https://blog.tensorflow.org/2018/04/a-gentle-introduction-to-tensorflowjs.html

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TensorFlow.js

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April 03, 2018 —
*Posted by Zaid Alyafeai*

Tensorflow.js is a library built on deeplearn.js to create deep learning modules directly on the browser. Using that you can create CNNs, RNNs , etc … on the browser and train these modules using the client’s GPU processing power. Hence, a server GPU is not needed to train the NN. This tutorial starts by explaining the basic building blocks of TensorFlow.js and the operatio…

A Gentle Introduction to TensorFlow.js

Tensorflow.js is a library built on deeplearn.js to create deep learning modules directly on the browser. Using that you can create CNNs, RNNs , etc … on the browser and train these modules using the client’s GPU processing power. Hence, a server GPU is not needed to train the NN. This tutorial starts by explaining the basic building blocks of TensorFlow.js and the operations on them. Then, we describe how to create some complicated models.

`<script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs@latest"> </script>`

This will load the latest published version of the bundle.`const tensor = tf.scalar(2);`

This created a scalar tensor. We also can convert arrays to tensors`const input = tf.tensor([2,2]);`

This creates a constant tensor of the array `[2,2]`

. In other words we converted the one dimensional array to a tensor by a applying the tensor function. We can use `input.shape`

to retrieve the `size`

of the tensor.`const tensor_s = tf.tensor([2,2]).shape;`

This has the shape `[2]`

. We can also create a tensor with specific size. For instance, here we create a tensor of zeros with shape `[2,2]`

.`const input = tf.zeros([2,2]);`

```
const a = tf.tensor([1,2,3]);
a.square().print();
```

The value of `x2`* *

will be `[4,9,16]`

. TensorFlow.js also allows chaining operations. For example, to evaluate the 2nd power of a tensor we use```
const x = tf.tensor([1,2,3]);
const x2 = x.square().square();
```

The `x2`

tensor will have value `[1,16,81]`

.`x2`

we don’t need the value of `x`

. In order to do that we call `dispose()`

```
const x = tf.tensor([1,2,3]);
x.dispose();
```

Note that we can no longer use the tensor `x`* *

in later operations. Now, it might be a little inconvenient to do that for every tensor. Actually, not disposing tensors will be an overhead for the memory. TensorFlow.js offers a special operator `tidy()`

to dispose intermediary tensors automatically```
function f(x)
{
return tf.tidy(()=>{
const y = x.square();
const z = x.mul(y);
return z
});
}
```

Notice that the value of the tensor `y`

will be disposed since we don’t need it after we evaluate the value of `z`

.We will take a simple example were

`[-0.5,0]`

. We will use an optimizer to find the exact value.graph of the function f(x) |

```
function f(x)
{
const f1 = x.pow(tf.scalar(6, 'int32')) //x^6
const f2 = x.pow(tf.scalar(4, 'int32')).mul(tf.scalar(2)) //2x^4
const f3 = x.pow(tf.scalar(2, 'int32')).mul(tf.scalar(3)) //3x^2
const f4 = tf.scalar(1) //1
return f1.add(f2).add(f3).add(x).add(f4)
}
```

Now we can iteratively minimize the function to find the value of the minimum. We will start by an initial value of ```
function minimize(epochs , lr)
{
let y = tf.variable(tf.scalar(2)) //initial value
const optim = tf.train.adam(lr); //gadient descent algorithm
for(let i = 0 ; i < epochs ; i++) //start minimiziation
optim.minimize(() => f(y));
return y
}
```

Using a learning rate with value `0.9`

we find the value of the minimum after `200`

iterations to be `-0.16092407703399658`

.```
xs = tf.tensor2d([[0,0],[0,1],[1,0],[1,1]])
ys = tf.tensor2d([[0],[1],[1],[0]])
```

Then we create two dense layers with two different nonlinear activation functions. We use stochastic gradient descent with cross entropy loss. The learning rate is `0.1`

```
function createModel()
{
var model = tf.sequential()
model.add(tf.layers.dense({units:8, inputShape:2, activation: 'tanh'}))
model.add(tf.layers.dense({units:1, activation: 'sigmoid'}))
model.compile({optimizer: 'sgd', loss: 'binaryCrossentropy', lr:0.1})
return model
}
```

Then we fit the model for `5000iterations`

```
await model.fit(xs, ys, {
batchSize: 1,
epochs: 5000
})
```

