Up-sampling in convolutional neural network

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hoi on 26 Dec 2018
Commented: Ali Riza Durmaz on 15 Apr 2019
Hi everyone,
for a project at university I am trying to rebuild a NN described in a paper. It was orininally designed in Keras (I don't have any code, only its rough describtion) and I'm struggling with one specific layer they're using. To up-sample their data, they use a layer which takes a single entry of its input and replicates it to a 2x2-region of the output. This results in a matrix with doubled dimensions, without zero-entries (assuming there was none in input) and same entry in each 2x2-block. It is an approximation to the inverse of the maxPooling-Layer of MATLAB. It is similar, but NOT the same as maxUnpooling-Layer, which keeps the position of an maximum-entry and fills up with zeros. For this specific "up-sampling-operation", there is no explicit NN-layer in MATLAB.
Does someone have an idea how I can do this operation?
An idea I had in mind: Just using the given maxUnpooling-Layer and hope there will be no big difference. I tried this and prepared my maxPooling-Layers with "HasUnpoolingOutputs", but it seems that maxUnpooling-Layer has to follow immediately after the maxPooling-Layer. I get unused outputs for my maxPooling-Layers and missing outputs for my maxUnpooling-Layers (seen via analyzeNetwork) as I use convolution-layers in between (see code for example).
layers = [
imageInputLayer([32 32 1])
convolution2dLayer(filterSize, 32, 'Padding', 'same')
maxPooling2dLayer(2,'Stride',2,'HasUnpoolingOutputs',true) %
convolution2dLayer(filterSize, 64, 'Padding', 'same')
maxUnpooling2dLayer() %
convolution2dLayer(filterSize, 32, 'Padding', 'same')
So in this case, one has to bring the outputs "indices" and "size" of the maxPooling-layer to the maxUnpooling-layer. But I don't know how this can be achieved :/
I'd be very thankful for any ideas.

Answers (1)

Teja Muppirala
Teja Muppirala on 27 Dec 2018
I think you may be able to do it using a transposedConv2dLayer. This works, (though I'm not entirely confident that this is the most appropriate implementation). Define a new layer like this and then use it in your layers instead of the maxUnpooling.
upsample2x2Layer = transposedConv2dLayer(2,1,'Stride',2, 'WeightLearnRateFactor',0,'BiasLearnRateFactor',0);
upsample2x2Layer.Weights = [1 1;1 1];
upsample2x2Layer.Bias = [0];
We set the learn rate factors to zero so that the parameters in this layer are not changed during training.
Ali Riza Durmaz
Ali Riza Durmaz on 15 Apr 2019
Hello Hoi,
I am dealing with a very similar problem currently. Did the approach of using transposedConv2dLayer with adjusted weight's, biases and their respective learn rate's set to zero solve the problem?
If I understand your last sentence right, you do need a way to apply this to all feature channel's separately so they won't be lost? If I understand the function correctly the second argument 'numFilters' specifies the number of output channels. So additionally to adjusting the weight, and biases you might need to adjust numFilters argument.
Best regards

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