# Fast forward videos with ffmpeg

> Learn how to speed up your videos effortlessly using ffmpeg.

January 18, 2023 · 9 min read · https://yasint.dev/fast-forward-videos-with-ffmpeg/
Tags: ffmpeg, tools

---

The `ffmpeg` command line tool is a hyper-fast video and audio converter that can also grab from a live audio/video source. You can use it to convert arbitrary sample audio/video rates and resize/trim videos on the fly with a high-quality _polyphase_ filter.

> **Installing ffmpeg**
>
> If you don't have `ffmpeg` installed in your system you can refer [how to install ffmpeg in your operating system](https://www.hostinger.com/tutorials/how-to-install-ffmpeg). Or if you're on macOS just do `brew install ffmpeg`[^1]

Suppose you want to fast-forward this masterpiece by `2x`:-

<video src="./hehe-peter.mp4" data-caption="Original video at normal speed" controls width="100%" />

## Fast-forward (video only)

```bash terminal lineNumsToHide="2-*"
ffmpeg -i hehe-peter.mp4 \
    -filter:v "setpts=0.5*PTS" \
    -preset fast \
    -crf 24 \
    -c:v libx264 \
    -an \
    output-without-audio-but-2x-speed.mp4
```

When you execute the above with the correct inputs, the output file will contain the **fast-forwarded video file without the audio stream**.

However, if you still want to fast-forward both audio and video simultaneously, you must use a _complex filter_. I will teach you [how we can do that](#fast-forward-video--audio) in the end. But first, let's understand what the above command does.

---

## Behind the scenes

### `-i hehe-peter.mp4`

The string `hehe-peter.mp4` followed by the `-i` flag describes location of the
source input file. Conversely, the last argument for the command is the name/target
file name for the output.

### `-filter:v "setpts=0.5*PTS"`

The **"setpts"** filter changes the "PTS" (presentation timestamps) of video frames.
This is the filter we use to change the frame rate and do _slow-motion_ or _fast-motion_
effects. The filter accepts an expression that specifies the new value of the
PTS. For example, `setpts=0.5*PTS` will double the frame rate, effectively
making the video play at `2x` speed.

### `-preset fast`

This argument specifies the speed-to-compression ratio. `ffmpeg` has a collection of options
providing certain encoding speeds such as `ultrafast`, `superfast`, `veryfast`,
`faster`, `fast`, `medium` (default), `slow`, `slower`,  and `veryslow`.

A slower preset will provide better compression (compression is quality per file size).
For example, if you target a specific file size or constant bit rate, you will achieve
better quality with a slower preset. Similarly, choosing a slower preset will save bitrate
for constant quality encoding. Use the slowest preset that you have patience for, haha.

### `-crf 24`

Constant Rate Factor configuration or `crf` is a procedure that allows the encoder to
attempt to achieve a certain output quality for the whole file while controlling the
output file size. A lower CRF value results in higher quality but larger file size.

The values for this argument will depend on which encoder you're using. For example,
I'm using the [`x264` encoder](https://trac.ffmpeg.org/wiki/Encode/H.264) like in the
above command. Hence, the possible value is an integer from `0-51`.

### `-c:v libx264`

`-c:v libx264` specifies the codec to be used for the video stream of the output file.
The "v" in "c:v" stands for video. The `libx264` is the codec name, it's a codec library
for `H.264` video compression. It tells ffmpeg to use the `libx264` codec to encode the video
stream in the output file. [Also, `-vcodec` and `-codec:v`
is an alias for `-c:v`](https://lists.ffmpeg.org/pipermail/ffmpeg-user/2017-February/035335.html).

You can see the supported codecs list by running `ffmpeg -codecs`. Or, if you want a
more fine-grained list, you can see _encoders_ or _decoders_ using `ffmpeg -encoders` or
`ffmpeg -decoders`. And see more detailed information using `ffmpeg -h encoder=libx264` or
`ffmpeg -h decoder=aac`. Similarly, you can do the same for listing the formats
supported by ffmpeg using `ffmpeg -formats`.

### `-an`

In ffmpeg, the -an option tells ffmpeg to disable the audio stream in the output file. It
stands for **"audio disable"**. Alternatively, we also can:-

- Use the `-vcodec copy` option: This option tells ffmpeg to only include
the video stream in the output file. For example,
`ffmpeg -i input.mp4 -vcodec copy output.mp4`

- Use the `-map` option: This option allows you to specify which streams
from the input file should be included in the output file. For example,
`ffmpeg -i input.mp4 -map 0:v -c:v libx264 output.mp4`. This will only include
the first video stream from the input file in the output file.

- Use the `-codec:a none` instead of the `-an`
option In contrast, `-vn` refers to video disable..

When you include one of these flags in a command, ffmpeg will not process any
audio stream from the input file, and it will not include any audio stream
in the output file.

> **Multimedia Files**
>
> Video(s) typically includes visual information, such as
    **images** and **animation**, while the audio has **sound** and **speech**. By separating the video
    and audio streams, `ffmpeg` can process and manipulate them independently, allowing for
    more flexibility and control when working with multimedia files.