Finally we predict on the training set`model.predict(xs).print()`

the output should be `[[0.0064339], [0.9836861], [0.9835356], [0.0208658]]`

which should be expected.`model = tf.sequential();`

Now we can add different layers for the model. Let us add the first convolutional layer with input `[28,28,1]`

```
const convlayer = tf.layers.conv2d({
inputShape: [28,28,1],
kernelSize: 5,
filters: 8,
strides: 1,
activation: 'relu',
kernelInitializer: 'VarianceScaling'
});
```

Here we created a `conv`

layer that takes input of size `[28,28,1]`

. The input will be a gray image of size`28 x 28`

`8`

kernels of size `5x5`

and stride equals to `1`

initialized with `VarianceScaling`

. After that, we apply an activation function which basically takes the negative values in the tensor and replaces them with zeros. Now we can add this `model.add(convlayer);`

Now what is nice about Tensorflow.js we don’t need to specify the input size for the next layer as it will be evaluated automatically after we compile the model. We can also add max-pooling, dense layers , so on. Here is a simple model```
const model = tf.sequential();
//create the first layer
model.add(tf.layers.conv2d({
inputShape: [28, 28, 1],
kernelSize: 5,
filters: 8,
strides: 1,
activation: 'relu',
kernelInitializer: 'VarianceScaling'
}));
//create a max pooling layer
model.add(tf.layers.maxPooling2d({
poolSize: [2, 2],
strides: [2, 2]
}));
//create the second conv layer
model.add(tf.layers.conv2d({
kernelSize: 5,
filters: 16,
strides: 1,
activation: 'relu',
kernelInitializer: 'VarianceScaling'
}));
//create a max pooling layer
model.add(tf.layers.maxPooling2d({
poolSize: [2, 2],
strides: [2, 2]
}));
//flatten the layers to use it for the dense layers
model.add(tf.layers.flatten());
//dense layer with output 10 units
model.add(tf.layers.dense({
units: 10,
kernelInitializer: 'VarianceScaling',
activation: 'softmax'
}));
```

We can apply a tensor for any layer to inspect the output tensor. But here is a catch the input needs to be of shape `[BATCH_SIZE,28,28,1]`

where `BATCH_SIZE`

represents the number of dataset elements we apply to the model at a time. Here is an example of how to evaluate a convolutional layer```
const convlayer = tf.layers.conv2d({
inputShape: [28, 28, 1],
kernelSize: 5,
filters: 8,
strides: 1,
activation: 'relu',
kernelInitializer: 'VarianceScaling'
});
const input = tf.zeros([1,28,28,1]);
const output = convlayer.apply(input);
```

After inspecting the shape of the `output`

tensor we see it has shape`[1,24,24,8]`

. This is evaluated using the formula`const outputSize = Math.floor((inputSize-kernelSize)/stride +1);`

Which will result in `24`

in our case. Returning to our model we realize that we used `flatten()`

which basically convert the input from the shape `[BATCH_SIZE,a,b,c]`

to the shape `[BATCH_SIZE,axbxc]`

. This is important because in the dense layers we cannot apply `2d`

arrays. Finally, we used the dense layer with output units `10`* *

which represents the number of classes we need in our recognition system. Actually this model is used for the recognizing hand-written digits in the so called MNIST dataset.```
const LEARNING_RATE = 0.0001;
const optimizer = tf.train.adam(LEARNING_RATE);
```

This will create an Adam optimizer using the specified learning rate. Now, we are ready to compile the model (attaching the model with the optimizer)```
model.compile({
optimizer: optimizer,
loss: 'categoricalCrossentropy',
metrics: ['accuracy'],
});
```

Here we created model that uses Adam to optimize the loss function that evaluates a cross entropy of the predicted output and the true label.`fit()`

function for that```
const batch = tf.zeros([BATCH_SIZE,28,28,1]);
const labels = tf.zeros([BATCH_SIZE, NUM_CLASSES]);
const h = await model.fit(batch, labels,
{
batchSize: BATCH_SIZE,
validationData: validationData,
epochs: BATCH_EPOCHs
});
```

Note that we are feeding to the fit function a batch of training set. The second variable for the `fit`

function represents the true labels of the model. Lastly, we have the configuration parameters like the `batchSize`

and `epochs`

. Note that `epochs`

represents how many times we iterate over the current batch NOT the whole dataset. Hence we can for example wrap that code inside a `for`* *

loop that iterates over all the batches of the training set.Note that we used the special keyword

`await`

which basically blocks and waits for the function to finish executing the code. It is like running a another thread and the main thread is waiting for the fitting function to finish execution.`0`

and the apple class label `1`

. But, our network accepts a tensor of size `[BATCH_SIZE,NUM_CLASSES]`

. Hence we need to use what we call one hot encoding```
const output = tf.oneHot(tf.tensor1d([0,1,0]), 2);
//the output will be [[1, 0],[0, 1],[1, 0]]
```

