LocalNet Upsampling

[fvlntn]

Hello,

I've been using your LocalNet/GlobalNet and there was something off with my DDFs for a long time but couldn't think there was a direct error in your implementation. After checking it, there indeed is one small mistake.

Original is using Trilinear Upsampling (look at papers or DeepReg).
But here it is using F.interpolate which uses "Nearest" interpolation by default, both in

MONAI/monai/networks/blocks/regunet_block.py

Line 189 in 97918e4

class RegistrationExtractionBlock(nn.Module):

and in

MONAI/monai/networks/nets/regunet.py

Line 339 in 97918e4

class AdditiveUpSampleBlock(nn.Module):

.

Try looking at your output DDFs with LocalNet, it is really non-linear.
Changing both method to "trilinear" for 3D data fixes the problem and makes the DDFs smooth like in the LocalNet paper.

F.interpolate() => F.interpolate(mode="trilinear")
Linear/Bilinear/Trilinear depending on dimensions.

Thanks a lot for your hard work,
Franck

[fvlntn]

Maybe torch updated F.interpolate to use "nearest" by default in a recent update. In such case you should also check all F.interpolate uses?

[wyli]

thanks for reporting, I'll make a PR to make it flexible.

cc @yiheng-wang-nv do you have any experience of different interpolating modes here:

MONAI/monai/networks/nets/dynunet.py

Line 272 in c5ea694

out_all.append(interpolate(feature_map, out.shape[2:]))