PlaceholderLayer

Layer replacing an unsupported Keras or ONNX layer

Description

PlaceholderLayer is a layer that importKerasLayers and importONNXLayers insert into a layer array or layer graph in place of an unsupported Keras or ONNX™ layer.

Creation

Importing layers from a Keras or ONNX network that has layers that are not supported by Deep Learning Toolbox™ creates PlaceholderLayer objects.

Properties

Name — Layer name

character vector | string scalar

Layer name, specified as a character vector or a string scalar.

Data Types: char | string

Description — Layer description

character vector | string scalar

Layer description, specified as a character vector or a string scalar.

Data Types: char | string

Type — Layer type

character vector | string scalar

Layer type, specified as a character vector or a string scalar.

Data Types: char | string

KerasConfiguration — Keras configuration of layer

structure

Keras configuration of a layer, specified as a structure. The fields of the structure depend on the layer type.

Note

This property only exists if the layer was created when importing a Keras network.

Data Types: struct

ONNXNode — ONNX configuration of layer

structure

ONNX configuration of a layer, specified as a structure. The fields of the structure depend on the layer type.

Note

This property only exists if the layer was created when importing an ONNX network.

Data Types: struct

Weights — Imported weights

structure

Imported weights, specified as a structure.

Data Types: struct

Examples

Find and Explore Placeholder Layers

Specify the Keras network file to import layers from.

modelfile = 'digitsDAGnetwithnoise.h5';

Import the network architecture. The network includes some layer types that are not supported by Deep Learning Toolbox. The importKerasLayers function replaces each unsupported layer with a placeholder layer and returns a warning message.

lgraph = importKerasLayers(modelfile)

Warning: Unable to import some Keras layers, because they are not supported by the Deep Learning Toolbox. They have been replaced by placeholder layers. To find these layers, call the function findPlaceholderLayers on the returned object.

lgraph = 
  LayerGraph with properties:

     InputNames: {'input_1'}
    OutputNames: {'ClassificationLayer_activation_1'}
         Layers: [15x1 nnet.cnn.layer.Layer]
    Connections: [15x2 table]

Display the imported layers of the network. Two placeholder layers replace the Gaussian noise layers in the Keras network.

lgraph.Layers

ans = 
  15x1 Layer array with layers:

     1   'input_1'                            Image Input             28x28x1 images
     2   'conv2d_1'                           2-D Convolution         20 7x7 convolutions with stride [1  1] and padding 'same'
     3   'conv2d_1_relu'                      ReLU                    ReLU
     4   'conv2d_2'                           2-D Convolution         20 3x3 convolutions with stride [1  1] and padding 'same'
     5   'conv2d_2_relu'                      ReLU                    ReLU
     6   'gaussian_noise_1'                   PLACEHOLDER LAYER       Placeholder for 'GaussianNoise' Keras layer
     7   'gaussian_noise_2'                   PLACEHOLDER LAYER       Placeholder for 'GaussianNoise' Keras layer
     8   'max_pooling2d_1'                    2-D Max Pooling         2x2 max pooling with stride [2  2] and padding 'same'
     9   'max_pooling2d_2'                    2-D Max Pooling         2x2 max pooling with stride [2  2] and padding 'same'
    10   'flatten_1'                          Keras Flatten           Flatten activations into 1-D assuming C-style (row-major) order
    11   'flatten_2'                          Keras Flatten           Flatten activations into 1-D assuming C-style (row-major) order
    12   'concatenate_1'                      Depth concatenation     Depth concatenation of 2 inputs
    13   'dense_1'                            Fully Connected         10 fully connected layer
    14   'activation_1'                       Softmax                 softmax
    15   'ClassificationLayer_activation_1'   Classification Output   crossentropyex

Find the placeholder layers using findPlaceholderLayers. The output argument contains the two placeholder layers that importKerasLayers inserted in place of the Gaussian noise layers of the Keras network.

placeholders = findPlaceholderLayers(lgraph)

placeholders = 
  2x1 PlaceholderLayer array with layers:

     1   'gaussian_noise_1'   PLACEHOLDER LAYER   Placeholder for 'GaussianNoise' Keras layer
     2   'gaussian_noise_2'   PLACEHOLDER LAYER   Placeholder for 'GaussianNoise' Keras layer

Specify a name for each placeholder layer.

gaussian1 = placeholders(1);
gaussian2 = placeholders(2);

Display the configuration of each placeholder layer.

gaussian1.KerasConfiguration

ans = struct with fields:
    trainable: 1
         name: 'gaussian_noise_1'
       stddev: 1.5000

gaussian2.KerasConfiguration

ans = struct with fields:
    trainable: 1
         name: 'gaussian_noise_2'
       stddev: 0.7000

Assemble Network from Pretrained Keras Layers

This example shows how to import the layers from a pretrained Keras network, replace the unsupported layers with custom layers, and assemble the layers into a network ready for prediction.

Import Keras Network

Import the layers from a Keras network model. The network in 'digitsDAGnetwithnoise.h5' classifies images of digits.

filename = 'digitsDAGnetwithnoise.h5';
lgraph = importKerasLayers(filename,'ImportWeights',true);

Warning: Unable to import some Keras layers, because they are not supported by the Deep Learning Toolbox. They have been replaced by placeholder layers. To find these layers, call the function findPlaceholderLayers on the returned object.

