Keras - How are batches and epochs used in fit_generator()?

Question

I have a video of 8000 frames, and I'd like to train a Keras model on batches of 200 frames each. I have a frame generator that loops through the video frame-by-frame and accumulates the (3 x 480 x 640) frames into a numpy matrix X of shape (200, 3, 480, 640) -- (batch size, rgb, frame height, frame width) -- and yields X and Y every 200th frame:

import cv2
...
def _frameGenerator(videoPath, dataPath, batchSize):
    """
    Yield X and Y data when the batch is filled.
    """
    camera = cv2.VideoCapture(videoPath)
    width = camera.get(3)
    height = camera.get(4)
    frameCount = int(camera.get(7))  # Number of frames in the video file.

    truthData = _prepData(dataPath, frameCount)

    X = np.zeros((batchSize, 3, height, width))
    Y = np.zeros((batchSize, 1))

    batch = 0
    for frameIdx, truth in enumerate(truthData):
        ret, frame = camera.read()
        if ret is False: continue

        batchIndex = frameIdx%batchSize

        X[batchIndex] = frame
        Y[batchIndex] = truth

        if batchIndex == 0 and frameIdx != 0:
            batch += 1
            print "now yielding batch", batch
            yield X, Y

Here's how run fit_generator():

        batchSize = 200
        print "Starting training..."
        model.fit_generator(
            _frameGenerator(videoPath, dataPath, batchSize),
            samples_per_epoch=8000,
            nb_epoch=10,
            verbose=args.verbosity
        )

My understanding is an epoch finishes when samples_per_epoch samples have been seen by the model, and samples_per_epoch = batch size * number of batches = 200 * 40. So after training for an epoch on frames 0-7999, the next epoch will start training again from frame 0. Is this correct?

With this setup I expect 40 batches (of 200 frames each) to be passed from the generator to fit_generator, per epoch; this would be 8000 total frames per epoch -- i.e., samples_per_epoch=8000. Then for subsequent epochs, fit_generator would reinitialize the generator such that we begin training again from the start of the video. Yet this is not the case. After the first epoch is complete (after the model logs batches 0-24), the generator picks up where it left off. Shouldn't the new epoch start again from the beginning of the training dataset?

If there is something incorrect in my understanding of fit_generator please explain. I've gone through the documentation, this example, and these related issues. I'm using Keras v1.0.7 with the TensorFlow backend. This issue is also posted in the Keras repo.

Answer 1

After the first epoch is complete (after the model logs batches 0-24), the generator picks up where it left off

This is an accurate description of what happens. If you want to reset or rewind the generator, you'll have to do this internally. Note that keras's behavior is quite useful in many situations. For example, you can end an epoch after seeing 1/2 the data then do an epoch on the other half, which would be impossible if the generator status was reset (which can be useful for monitoring the validation more closely).

Answer 2

You can force your generator to reset itself by adding a while 1: loop, that's how I proceed. Thus your generator can yield batched data for each epochs.