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forward

Compute video classifier outputs for training

Since R2021b

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Syntax

dLYVideo = forward(classifier,dlXVideo)
[dLYVideo,stateVideo] = forward(classifier,dlXVideo)
[dLYVideo,dlYFlow] = forward(classifier,dlXVideo,dlXFlow)
[dLYVideo,dlYFlow,stateVideo,stateFlow] = forward(classifier,dlXVideo,dlXFlow)

Description

dLYVideo = forward(classifier,dlXVideo) computes the video classifier outputs for training. You can use this function with dlfeval (Deep Learning Toolbox) to automatically compute gradients for updating the learnable parameters of the video classifier. classifier is specified as a r2plus1dVideoClassifier or slowFastVideoClassifier classifier object.

example

[dLYVideo,stateVideo] = forward(classifier,dlXVideo) also returns the updated video network state. The output, stateVideo, contains information maintained by the classifier between training iterations. For example, the state of batch normalization operation.

[dLYVideo,dlYFlow] = forward(classifier,dlXVideo,dlXFlow) also returns the optical flow outputs from the classifier for training. Use this syntax when you set the OpticalFlowMethod property of the classifier object to "Farneback".

[dLYVideo,dlYFlow,stateVideo,stateFlow] = forward(classifier,dlXVideo,dlXFlow) also returns the updated video network and the optical flow network states.

Examples

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This example uses:

Open Live Script

This example shows how to compute video classifier outputs for training. To learn more about how to train a video classifier network for your dataset, see Gesture Recognition using Videos and Deep Learning.

Load a video classifier pretrained on the Kinetics-400 data set.

sf = slowFastVideoClassifier;

Specify the video file name.

videoFilename = "washingHands.avi";

Create a VideoReader to read the video frames.

reader = VideoReader(videoFilename);

Read the required number of video frames corresponding to the video classifier network from the beginning of the video file. The required number of frames is defined by the value of the 4th element of the InputSize property of the video classifier.

sequenceLength = sf.InputSize(4);
sequenceRange = [1,sequenceLength];
videoFrames = read(reader,sequenceRange);

Resize the video frames for training. The required height and width are defined by the first two elements of the InputSize property of the video classifier.

heightWidth = sf.InputSize(1:2);
resized = imresize(videoFrames,heightWidth);

Convert the input to type single.

resized = single(resized);

Rescale the input between 0 and 1.

minValue = sf.InputNormalizationStatistics.Min;
maxValue = sf.InputNormalizationStatistics.Max;
minValue = reshape(minValue,1,1,3);
maxValue = reshape(maxValue,1,1,3);
resized = rescale(resized,0,1,InputMin=minValue,InputMax=maxValue);

Normalize the video data using the mean and standard deviation.

meanValue = sf.InputNormalizationStatistics.Mean;
stdValue = sf.InputNormalizationStatistics.StandardDeviation;
meanValue = reshape(meanValue,1,1,3);
stdValue = reshape(stdValue,1,1,3);
resized = resized - meanValue;
resized = resized./stdValue;

Convert the input to a dlarray object.

dlVideo = dlarray(resized,"SSCTB");
trainingActivations = forward(sf,dlVideo);

Find the class label corresponding to the maximum score.

[score,idx] = max(trainingActivations);
label = sf.Classes(idx)
label = categorical
     washing hands 


score
score = 
  1(S) × 1(S) × 1(S) × 1(C) × 1(B) single dlarray

    0.0026

Display the class label.

frame = videoFrames(:,:,:,end);
frame = insertText(frame,[2,2],string(label),FontSize=24);
imshow(frame)

Input Arguments

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Classifier object, specified as a r2plus1dVideoClassifier or slowFastVideoClassifier object.

Video input, specified as an H-by-W-by-C-by-T-by-B SSCTB formatted dlarray (Deep Learning Toolbox) object that corresponds to the video input of the classifier.

  • H — Height.

  • W — Width.

  • C — Number of channels. The number of channels must be equal to the channels value of the InputSize property of the classifier object.

  • T — Number of frames. The number of frames must be equal to the frames value of the InputSize property of the classifier object.

  • B — Batch size.

Video and optical flow input, specified as an H-by-W-by-C-by-T-by-B SSCTB formatted dlarray (Deep Learning Toolbox) object that corresponds to the video and optical flow input of the classifier.

  • H — Height.

  • W — Width.

  • C — Number of channels. The number of channels must be equal to the channels value of the InputSize property of the classifier object.

  • T — Number of frames. The number of frames must be equal to the frames value of the InputSize property of the classifier object.

  • B — Batch size.

Output Arguments

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Activations of the video network, returned as a formatted dlarray (Deep Learning Toolbox) object.

Updated video network state, returned as a table with three columns:

  • Layer — Layer name, specified as a string scalar.

  • Parameter — Parameter name, specified as a string scalar.

  • Value — Value of the parameter, specified as a dlarray object.

The network state contains information remembered by the network between iterations. For example, the state of the LSTM and batch normalization layers.

During training or inference, you can update the network state using the output of the forward and predict functions.

Activations of the optical flow network, returned as a formatted dlarray (Deep Learning Toolbox) object.

Updated optical flow network state, returned as a table with three columns:

  • Layer — Layer name, specified as a string scalar.

  • Parameter — Parameter name, specified as a string scalar.

  • Value — Value of the parameter, specified as a dlarray object.

The network state contains information remembered by the network between iterations. For example, the state of LSTM and batch normalization layers.

During training or inference, you can update the network state using the output of the forward and predict functions.

Version History

Introduced in R2021b

See Also

Functions

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