L1 loss or SmoothL1Loss?

[Jonathan]

Hi, I've been reading through the code and I found that L1 loss is used instead of Smooth L1 loss for localization loss. This is quite different from the paper's procedure, where as far as I know SSD uses Smooth L1 loss.

https://github.com/sgrvinod/a-PyTorch-Tutorial-to-Object-Detection/blob/master/model.py#L549

self.smooth_l1 = nn.L1Loss()

https://github.com/sgrvinod/a-PyTorch-Tutorial-to-Object-Detection/blob/master/model.py#L612

loc_loss = self.smooth_l1(predicted_locs[positive_priors], true_locs[positive_priors]) # (), scalar

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My questions are:

1. Has anyone tried changing the loss function to SmoothL1Loss as implemented in PyTorch as of right now?
2. If it has been tried, is the result similar to what SSD achieves?

Thank you in advance.

[Aditya Gupta]

@jonathan016 did you try it yet ? I'll try and post result here once I have some results.

[Jonathan]

Hi @adityag6994 , I didn't try it due to my research's limited resources and time. However, using L1Loss seems to still help the learning process from what I observed in my experiments. I look forward to seeing the results you obtained from using SmoothL1Loss

[Aditya Gupta]

Okay, I started the experiment will have something by tomorrow. Same, L1Loss worked for me on the dateset I tried it on.

Side question, I just noticed there is no Softmax being used when calculating cross Entropy https://github.com/sgrvinod/a-PyTorch-Tutorial-to-Object-Detection/blob/master/model.py#L629, do you have any idea, or am I missing something here ?

Thanks,
Aditya

[Aditya Gupta]

@jonathan016 so I changed https://github.com/sgrvinod/a-PyTorch-Tutorial-to-Object-Detection/blob/master/model.py#L549

self.smooth_l1 = nn.L1Loss()

to this

self.smooth_l1 = nn.SmoothL1Loss()

and there was less than 1% drop on Mean Average Precision (mAP) for me.

[Jonathan]

Wow, that's interesting @adityag6994, thanks for the experiment! At least now we know which works best for your case, since different datasets may require different approach 😁

Anyway, softmax is calculated implicitly in CrossEntropyLoss as CrossEntropyLoss is actualy LogSoftmax applied with NLLLoss if I'm not mistaken (see https://pytorch.org/docs/master/nn.html#torch.nn.CrossEntropyLoss). With that said, when inferencing, you need to explicitly apply softmax function to the model's output if you're going after probability values.

[Aditya Gupta]

That makes sense now. Thank you @jonathan016

[Jonathan]

With the experiment results provided by @adityag6994, I believe my questions have been answered. Closing this issue for now. Thanks a lot @adityag6994!