    When we ran the previous command, we did not specify that we wanted audio streams
    included in the output file. We can do that via the following:-

    ```bash terminal highlight="6" obscure="1-6,7-*" lineNumsToHide="2-*"
    ffmpeg -i hehe-peter.mp4 \
        -filter:v "setpts=0.5*PTS" \
        -preset fast \
        -crf 24 \
        -c:v libx264 \
        -c:a copy \
        output.mp4
    ```

    The `-c:a` option or alias `-acodec copy` in ffmpeg specifies the codec to be used
    for the audio stream. It stands for codec:audio.

        We can use it to set the codec for the output audio file. For example, `-c:a aac`
        tells ffmpeg to use the `AAC` codec for the audio stream, while `-c:a mp3` is for
        `MP3` codec.

        `-acodec copy` or `-c:a copy` is a command line option for the `ffmpeg` tool that tells
        it to copy the audio stream from the input file to the output file without
        re-encoding. In other words, it copies the audio stream "as is" from the input to
        the output file. This can save time and preserve the original audio quality. This
        option is used when you want to remux a video file or change the container format
        without changing the audio codec or when you want to extract audio from a video
        file and save it as a separate audio file.

        If this option is not specified, ffmpeg will use the default codec for the output
        container format. However, you can avoid passing this argument if you don't
        want to copy all the audio streams.

Well, what if you want to fast-forward both audio and video streams? would appending
`-acodec copy` work? Let's see what's the outcome.

Did you see? The video is fast-forwarded but the audio isn't. And we are still
left with the original duration. Why? you may ask.

    In `ffmpeg`, video and audio are treated as separate streams because they
    are typically encoded and compressed differently and may have different characteristics,
    such as _frame rate_ and _bit rate_. This is because video and audio are two distinct
    types of media that are often combined in a single file for **playback**. So this why
    we need to take care both the _audio_ and _video_ streams separately when fast-forwarding
    or slow-downing multimedia files.

---

## Fast forward (video & audio)

```bash terminal lineNumsToHide="2-*"
ffmpeg -i hehe-peter.mp4 \
    -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" \
    -map "[v]" \
    -map "[a]" \
    -preset fast \
    -crf 24 \
    -c:v libx264 \
    output-2x-audio-and-video.mp4
```

Executing the above command with the correct input and a targeted output file
will yield the fast-forwarded video and audio.

And would you look at that! I've just ruined that masterpiece, lol.
But seriously though, I think `ffmpeg` is pretty cool because it has all these
fancy features :)

---

## Behind the scenes (cont.)

### `-filter_complex "..."`

In `ffmpeg`, the -filter-complex option specifies a complex filtergraph.
A filtergraph is a _directed acyclic graph_ of connected filters. Each filter
takes one or more inputs, processes them, and produces one or more outputs.

The above command has two filter graph nodes separated by a semicolon.
`[0:v]setpts=0.5*PTS[v]` is what allows us to fast-forward the video by
`2x` speed. In this argument, `[0:v]` tells `ffmpeg` which video stream
of the input file to use. "0" is the index of the input file, and "v"
specifies that we want to use the video stream.

> Note that we use the `atempo` filter to change [tempo](https://ffmpeg.org/ffmpeg-filters.html#atempo)
> of the audio. However, the value of it are limited to using values between `0.5` and `2.0`
> (so it can slow it down to no less than half the original speed, and speed up to no more than
> double the input). If you need to, you can get around this limitation by stringing multiple
> `atempo` filters together (i.e., `-filter:a "atempo=2.0,atempo=2.0"`).

Essentially, where `0:v` specifies the first video stream of the input file
as the input link for the filter `setpts`. And the `[v]` appended at the end
is an output link specification in filtergraphs. It tells ffmpeg to use the
output of a filter as the input for the next filter in the chain or as the
final output if it's the last filter in the chain.

> The brackets `[]` are used to specify the inputs and outputs of a filter
    in a filtergraph. The name inside the brackets (e.g. "v") is a **label or an
    identifier** that can be used to refer to that _input_ or _output_ later in the
    filtergraph.

### `-map "..."`

In `ffmpeg`, the -map `"[v]"` option specifies which streams from the input
file should be included in the output file. It tells `ffmpeg` to map the output
link set inside the brackets (in this case, `"[v]"` maps the video and `"[a]"`
maps the audio) to the output file.

We often use it with filtergraphs, where there can output of the filtergraph
is used as the input for the mapping. We can use the `-map` option multiple
times to select various streams from the input files and to specify
the order in which they will appear in the output file. Also, note
that if you want to use the entire input file, you can use `-map 0`
instead of -map `"[v]"`.

![Filtergraph](./filtergraph.png)

Essentially, where `0:v` specifies the first video stream of the input file.

---

If you need a more coarse-grained [guide on how to slow-motion or fast-forward
videos, you can refer to the official guide on ffmpeg
wiki](https://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video).
But for me, [`ffmpeg` documentation](https://ffmpeg.org/documentation.html) is the primary
and most reliable source of information on the tool's features and capabilities.

Well, that's it, folks. Thanks a bunch for reading!

[^1]: Official guide to installing [homebrew](https://brew.sh/)