MATLAB: Error when using convolution2dLayer between connected maxPooling2dLayer and maxUnpooling2dLayer

Deep Learning Toolboxmaxpooling2dlayermaxunpooling2dlayer

I'm trying to create a modified UNet using connected max pooling and max unpooling layers. However, if I put a convolution layer between the pooling and unpooling layers, the network isn't valid. The error is reported for the unpooling layer:
Input size mismatch. Size of input to this layer is different from the expected input size.
The sizes for the CONV layer, MAXPOOL indices, and MAXPOOL size inputs are all different. Minimum working example below. Am I missing something obvious, or is it not possible to use other layers between maxpool and maxunpool?
% define layers
layers = [
    imageInputLayer([128, 128], 'Name', 'INPUTLAYER')
    maxPooling2dLayer([2 2], 'HasUnpoolingOutputs', true, 'Stride', [2 2], 'Name', 'MAXPOOL')
    convolution2dLayer([3 3], 32, 'Padding', 'same', 'Stride', [1 1], 'Name', 'CONV')
    maxUnpooling2dLayer('Name', 'UNPOOL')
    regressionLayer('Name', 'MSE')
    ]; 
% define network
lgraph = layerGraph(layers);
% define connections
lgraph = connectLayers(lgraph, 'MAXPOOL/indices', 'UNPOOL/indices');
lgraph = connectLayers(lgraph, 'MAXPOOL/size', 'UNPOOL/size');
% plot and check
analyzeNetwork(lgraph);

Best Answer

You convolution layer is changing the number of channels in the output after the max pooling.
This causes the input size mismatch. You need to match the number of channels output by convolution layer to the output by maxpooling layer.
% define layers
layers = [
    imageInputLayer([128, 128], 'Name', 'INPUTLAYER')
    maxPooling2dLayer([2 2], 'HasUnpoolingOutputs', true, 'Stride', [2 2], 'Name', 'MAXPOOL')
    convolution2dLayer([3 3], 1, 'Padding', 'same', 'Stride', [1 1], 'Name', 'CONV')
    maxUnpooling2dLayer('Name', 'UNPOOL')
    regressionLayer('Name', 'MSE')
    ]; 
% define network
lgraph = layerGraph(layers);
% define connections
lgraph = connectLayers(lgraph, 'MAXPOOL/indices', 'UNPOOL/indices');
lgraph = connectLayers(lgraph, 'MAXPOOL/size', 'UNPOOL/size');
% plot and check
analyzeNetwork(lgraph);
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