Video Pipeline Reading Labelled Videos from a Directory#

In this example, we will go through the creation of a pipeline using the operator to read videos along with their labels. The pipeline will return a pair of outputs: a batch of sequences and respective labels.

For more information on the parameters, please look at the documentation.

Setting Up#

First let’s start with the imports:

import os
import numpy as np

from nvidia.dali import pipeline_def
import nvidia.dali.fn as fn
import nvidia.dali.types as types

We need some video containers to process. We can use Sintel trailer, which is an mp4 container containing an h.264 video and distributed under the Create Common license. We’ve split it into 5 second clips and divided the clips into labelled groups. This can be done easily with the ffmpeg standalone tool.

Then we can set the parameters that will be used in the pipeline. The count parameter will define how many frames we want in each sequence sample.

We can replace video_directory with any other directory containing labelled subdirectories and video container files recognized by FFmpeg.

batch_size = 2
sequence_length = 8
initial_prefetch_size = 11
video_directory = os.path.join(
    os.environ["DALI_EXTRA_PATH"], "db", "video", "sintel", "labelled_videos"
shuffle = True
n_iter = 6

Note: DALI_EXTRA_PATH environment variable should point to the place where data from DALI extra repository is downloaded. Please make sure that the proper release tag is checked out.

Running the Pipeline#

We can then define a minimal Pipeline that will output directly the readers.Video outputs:

def video_pipe(file_root):
    video, labels =
    return video, labels

Caution: One important here is tuning initial_fill, that correspond to the Loader prefetch buffer intial size. Since this buffer will be filled of initial_fill sequences, the total number of frames can be really huge! So set it consequently to not OOM during training.

Let’s try to build and run a video_pipe instance on device 0 that will output batch_size sequences of count frames and batch_size labels at each iteration.

pipe = video_pipe(
for i in range(n_iter):
    sequences_out, labels =
    sequences_out = sequences_out.as_cpu().as_array()
    labels = labels.as_cpu().as_array()
(2, 8, 720, 1280, 3)
(2, 1)
(2, 8, 720, 1280, 3)
(2, 1)
(2, 8, 720, 1280, 3)
(2, 1)
(2, 8, 720, 1280, 3)
(2, 1)
(2, 8, 720, 1280, 3)
(2, 1)
(2, 8, 720, 1280, 3)
(2, 1)

Visualizing the Results#

The previous iterations seems to have the yield batches of the expected shape. But let’s visualize the results to be

sequences_out, labels =
sequences_out = sequences_out.as_cpu().as_array()
labels = labels.as_cpu().as_array()

We will use matplotlib to display the frames we obtained in the last batch.

%matplotlib inline
from matplotlib import pyplot as plt
import matplotlib.gridspec as gridspec
def show_sequence(sequence, label):
    columns = 4
    rows = (sequence_length + 1) // (columns)
    fig = plt.figure(figsize=(32, (16 // columns) * rows))
    gs = gridspec.GridSpec(rows, columns)
    for j in range(rows * columns):
        plt.suptitle("label " + str(label[0]), fontsize=30)

And now let’s generate 5 batches of sequence, label pairs:

ITER = 5
for i in range(ITER):
    sequences_out, labels =
    sequences_out = sequences_out.as_cpu().as_array()
    labels = labels.as_cpu().as_array()
    show_sequence(sequences_out[1], labels[1])