In this FFmpeg tutorial, we learn to change the resolution of a video (or resize/scale a video) using FFmpeg’s commandline tool.
Changing a video’s resolution (also known as resizing or scaling) is a very common operation in video editing, processing, and compression. This is particularly true for ABR video streaming where a single video is taken as the source and compressed to several different bitrate-resolution combinations. For example, the input video’s resolution could be
1920x1080
and ABR bitstreams could be
1280x720
,
640x480
, etc.
So, as the very first step, let’s find out what the input video’s resolution is. Using the
ffprobe
tool that’s shipped with the FFmpeg builds, let’s determine the resolution of an input video. Here’s the command line using
ffprobe
. If you don’t have access to
ffprobe
, you can download it from OTTVerse’s
FFmpeg build page
.
The output of this command should be something like this
1920x1080
displayed on your console. That’s great – you now know the video’s resolution and can scale or change the resolution now.
Note:
You must note that the act of up/down scaling is a lossy process and will result in some loss of video quality.
Table of Contents
How do you Resize/Scale/Change the Resolution of a Video using FFmpeg?
Using FFmpeg to scale or change the resolution of a video is done by the
scale
filter in FFmpeg. To use the
scale
filter, use the following command –
where,
$w
and
$h
denote the required width and height of the destination video. For example, you could use
-vf scale=640:480
to resize your video to
480p
.
That’s it! With this simple command, you can change the video’s resolution with FFmpeg.
And, after FFmpeg changes the resolution of the video, it will re-encode it at that resolution. In the command line above, you can supply encoding parameters to FFmpeg and encode the scaled video using those parameters. For example, you could tell FFmpeg to encode it using
crf=18
for pretty high-quality H.264/AVC encoding, or choose something else!
All good? Okay, let’s tackle the next subject which is changing a video’s resolution but retaining/keepings it’s aspect ratio.
How to Resize Video While Keeping the Quality High with FFmpeg
After resizing, you might notice that the quality of the output video is pretty bad or not what you expected. This can be easily fixed by telling FFmpeg the video encoding parameters that you would like to use after the resizing process.
Here, you are telling FFmpeg to scale the video to
720p
and then encode it using
crf=18
with
libx264
‘s
slow
preset that usually provides very good quality due to the number of coding tools that it turns on.
How to Change the Video’s Resolution but Keep the Aspect Ratio?
The aspect ratio of an image is very well defined in Wikipedia as follows:
The aspect ratio of an image is the ratio of its width to its height. It is commonly expressed as two numbers separated by a colon, as in
16:9
. For an
x:y
aspect ratio, the image is x units wide and y units high.
It is very common to run into this problem while working with videos:
How do I change a video’s resolution (or scaling a video) but keeping or retaining the video’s original aspect ratio.
In
FFmpeg
, if you want to scale a video while retaining its aspect ratio, you need to set either one of the
height
or
width
parameter and set the other parameter to
-1
. That is if you set the
height
, then set the width to
-1
and vice-versa.
To demonstrate, assume the following commands take a HD video (
1920x1080
) as its input. And, let’s assume that we want to change its resolution. This can be done in two ways as discussed above, so let’s try both ways.
1. Specify the Width To Retain the Aspect Ratio
ffmpeg -i input.mp4 -vf scale=320:-1 output.mp4
The resulting video will have a resolution of
320x180
. This is because
1920 / 320
=
6
. Thus, the
height
is scaled to
1080 / 6 = 180
pixels.
2. Specify the Height To Retain the Aspect Ratio
ffmpeg -i input.mp4 -vf scale=-1:720 output.mp4
The resulting video will have a resolution of
1280x720
. This is because
1080 / 720
=
1.5
. Thus, the
width
is scaled to
1920 / 1.5 = 1280
pixels.
Use Variables to Scale/Change Resolution of a Video in FFmpeg
We can implement the same scaling commands using variables that denote the video parameters. The input video’s
width
and
height
are denoted by
iw
and
ih
respectively.
Let’s see what a command to scale the video’s width two times (
2x
) looks like.
ffmpeg -i input.mp4 -vf scale=iw*2:ih output.mp4
If you want to divide either the height or width by a number, the syntax changes a little as the
scale=iw/2:ih/2
argument need to be enclosed within double quotes.
Avoid Upscaling a Video based on the Input Video’s Dimensions
As we mentioned right at the start of the article, every up/down scaling action will usually not produce the same level of video quality as the input video. There is bound to be a few compression losses during the scaling process. If the input resolution is too low,
FFmpeg
offers a neat trick to prevent upscaling.
In the command line above, the minimum width/height to perform scaling is set to
320
and
240
pixels respectively. This is a very simple way to guard against poor quality scaling.
If you enjoyed this post, do check out the rest of
OTTVerse’s FFmpeg tutorials
to learn more about how to use FFmpeg to improve your video editing, compression, and processing skills!
Akshay is a student at the NYU Tandon School of Engineering, completing his M.S. in Computer Engineering. His area of interest includes software system design, machine learning, and cybersecurity.
Akshay also serves as a Contributing Author on OTTVerse.com
” ffmpeg -i input.mp4 -bf scale=iw*2:ih output.mp4 ” & ” ffmpeg -i input.mp4 -bf “scale=iw/2:ih/2” output.mp4 “,
Perhaps you meant to use -vf.
I did it as author recommended in y=this article BUT I got a strange effect. I lost a contrast in every decoded video. Not too significant but noticiable. Moreover I got the same effect in H264 & H265. I tried to resize 1280×720 into 858×452 and I tried different crf value (and keep default) and include/use default -preset parameter.
By the way if you try to join video using traditional joiners you can get in joined mp4 some segments are not visible. I used this trick to solve this problem
