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Create Simple Semantic Segmentation Network in Deep Network Designer

This example shows how to create a simple semantic segmentation network using Deep Network Designer.

Semantic segmentation describes the process of associating each pixel of an image with a class label (such as flower, person, road, sky, ocean, or car). Applications for semantic segmentation include road segmentation for autonomous driving and cancer cell segmentation for medical diagnosis. To learn more, see Getting Started with Semantic Segmentation Using Deep Learning.

Load Training Data

To train a semantic segmentation network, you need a collection of images and its corresponding collection of pixel-labeled images. A pixel-labeled image is an image where every pixel value represents the categorical label of that pixel. This example uses a simple data set of 32-by-32 images of triangles for illustration purposes. You can interactively label pixels and export the label data for computer vision applications using Image Labeler. For more information on creating training data for semantic segmentation applications, see Label Pixels for Semantic Segmentation.

Load the training data.

dataFolder  = fullfile(toolboxdir("vision"),"visiondata","triangleImages");

Prepare Data for Training

imageDir = fullfile(dataFolder,"trainingImages");
labelDir = fullfile(dataFolder,"trainingLabels");

Create an ImageDatastore containing the images.

imds = imageDatastore(imageDir);

Create a PixelLabelDatastore containing the ground truth pixel labels. This data set has two classes: "triangle" and "background".

classNames = ["triangle","background"];
labelIDs   = [255 0];

pxds = pixelLabelDatastore(labelDir,classNames,labelIDs);

Combine the image datastore and the pixel label datastore into a CombinedDatastore object using the combine function. A combined datastore maintains parity between the pair of images in the underlying datastores.

cds = combine(imds,pxds);

Build Network Architecture

Open Deep Network Designer (Deep Learning Toolbox).


In Deep Network Designer, you can build deep neural networks. Pause on Blank Network and click New.

Create a semantic segmentation network by dragging layers from the Layer Library to the Designer pane.

Connect the layers in this order:

  1. imageInputLayer with InputSize set to 32,32,1

  2. convolution2dLayer with FilterSize set to 3,3, NumFilters set to 64, and Padding set to 1,1,1,1

  3. reluLayer

  4. maxPooling2dLayer with PoolSize set to 2,2, Stride set to 2,2, and Padding set to 0,0,0,0

  5. convolution2dLayer with FilterSize set to 3,3, NumFilters set to 64, and Padding set to 1,1,1,1

  6. reluLayer

  7. transposedConv2dLayer with FilterSize set to 4,4, NumFilters set to 64, Stride set to 2,2, and Cropping set to 1,1,1,1

  8. convolution2dLayer with FilterSize set to 1,1, NumFilters set to 2, and Padding set to 0,0,0,0

  9. softmaxLayer

You can also create this network at the command line and then import the network into Deep Network Designer using deepNetworkDesigner(layers).

layers = [
    imageInputLayer([32 32 1])

This network is a simple semantic segmentation network based on a downsampling and upsampling design. For more information on constructing a semantic segmentation network, see Create a Semantic Segmentation Network.

To check that the network is ready for training, click Analyze. The Deep Learning Network Analyzer reports zero errors or warnings, therefore, the network is ready for training. To export the network, click Export. The app saves the network in the variable net_1.

Specify Training Options

Specify the training options. Choosing among the options requires empirical analysis. To explore different training option configurations by running experiments, you can use the Experiment Manager app.

  • Train using the SGDM optimizer.

  • Use an initial learning rate of 0.001.

  • Train for 100 epochs.

  • Use a mini-batch size of 64.

  • Display the training progress in a plot and monitor the accuracy.

  • Disable the verbose output.

options = trainingOptions("sgdm", ...
    MaxEpochs=100, ...
    InitialLearnRate=0.001, ...
    MiniBatchSize=64, ...
    Plots="training-progress", ...
    Metrics="accuracy", ...

Train Network

Train the neural network using the trainnet function. For semantic segmentation tasks, use cross-entropy loss. By default, the trainnet function uses a GPU if one is available. Training on a GPU requires a Parallel Computing Toolbox™ license and a supported GPU device. For information on supported devices, see GPU Computing Requirements (Parallel Computing Toolbox). Otherwise, the trainnet function uses the CPU. To specify the execution environment, use the ExecutionEnvironment training option.

net = trainnet(cds,net_1,"crossentropy",options);

Segment Triangles in an Image

Make predictions using test data and the trained network.

Segment the test image using semanticseg. Display the labels over the image by using the labeloverlay function.

imgTest = imread("triangleTest.jpg");
testSeg = semanticseg(imgTest,net);
testImageSeg = labeloverlay(imgTest,testSeg);

Display the results.


The network successfully labels the triangles in the test image.

The semantic segmentation network trained in this example is very simple. For an example showing how to train a more complex semantic segmentation network, see Semantic Segmentation Using Deep Learning.