output*mask in warp

[cxy7tv]

Hi, I have no idea why mask is needed following code in PWC-Net/PyTorch/models/PWCNet.py
mask = torch.autograd.Variable(torch.ones(x.size())).cuda()
mask = nn.functional.grid_sample(mask, vgrid)

    # if W==128:
        # np.save('mask.npy', mask.cpu().data.numpy())
        # np.save('warp.npy', output.cpu().data.numpy())
    
    mask[mask<0.9999] = 0
    mask[mask>0] = 1
    
    return output*mask

[poppinjie]

It seems that getting rid of it has the same effect.