why setting evaluate mode in test.lua yeilding black images?

[sunshineatnoon]

Thanks for open sourcing this great project.
I saw this line of code in test.lua: --netG:evaluate(), I guess this is the code that enables dropout at testing time. I tried to set evaluate mode when testing, but the model generates black images.

Why is enabling dropout important when testing? Or is it possible that the black images are caused by BN layers, Anybody has insight on this? Thanks.

[phillipi]

Yeah, setting the net to evaluate mode does two things: 1) enables dropout, 2) makes batchnorm use aggregated training set statistics. I'm not entirely sure why it results in black images, but I think it's more due to 1 than to 2 (or possibly the interaction of the two effects).

The goal of enabling dropout during testing was to make the outputs stochastic (though it only has a very minor effect). Also, our strategy was to run the net identically during training and test, to avoid any "domain gap" between the two phases. This makes a big difference for batchnorm. It could be also that the way we used dropout causes a problematic gap if you turn it off at test time.

[Quasimondo]

Maybe worth mentioning: if you are creating animations or videos with pix2pix I recommend to un-comment that line and thus disable dropout in test mode. This will give you much better frame-to-frame consistency and reduce flickering a lot.