[Feat] kornia.Resize has a bad default for align_corners

[pmeier]

🐛 Bug

kornia.Resize has a bad default for align_corners if interpolation=="nearest" or interpolation=="area".

To Reproduce

import torch
import kornia

transform = kornia.Resize(32, interpolation="nearest")

input = torch.rand(1, 3, 16, 16)
transform(input)

ValueError: align_corners option can only be set with the interpolating modes: linear | bilinear | bicubic | trilinear

Expected behavior

This should work out of the box. You need to explicitly set align_corners=None to get this working. This is neither documented nor even accepted by the type annotations.

Environment

kornia==0.4.1

[pmeier]

I only know that you guys had problems with align_corners before so here is my proposal to fix this:

def parse_align_corners(
    align_corners: Optional[bool], interpolation: str
) -> Optional[bool]:
    if align_corners is not None:
        return align_corners

    return (
        False
        if interpolation in ("linear", "bilinear", "bicubic", "trilinear")
        else None
    )

If you use that in the constructor of every transformation that has the align_corners parameter together with align_corners=None as default, you should be good. I imagine most users don't care about align_corners at all as long as it works.

[shijianjian]

Yes, you are right. We should make a better default.

Should we set it to None directly for resize?

[pmeier]

@shijianjian

Should we set it to None directly

I wouldn't recommend that:

import torch
import kornia

transform = kornia.Resize(32, interpolation="bilinear", align_corners=None)

input = torch.rand(1, 3, 16, 16)
transform(input)

UserWarning: Default upsampling behavior when mode=bilinear is changed to align_corners=False since 0.4.0. 
Please specify align_corners=True if the old behavior is desired. 
See the documentation of nn.Upsample for details.

To avoid the error and the warning, you need to set align_corners=False if interpolation in ("linear", "bilinear", "bicubic", "trilinear") and align_corners=None otherwise. That is exactly what the parse_align_corners function does that I suggested in #777 (comment).

for resize?

Again, I wouldn't recommend that. What is special about resize? Shouldn't this behavior consistent throughout the codebase? I would recommend doing something along the lines of

def resize(..., interpolation="bilinear", align_corners=None):
    align_corners = parse_align_corners(align_corners, interpolation)
    ...

in every user-facing function or module that takes the align_corners keyword.

[shijianjian]

I see. That is reasonable.

To avoid the error and the warning, you need to set align_corners=False if interpolation in ("linear", "bilinear", "bicubic", "trilinear") and align_corners=None otherwise. That is exactly what the parse_align_corners function does that I suggested in #777 (comment).

From what you proposed, this is a general setting that fits all functions with (align_corners, interpolation), right? I mean it is not only for resize.

[edgarriba]

@pmeier do you think you can send a PR with it ? we are planning the release for v0.5 in a couple of weeks and this definitely can be good sanity patch. I guess this will also help with this #747

[pmeier]

Sorry for the delay. I can give it a shot.

[edgarriba]

@pmeier can you check #941 ? to see the next steps for augmentations and see hwo we could proceed with this ? /cc @shijianjian

[adamjstewart]

This appears to be fixed in the latest release. The following has no errors:

transform = kornia.geometry.transform.Resize(32, interpolation="nearest")

although the original error can be reproduced with:

transform = kornia.geometry.transform.Resize(32, interpolation="nearest", align_corners=True)

[edgarriba]

@pmeier shall we close this issue?