The Keras network contains some layers that are not supported by Deep Learning Toolbox. The importKerasLayers function displays a warning and replaces the unsupported layers with placeholder layers.

Plot the layer graph using plot.

figure
plot(lgraph)
title("Imported Network")

Replace Placeholder Layers

To replace the placeholder layers, first identify the names of the layers to replace. Find the placeholder layers using findPlaceholderLayers.

placeholderLayers = findPlaceholderLayers(lgraph)

placeholderLayers = 
  2x1 PlaceholderLayer array with layers:

     1   'gaussian_noise_1'   PLACEHOLDER LAYER   Placeholder for 'GaussianNoise' Keras layer
     2   'gaussian_noise_2'   PLACEHOLDER LAYER   Placeholder for 'GaussianNoise' Keras layer

Display the Keras configurations of these layers.

placeholderLayers.KerasConfiguration

ans = struct with fields:
    trainable: 1
         name: 'gaussian_noise_1'
       stddev: 1.5000

ans = struct with fields:
    trainable: 1
         name: 'gaussian_noise_2'
       stddev: 0.7000

Create two Gaussian noise layers with the same configurations as the imported Keras layers using the helper gaussianNoiseLayer function.

gnLayer1 = gaussianNoiseLayer(1.5,'new_gaussian_noise_1');
gnLayer2 = gaussianNoiseLayer(0.7,'new_gaussian_noise_2');

Replace the placeholder layers with the custom layers using replaceLayer.

lgraph = replaceLayer(lgraph,'gaussian_noise_1',gnLayer1);
lgraph = replaceLayer(lgraph,'gaussian_noise_2',gnLayer2);

Plot the updated layer graph using plot.

figure
plot(lgraph)
title("Network with Replaced Layers")

Specify Class Names

If the imported classification layer does not contain the classes, then you must specify these before prediction. If you do not specify the classes, then the software automatically sets the classes to 1, 2, ..., N, where N is the number of classes.

Find the index of the classification layer by viewing the Layers property of the layer graph.

lgraph.Layers

ans = 
  15x1 Layer array with layers:

     1   'input_1'                            Image Input             28x28x1 images
     2   'conv2d_1'                           2-D Convolution         20 7x7x1 convolutions with stride [1  1] and padding 'same'
     3   'conv2d_1_relu'                      ReLU                    ReLU
     4   'conv2d_2'                           2-D Convolution         20 3x3x1 convolutions with stride [1  1] and padding 'same'
     5   'conv2d_2_relu'                      ReLU                    ReLU
     6   'new_gaussian_noise_1'               Gaussian Noise          Gaussian noise with standard deviation 1.5
     7   'new_gaussian_noise_2'               Gaussian Noise          Gaussian noise with standard deviation 0.7
     8   'max_pooling2d_1'                    2-D Max Pooling         2x2 max pooling with stride [2  2] and padding 'same'
     9   'max_pooling2d_2'                    2-D Max Pooling         2x2 max pooling with stride [2  2] and padding 'same'
    10   'flatten_1'                          Keras Flatten           Flatten activations into 1-D assuming C-style (row-major) order
    11   'flatten_2'                          Keras Flatten           Flatten activations into 1-D assuming C-style (row-major) order
    12   'concatenate_1'                      Depth concatenation     Depth concatenation of 2 inputs
    13   'dense_1'                            Fully Connected         10 fully connected layer
    14   'activation_1'                       Softmax                 softmax
    15   'ClassificationLayer_activation_1'   Classification Output   crossentropyex

The classification layer has the name 'ClassificationLayer_activation_1'. View the classification layer and check the Classes property.

cLayer = lgraph.Layers(end)

cLayer = 
  ClassificationOutputLayer with properties:

            Name: 'ClassificationLayer_activation_1'
         Classes: 'auto'
    ClassWeights: 'none'
      OutputSize: 'auto'

   Hyperparameters
    LossFunction: 'crossentropyex'

Because the Classes property of the layer is 'auto', you must specify the classes manually. Set the classes to 0, 1, ..., 9, and then replace the imported classification layer with the new one.

cLayer.Classes = string(0:9)

cLayer = 
  ClassificationOutputLayer with properties:

            Name: 'ClassificationLayer_activation_1'
         Classes: [0    1    2    3    4    5    6    7    8    9]
    ClassWeights: 'none'
      OutputSize: 10

   Hyperparameters
    LossFunction: 'crossentropyex'

lgraph = replaceLayer(lgraph,'ClassificationLayer_activation_1',cLayer);

Assemble Network

Assemble the layer graph using assembleNetwork. The function returns a DAGNetwork object that is ready to use for prediction.

net = assembleNetwork(lgraph)

net = 
  DAGNetwork with properties:

         Layers: [15x1 nnet.cnn.layer.Layer]
    Connections: [15x2 table]
     InputNames: {'input_1'}
    OutputNames: {'ClassificationLayer_activation_1'}

Version History

Introduced in R2017b