Validation failed for ConvolutionLayer message?

Question

I have a simple fully convolutional network and I'm trying to figure out why I get this error

NetChain::valfail: Validation failed for ConvolutionLayer: kernel size 3*3 plus padding size 0*0 cannot exceed input size 2*2.

when I uncomment the 6-8th layers (which should be totally valid) in the following net:

el = ElementwiseLayer["SELU"];
bn = BatchNormalizationLayer[];
n = NetInitialize[NetChain[{
    ConvolutionLayer[24, {3, 3}], bn, el, 
    ConvolutionLayer[24, {3, 3}, "Dilation" -> 2], bn, el,
    ConvolutionLayer[24, {3, 3}, "Dilation" -> 4], bn, el, 
    ConvolutionLayer[24, {3, 3}, "Dilation" -> 8], bn, el, 
    ConvolutionLayer[24, {3, 3}, "Dilation" -> 16], bn, el, 
    (*
     ConvolutionLayer[24, {3,3},"Dilation" -> 32], bn, el,
     ConvolutionLayer[24, {3,3},"Dilation" -> 64], bn, el,
     ConvolutionLayer[24, {3,3},"Dilation" -> 128], bn, el,
    *) 
    ConvolutionLayer[24, {3, 3}, "Dilation" -> 1], bn, el,
    ConvolutionLayer[3, {1, 1}, "Dilation" -> 1]
    },
   "Input" -> NetEncoder[{"Image", ColorSpace -> "RGB"}], 
   "Output" -> NetDecoder[{"Image", ColorSpace -> "RGB"}]], 
  Method -> "Identity"];
n[ExampleData[{"TestImage", "Lena"}]]

It works without those three (dilations 32,64,128) layers:

Answer 1

By default, a ConvolutionLayer only returns the pixels where the whole convolution kernel "fits" (this is sometimes called a "valid" convolution). So for a 3x3 kernel, the output is 2 pixels smaller in both dimensions:

If you use dilation, the output gets even smaller:

So in your net, there's nothing left at the 6th layer.

If you use PaddingSize:

ConvolutionLayer[24, {3, 3}, "Dilation" -> 2, "PaddingSize" -> 2], bn, el, 
ConvolutionLayer[24, {3, 3}, "Dilation" -> 4, "PaddingSize" -> 4], bn, el, 
ConvolutionLayer[24, {3, 3}, "Dilation" -> 8, "PaddingSize" -> 8], bn, el, 
ConvolutionLayer[24, {3, 3}, "Dilation" -> 16, "PaddingSize" -> 16], bn, el,

you can add as many layers as you want. But the input to each layer will be padded with zeros.