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I'm currently building a video classification model for engagement detection but I'm having some trouble training it. The model takes in two tensors as inputs: a 10x48x48x1 tensor which holds a stack of 10 sampled frames each of size 48x48 from the input video and a 10x392 tensor that contains high level facial features that are extracted from each second of the video using OpenFace which is a facial landmark detection tool (each video in the dataset I am training with is 10 seconds long).

The first input tensor is fed into an emotion recognition model that was pre-trained on the FER+ dataset. However, since emotion recognition model was trained on images instead of videos I wrapped the emotion classifier in an "aggregation layer" so that each 48x48x1 slice in the 1st tensor is fed into the emotion classifier and then all of those outputs are averaged together.

The second input tensor is fed into a feed forward neural network. And just like with the emotion classifier, the 329 OpenFace features for each second is fed into the network one at a time and then the output from the network is averaged together.

Finally, the aggregated outputs from these two models are stacked on top of one another and then passed through a final dense layer.

Unfortunately, the model ended up not being able to learn past a certain point. The training loss (sparse categorical cross entropy) decreased to around 1.06 and wouldn't go further. My dataset only has three classes so it looks like the model is essentially randomly guessing.

I can't figure out why it's unable to decrease further however. I checked to make sure that the gradients existed for the two sub models and the entire integrated model and they do as can be seen below:

import numpy as np
import tensorflow as tf
from tensorflow import keras
from tensorflow.keras.models import Model
from tensorflow.keras.layers import Layer, Input, Dense
from keras import models


#Custom attention layer used in the patt lite model. Tensorflow's attention layer was giving us
#nan gradients so we decided to make our own attention layer
class Custom_Attention(tf.keras.layers.Layer):

        def call(self, x):
            size = tf.shape(x)
            x = tf.reshape(x, [size[0], size[1], 1])
            y = tf.einsum("...ij,...jk->...ik", x, tf.transpose(x, perm = [0, 2, 1]))/16.0
            y = tf.matmul(tf.nn.softmax(y), x)
            return tf.reshape(y, [size[0], size[1]])

#We implemented the Patt Lite model described in https://arxiv.org/pdf/2306.09626v1.pdf for our emotion recognition classifier
class Patt_Lite:

    def __init__(self):
        mobile_net = tf.keras.applications.mobilenet.MobileNet(
        include_top = False)


        mobile_net.trainable = False
        inputs = tf.keras.Input(shape = (48, 48, 1), name = "image")
        x = tf.keras.layers.Conv2D(3, (1, 1))(inputs)
        x = tf.keras.layers.Resizing(224, 224)(x)
        x = tf.keras.applications.mobilenet.preprocess_input(x)

        for layer in mobile_net.layers[:56]:
            x = layer(x)

        x = tf.keras.layers.ZeroPadding2D(padding=(1, 1))(x)
        x = tf.keras.layers.Dropout(0.2)(x)
        x = tf.keras.layers.DepthwiseConv2D( (10,10), strides = 2)(x)
        x = tf.keras.layers.Conv2D(256, (1,1), activation = "relu")(x)
        x = tf.keras.layers.Dropout(0.2)(x)
        x = tf.keras.layers.DepthwiseConv2D((3,3))(x)
        x = tf.keras.layers.Conv2D(16, (1,1), activation = "relu")(x)
        x = tf.keras.layers.GlobalAveragePooling2D()(x)
        x = tf.keras.layers.Dense(256, activation = "relu")(x)
        x = Custom_Attention()(x)
        outputs = tf.keras.layers.Dense(9, activation = "softmax")(x)

        self.model = tf.keras.Model(inputs, outputs)
    


#we implement our focus and emotion classifiers as layers in our model
class Emotion_Classifier(Layer):
    def __init__(self, **kwargs):
        super(Emotion_Classifier, self).__init__(**kwargs)

        emoti_model = Patt_Lite().model
        tf.keras.utils.get_custom_objects()["Custom_Attention"] = Custom_Attention
        #we pre-trained the Patt Lite model on the FER+ dataset but for an MRE this isn't necessary
        #emoti_model.load_weights(os.path.join("..", "..", "Models", "Emotion_Rec", "PAtt_Lite_weights.h5"))

        inputs = keras.Input(shape = (48, 48, 1))

        y = emoti_model(inputs)

        #we attach a dense layer to our emotion classifier so that our emotion classifier can be trained
        #to detect engagement
        y = keras.layers.Dense(3, activation = "relu")(y)

        self.model = Model(inputs, y)

    def __call__(self, x):
        return self.model(x)
    


#for our focus classifier, we implemented it as an MLP. Since we're already working with high level features,
#our classifier doesn't need to be as complex as the emotion classifier
class Focus_Classifier(Layer):
    def __init__(self, **kwargs):
        super(Focus_Classifier, self).__init__(**kwargs)
        inputs = keras.Input(shape = (329))
        y = keras.layers.Dense(64, activation = "relu")(inputs)
        y = keras.layers.Dense(64, activation = "relu")(y)
        y = keras.layers.Dense(128, activation = "relu")(y)
        y = keras.layers.Dense(3, activation = "relu")(y)
        self.model = Model(inputs, y)

    def __call__(self, x):
        return self.model(x)
    

