Make Predictions Using dlnetwork Object

This example shows how to make predictions using a dlnetwork object by splitting data into mini-batches.

For large data sets, or when predicting on hardware with limited memory, make predictions by splitting the data into mini-batches. When making predictions with SeriesNetwork or DAGNetwork objects, the predict function automatically splits the input data into mini-batches. For dlnetwork objects, you must split the data into mini-batches manually.

Load dlnetwork Object

Load a trained dlnetwork object and the corresponding classes.

s = load("digitsCustom.mat");
dlnet = s.dlnet;
classes = s.classes;

Load Data for Prediction

Load the digits data for prediction.

digitDatasetPath = fullfile(matlabroot,'toolbox','nnet','nndemos', ...
    'nndatasets','DigitDataset');
imds = imageDatastore(digitDatasetPath, ...
    'IncludeSubfolders',true);

Make Predictions

Loop over the mini-batches of the test data and make predictions using a custom prediction loop.

Use minibatchqueue to process and manage the mini-batches of images. Specify a mini-batch size of 128. Set the read size property of the image datastore to the mini-batch size.

For each mini-batch:

• Use the custom mini-batch preprocessing function preprocessMiniBatch (defined at the end of this example) to concatenate the data into a batch and normalize the images.

• Format the images with the dimensions 'SSCB' (spatial, spatial, channel, batch). By default, the minibatchqueue object converts the data to dlarray objects with underlying type single.

• Make predictions on a GPU if one is available. By default, the minibatchqueue object converts the output to a gpuArray if a GPU is available. Using a GPU requires Parallel Computing Toolbox™ and a supported GPU device. For information on supported devices, see GPU Support by Release (Parallel Computing Toolbox).

miniBatchSize = 128;
imds.ReadSize = miniBatchSize;

mbq = minibatchqueue(imds,...
    "MiniBatchSize",miniBatchSize,...
    "MiniBatchFcn", @preprocessMiniBatch,...
    "MiniBatchFormat","SSCB");

Loop over the minibatches of data and make predictions using the predict function. Use the onehotdecode function to determing the class labels. Store the predicted class labels.

numObservations = numel(imds.Files);
YPred = strings(1,numObservations);

predictions = [];

% Loop over mini-batches.
while hasdata(mbq)
    
    % Read mini-batch of data.
    dlX = next(mbq);
       
    % Make predictions using the predict function.
    dlYPred = predict(dlnet,dlX);
   
    % Determine corresponding classes.
    predBatch = onehotdecode(dlYPred,classes,1);
    predictions = [predictions predBatch];
  
end

Visualize some of the predictions.

idx = randperm(numObservations,9);

figure
for i = 1:9
    subplot(3,3,i)
    I = imread(imds.Files{idx(i)});    
    label = predictions(idx(i));
    imshow(I)
    title("Label: " + string(label))
  
end

Mini-Batch Preprocessing Function

The preprocessMiniBatch function preprocesses the data using the following steps:

1. Extract the data from the incoming cell array and concatenate into a numeric array. Concatenating over the fourth dimension adds a third dimension to each image, to be used as a singleton channel dimension.

2. Normalize the pixel values between 0 and 1.

function X = preprocessMiniBatch(data)    
    % Extract image data from cell and concatenate
    X = cat(4,data{:});
    
    % Normalize the images.
    X = X/255;
end