Hence we converted the `1d`

tensor off labels into a tensor of shape `[BATCH_SIZE,NUM_CLASSES]`

.```
//h is the output of the fitting module
const loss = h.history.loss[0];
const accuracy = h.history.acc[0];
```

Note that we are evaluating the loss and accuracy of the `validationData`

that was an input to the `fit()`

function.```
//retrieve the canvas
const canvas = document.getElementById("myCanvas");
const ctx = canvas.getContext("2d");
//get image data
imageData = ctx.getImageData(0, 0, 28, 28);
//convert to tensor
const tensor = tf.fromPixels(imageData);
```

Here we created a `canvas`* *

and retrieved `imageData`* *

from it and then we converted to a tensor. Now the tensor will have size `[28,28,3]`* *

but the model takes 4-dimensional vectors. Hence we need to add an extra dimension for the tensor using `expandDims`

`const eTensor = tensor.expandDims(0);`

Hence the output tensor will have size `[1,28,28,3]`* *

since we have added a dimension at index `0`

. Now for prediction we simply use `predict()`

`model.predict(eTensor);`

The function `predict`

will return the value of the last layer in our network usually a `softmax`

activation function.`1,000`

different classes.To load the model we use the following

```
const mobilenet = await tf.loadModel(
'https://storage.googleapis.com/tfjs-models/tfjs/mobilenet_v1_0.25_224/model.json');
```

We can use inputs , outputs to inspect the structure of the model```
//The input size is [null, 224, 224, 3]
const input_s = mobilenet.inputs[0].shape;
//The output size is [null, 1000]
const output_s = mobilenet.outputs[0].shape;
```

Hence we need images of size `[1,224,224,3]`

and the output will be a tensor of size `[1,1000]`

which holds the probability of each class in the ImageNet dataset.For the sake of simplicity we will take an array of zeros and try to predict the class number out of

`1,000`

classes```
var pred = mobilenet.predict(tf.zeros([1, 224, 224, 3]));
pred.argMax().print();
```

After running the code I get class = 21 which represents a kite o:Now we need to inspect the contents of the model. To do that we can get the models layers and names

```
//The number of layers in the model '88'
const len = mobilenet.layers.length;
//this outputs the name of the 3rd layer 'conv1_relu'
const name3 = mobilenet.layers[3].name;
```

We see that we have `88`

layers which would be so expensive to train again on another dataset. Hence, the basic trick is to use this model just to evaluate the activations (we will not retrain) but we will create dense layers that we can train on another number of classes.For instance, suppose we need a model to differentiate between carrots and cucumbers. We will use mobilene tmodel to calculate the activation up to some layer we choose. Then we use dense layers with output size

`2`

to predict the correct class. Hence the `mobilenet`

model will be in some sense ‘freezed’ and we just train the dense layers.First, we need to get rid of the dense layers of the model. We choose to extract a random layer let us say number

`81`

with name `conv_pw_13_relu`

`const layer = mobilenet.getLayer('conv_pw_13_relu');`

Now let us update our model to have this layer is an output`mobilenet = tf.model({inputs: mobilenet.inputs, outputs: layer.output});`

Finally, we create the trainable model but we need to know the last layer output shape```
//this outputs a layer of size [null, 7, 7, 256]
const layerOutput = layer.output.shape;
```

We see that the shape `[null,7,7,256]`

Now we can input this to our dense layers```
trainableModel = tf.sequential({
layers: [
tf.layers.flatten({inputShape: [7, 7, 256]}),
tf.layers.dense({
units: 100,
activation: 'relu',
kernelInitializer: 'varianceScaling',
useBias: true
}),
tf.layers.dense({
units: 2,
kernelInitializer: 'varianceScaling',
useBias: false,
activation: 'softmax'
})
]
});
```

As you can see we created a dense layer with `100`

neurons and the output layer with size `2`

.```
const activation = mobilenet.predict(input);
const predictions = trainableModel.predict(activation);
```

And we can use the previous sections to train the last model using a certain optimizer.Next post

Community
**·**
TensorFlow.js

A Gentle Introduction to TensorFlow.js

April 03, 2018
—
*Posted by Zaid Alyafeai*

Tensorflow.js is a library built on deeplearn.js to create deep learning modules directly on the browser. Using that you can create CNNs, RNNs , etc … on the browser and train these modules using the client’s GPU processing power. Hence, a server GPU is not needed to train the NN. This tutorial starts by explaining the basic building blocks of TensorFlow.js and the operatio…

Build, deploy, and experiment easily with TensorFlow