IdentityLayer

docs/user-guide/layers/util-layer.rst

@@ -49,6 +49,20 @@ Utility Layers
       file. If this attribute is given, the corresponding symbol in this list is
       used as the dataset name instead of the original blob's name.
 
+.. class:: IdentityLayer
+
+   Identity layer maps inputs to outputs without changing anything. This could
+   be useful as glue layers to rename some blobs. There is no data-copying for
+   this layer.
+
+   .. attribute::
+      tops
+      bottoms
+
+      Blob names for output and input. This layer could take multiple input
+      blobs and produce the corresponding number of output blobs. The shapes of
+      the input blobs do not need to be the same.
+
 .. class:: ReshapeLayer
 
    Reshape a blob. Can be useful if, for example, you want to make the *flat*

src/layers.jl

@@ -5,6 +5,7 @@ export InnerProductLayer, ConvolutionLayer, PoolingLayer, SoftmaxLayer
 export PowerLayer, SplitLayer, ElementWiseLayer, ChannelPoolingLayer
 export LRNLayer, DropoutLayer, ReshapeLayer, ArgmaxLayer, HDF5OutputLayer
 export CropLayer, ConcatLayer, RandomMaskLayer, TiedInnerProductLayer
+export IdentityLayer
 export SquareLossLayer, SoftmaxLossLayer, MultinomialLogisticLossLayer
 export AccuracyLayer
 
@@ -201,6 +202,7 @@ include("layers/crop.jl")
 include("layers/concat.jl")
 include("layers/random-mask.jl")
 include("layers/tied-inner-product.jl")
+include("layers/identity.jl")
 
 #############################################################
 # Utility layers

src/layers/identity.jl

@@ -0,0 +1,34 @@
+############################################################
+# Identity Layer
+############################################################
+@defstruct IdentityLayer Layer (
+  name :: String = "identity",
+  (tops :: Vector{Symbol} = Symbol[], length(tops) > 0),
+  (bottoms :: Vector{Symbol} = Symbol[], length(bottoms) == length(tops)),
+)
+@characterize_layer(IdentityLayer,
+  can_do_bp => true
+)
+
+type IdentityLayerState <: LayerState
+  layer      :: IdentityLayer
+  blobs      :: Vector{Blob}
+  blobs_diff :: Vector{Blob}
+end
+
+function setup(backend::Backend, layer::IdentityLayer, inputs::Vector{Blob}, diffs::Vector{Blob})
+  blobs      = inputs[:] # shallow copy
+  blobs_diff = diffs[:] # shallow_copy
+
+  IdentityLayerState(layer, blobs, blobs_diff)
+end
+function shutdown(backend::Backend, state::IdentityLayerState)
+end
+
+function forward(backend::Backend, state::IdentityLayerState, inputs::Vector{Blob})
+  # do nothing
+end
+
+function backward(backend::Backend, state::IdentityLayerState, inputs::Vector{Blob}, diffs::Vector{Blob})
+  # do nothing
+end

test/layers/identity.jl

@@ -0,0 +1,32 @@
+function test_identity_layer(backend::Backend, T)
+  println("-- Testing IdentityLayer on $(typeof(backend)){$T}...")
+
+  eps = 1e-10
+  input = rand(T, 3, 4, 5)
+  input_blob = make_blob(backend, input)
+
+  println("    > Setup")
+  layer = IdentityLayer(tops=[:foo], bottoms=[:bar])
+  state = setup(backend, layer, Blob[input_blob], Blob[NullBlob()])
+  @test all(map(x -> isa(x, NullBlob), state.blobs_diff))
+
+  println("    > Forward")
+  forward(backend, state, Blob[input_blob])
+  got_output = to_array(state.blobs[1])
+  @test all(abs(got_output-input) .< eps)
+
+  shutdown(backend, state)
+end
+
+function test_identity_layer(backend::Backend)
+  test_identity_layer(backend, Float32)
+  test_identity_layer(backend, Float64)
+end
+
+if test_cpu
+  test_identity_layer(backend_cpu)
+end
+if test_gpu
+  test_identity_layer(backend_gpu)
+end
+