0.4.0

[edgarriba]

hi @magnux , thanks for reporting this issue.

I believe that this is related to #747 that refers to the align_corners flag for the grid_sample function. Check my comment here: #747 (comment)

Ideally we should find a proper configuration for aligning corners. /cc @pmeier was trying to normalize this across different functions too.

[magnux]

Calling HomographyWarper with align_corners=True indeed solves the issue.
It seems related with #747