#Finally our aggregation layer goes through each of the 10 sampled / averaged frames
#and evaluates the focus and emotion classifiers on each of them
class AggregationLayer(Layer):

    def __init__(self, emo_model, open_model, num_frames, **kwargs):
        super(AggregationLayer, self).__init__(**kwargs)
        self.emoti_model = emo_model
        self.open_model = open_model
        self.num_frames = num_frames

    def __call__(self, inputs):

        #we evaluate our two classifiers on each frame
        emot_outputs = [self.emoti_model(frame) for frame in tf.unstack(inputs[0], axis=1)]
        focus_outputs = [self.open_model(frame) for frame in tf.unstack(inputs[1], axis=1)]

        #average the outputs
        aver_emot_output = tf.reduce_mean(tf.stack(emot_outputs, axis = 1), axis=1)
        aver_focus_output = tf.reduce_mean(tf.stack(focus_outputs, axis = 1), axis=1)

        #and then our output from the aggregation layer are the two 3x1 averaged outputs
        aggregate_output = tf.concat([aver_emot_output, aver_focus_output], axis = 1)
        return aggregate_output

def test_trainable(model, input_size):

    with tf.GradientTape() as tape:
        a = [tf.random.normal(size) for size in input_size]
        z = model(a)
        loss = tf.keras.losses.SparseCategoricalCrossentropy()([1], z)
        #loss = tf.math.reduce_mean()
        #loss = tf.math.reduce_mean(z**2)
    grads = tape.gradient(loss, model.trainable_variables)

    for grad, var in zip(grads, model.trainable_variables):
    #unfortunately all of the gradients are None and I'm not sure why
        try:
            tf.debugging.check_numerics(grad, message=f"{var.name}: ")
            tf.debugging.assert_greater(tf.sum(tf.math.abs(grad)), 0.0)
        except:
            continue


inp_emo = keras.Input((10, 48, 48, 1))
inp_open = keras.Input((10, 329))

emoti_model = Emotion_Classifier()
focus_model = Focus_Classifier()

y = AggregationLayer(emoti_model, focus_model, 10)([inp_emo, inp_open])

outputs = Dense(3, activation = "softmax")(y)

model = Model([inp_emo, inp_open], outputs)

model.summary()



test_trainable(Patt_Lite().model,  [(1, 48, 48, 1)])
test_trainable(emoti_model, [(1, 48, 48, 1)])
test_trainable(focus_model, [(1, 329)])
test_trainable(model, [(1, 10, 48, 48, 1), (1, 10, 329)])

def generator():
    feats = (tf.random.normal((1, 10, 48, 48, 1)),
             tf.random.normal((1, 10, 329)))
    labels = tf.reshape(tf.convert_to_tensor( [1], dtype=  tf.int16), [1, 1])

    yield feats, labels

output_signature_train = ( (tf.TensorSpec(shape = (None, 10, 48, 48, 1), dtype = tf.float32),
                      tf.TensorSpec(shape = (None, 10, 329), dtype = tf.float32)),
                    tf.TensorSpec(shape = (None, 1), dtype = tf.int16))

train_ds = tf.data.Dataset.from_generator(generator,
                                          output_signature = output_signature_train)

model.compile(loss = tf.keras.losses.SparseCategoricalCrossentropy(),
              optimizer = keras.optimizers.SGD(learning_rate = 1e-3),
              metrics = ["acc"])

a = train_ds.take(1)

model.fit(a, 
          epochs = 100)

I also trained my model on a small subset of the training set (64 samples) to see if it could overfit on it. Unfortunately, it wasn't able to and also was stuck at around the same training loss. Engagement Training Loss

Any and all help with figuring out why the model isn't training would be greatly appreciated. I also trained the model on a custom tensorflow data generator as the dataset I was using was too large to load into memory all at once. It was a little bit difficult to make a minimal reproducible example with that but if you feel that that might be the source of error, please let me know and I will be happy to make one.

EDIT: I tracked down the problem and it looks like the issue is in fact with the generator dataset. When I train on a normal dataset, it trains normally and overfits on a small dataset but not on the generator dataset. I updated the code above to include an example demonstrating this.

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1 Answer 1

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You may be facing the "dying ReLU" problem, where many neurons begin to output zeros past a certain point in the process.

Have you tried to change the activation function? If no, the link below gives you a handful of activation functions you can use in tensorflow.

https://www.tensorflow.org/api_docs/python/tf/keras/activations

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  • $\begingroup$ Hi, thank you so much for your help. I think I was able to track down the problem. It looks like the model trains normally on a regular dataset but not on the generator dataset. I updated the MRE to demonstrate this. If you have time, please let me know if you have an idea why it doesn't train on the generator dataset. $\endgroup$
    – snowball
    Jan 9 at 14:34
  • $\begingroup$ @snowball if you think my answer helped you in a any way, please upvote or mark it as the answer. As for your 2nd question, that question would be a perfect candidate for your next question here in the community. To me, random data generators are not truly random. But please ask the question. $\endgroup$
    – Full Array
    Jan 10 at 1:10

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