## Rotation of Video using FFmpeg With NVIDIA GPU Acceleration on Ubuntu

This tutorial deals with video rotation using GPU accelerated libraries supported by FFmpeg in Ubuntu 16.04.

## Introduction

FFmpeg is one of the most famous multimedia frameworks wich is widely used for processeing videos. In order to encode the video, certainly a video encoder must be used. The popular x264 is the one which is widely used however it is not super fast! The latest NVIDIA GPUs contain a hardware-based video encoder called NVENC which is much faster than traditional ones. In order to be able to utilize this gpu-accelerated encoder, FFmpeg must be installed with NVENC support. The full documentation of FFmpeg integrated with NVIDIA can be fount at here. documentation on NVENC can be found here. Moreover The NVENC programming guide can be found here. The full code for this tutorial is available at this github repository.

In this tutorial the main goal is to show how to do video rotation with GPU-accelerated libraries in Linux. In this tutorial we do not use the terminal commands directly for employing the FFmpeg with NVENC support. Instead the python interface is being used to run commands in the terminal. This can be done using subprocess python module. This module is employed for execution and dealing external commands, intended to supersede the os.sys module. The trivial method os its usage will be explained in this tutorial. Please refer to this documentation for further details.

The assumption of this tutorial is that the FFmpeg is already installed with NVENC support. The installation guide can be found in FFMPEG WITH NVIDIA ACCELERATION ON UBUNTU LINUX documentation provided by NVIDIA.

## Data Indicator

This tutorial is customized for processing multiple videos. The assumption is that the full path of each video is stored in a .txt file in line-by-line format. The example of the ".txt" file is as below:
Figure 1: The format of .txt file.
As a guidance if a recursive search for specific files in a directory and its subdirectories with extension ".png" is desired, the following method in command line is useful and it saves the output as a ".txt" file:

Search for Specific Files

• find /absolute/path/to/directory/to/be/search -type f -name "*.png" > /absolute/path/to/save/the/output/textfile.txt

## Video Rotation

From now on the assumption is that the ".txt" file is ready and well-formatted. The python script for processing videos is as below:
Rotation of video using FFmpeg with NVENC encoder
  import subprocess
import os

# Pre...
textfile_path = 'videos.txt'
output_prefix = 'front'

# Read the text file line by line.
with open(textfile_path) as f:

# Remove whitespace characters like \n at the end of each line
files_list = [x.strip() for x in content]

# Transpose 90 degree & Clockwise
# It already save the video file using the named defined by output_name.
for file_num, file_path in enumerate(files_list, start=1):

# Get the file name with extension
file_name = os.path.basename(file_path)

# Get the file name without extension
raw_file_name = os.path.basename(file_name).split('.')[0]

# Get the input file directory path.
file_dir = os.path.dirname(file_path)

# Form the output file full path.
output_file_path = file_dir + '/' + output_prefix + '_' + raw_file_name + '.mov'

print('processing file: %s' % file_path)
subprocess.call(
['ffmpeg', '-i', file_path, '-vf', 'transpose=1', '-vcodec', 'nvenc',
'-preset', 'slow', '-b:v', '5M',
'-acodec', 'copy', output_file_path])
print('file %s saved' % output_file_path)

  import subprocess
import os

# Pre...
textfile_path = 'videos.txt'
output_prefix = 'front'

# Read the text file line by line.
with open(textfile_path) as f:

# Remove whitespace characters like \n at the end of each line
files_list = [x.strip() for x in content]

# Transpose 90 degree & Counterclockwise
# It already save the video file using the named defined by output_name.
for file_num, file_path in enumerate(files_list, start=1):

# Get the file name with extension
file_name = os.path.basename(file_path)

# Get the file name without extension
raw_file_name = os.path.basename(file_name).split('.')[0]

# Get the input file directory path.
file_dir = os.path.dirname(file_path)

# Form the output file full path.
output_file_path = file_dir + '/' + output_prefix + '_' + raw_file_name + '.mov'

print('processing file: %s' % file_path)
subprocess.call(
['ffmpeg', '-i', file_path, '-vf', 'transpose=2', '-vcodec', 'nvenc',
'-preset', 'slow', '-b:v', '5M',
'-acodec', 'copy', output_file_path])
print('file %s saved' % output_file_path)


### I - Overall Code Description

The videos.txt file is saved in the absolute path. Lines 8-13 of the code reads the ".txt" file and stores each line as an item of a list called files_list. The loop starts at line 17 process each file with the subprocess.call command. In each loop the folder of the input file is found and the output file will be stored in the same directory but with different naming convention which is explaned by the comments in the code. Each , in the subprocess.call command in the python is correspondant to an empty space in the terminal. As an example the correspondant shell command is as below:

Build Essentials

• ffmpeg -i file_path -filter:v transpose=-1 -vcodec nvenc -preset slow -b:v 5M -acodec copy output_file_path

### II - FFmpeg Encoder

The command executed by FFmpeg needs to be described. Each of the elements started by - are calling specific operations and the command follows by them execute the desired operation. For example -vcodec indicator will specify the codec to be used by FFmpeg and nvenc which follows by that point to the codec. More details can be found at FFmpeg Filters Documentation. The fllowing Table, summarize the indicators:

# Table 1

Attribute Description Option Description
-i input argument file_path path to the input file
-vf alias for -filter:v transpose=1 clockwise rotation by 90
-vcodec Set the video codec nvenc Nvidia Gpu accelerated library
-preset increase the quality slow improves quality
-b:v set the video bitrate 5M Set to 5M
-acodec set the audio codec copy only copied & no encoding
The -vf is the main command which its full documentation is available at here and it has the filter options.

## Code Execution

In order to run the python file we go to the terminal and execute the following:

Build Essentials

• python /absolute/path/to/python/file
As a consideration, if we are working on any specific virtual environment it has to be activated at first.

## Summary

This tutorial demonstrated how to process a video and specifictly rotating that using FFmpeg and Nvidia GPU accelerated library called NVENC. The advantage of using python interface is to easily parse the .txt file and looping through all files. Moreover it enables the user with options which are more complex to be directly employed in the terminal environment.
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