Compressing videos

• Preamble
• Codecs
• Encoders
• Compressing videos
• Benchmarking codecs and their compression with vmaf
• Benchmark result interpretation
• About GPU encoding

Preamble

This article explains how to reduce the filesize of videos and some popular video codecs.
This is a simple comparison of my own experiments and methods on how to reduce filesizes.
To install the tools mentioned here:

• ffmpeg - via sudo apt-get install ffmpeg

Compressing videos

To simply compress a video you can use, for example, ffmpeg -i "input.mp4" -crf 35 output.mp4. crf means "constant rate factor" and the higher you set it the more compressed the videos get. The higher the "crf" the lower the quality and the lower the filesize. Every video codec has its own interpretation of the crf value and there is no fixed number that works for every codec.

Another example to compress further: You can change the codecs to more modern ones. Simply add -vcodec h265 -acodec aac inside the ffmpeg command to change the codecs of video/audio to h264/aac respectively. (This would make the command look like ffmpeg -i "input.mp4" -crf 25 -vcodec h265 -acodec aac output.mp4)

If the framerate of the video is 60 you could lower it to 30 to save space. Simply add -r 30 inside the ffmpeg command to limit the framerate to 30.

Some videos have an extremely high resolution (e.g. 4K) and you can save a LOT of space if you limit the resolution to a lower one. Simply add -vf "scale=1280:-1" to set the resolution of the output video to HD.
(The "-1" in the height means "auto set height to match aspect ratio of input video with 1280 width".)

With all the settings combined the command would look like:

ffmpeg -i "input.mp4" -crf 25 -vcodec h265 -acodec aac -r 30 -vf "scale=1280:-1" output.mp4

user@debian:~/Videos$ ls -ahl
-rwxrwxrwx 1 user user 341M Jan 22 16:41 'divinity2_20220122_16.mp4'
user@debian:~/Videos$ ffmpeg -i "divinity2_20220122_16.mp4" -crf 25 _divinity2_20220122_16.mp4
user@debian:~/Videos$ ls -ahl _divinity2_20220122_16.mp4
-rwxrwxrwx 1 user user 54M Feb  3 10:31 _divinity2_20220122_16.mp4

Codecs

There exist many different video codecs. The popular ones are:

• H264 (AVC)
• H265 (HEVC)
• H266 (VVC)
• VP8
• VP9
• AV1

Encoders

AV1 is a codec but to actually encode something to the format you need an encoder.

To list all AV1 encoders in ffmpeg:

$ ffmpeg -hide_banner -encoders | grep -i av1
V....D libaom-av1           libaom AV1 (codec av1)
V....D librav1e             librav1e AV1 (codec av1)
V..... libsvtav1            SVT-AV1(Scalable Video Technology for AV1) encoder (codec av1)
V....D av1_d3d12va          D3D12VA av1 encoder (codec av1)
V....D av1_nvenc            NVIDIA NVENC av1 encoder (codec av1)
V..... av1_qsv              AV1 (Intel Quick Sync Video acceleration) (codec av1)
V....D av1_amf              AMD AMF AV1 encoder (codec av1)
V....D av1_mf               AV1 via MediaFoundation (codec av1)
V....D av1_vaapi            AV1 (VAAPI) (codec av1)
V....D av1_vulkan           AV1 (Vulkan) (codec av1)
A....D wmav1                Windows Media Audio 1

Benchmarking codecs and their compression with vmaf

VMAF (Video Multi-Method Assessment Fusion) is a library to objectively compare video quality between 2 videos.
It can be used to compare different video codecs with each other in regards to "filesize-to-quality" ratio.
ffmpeg supports vmaf out-of-the-box and will be used for the comparisons below.
An example usage of libvmaf:

ffmpeg -i distorted.mpg -i reference.mpg -lavfi libvmaf=log_path=output.xml -f null -

12s tiktok dance

• The input was already x265 with high compression from tiktok, so the re-encoding without flags makes the input file much bigger

1min music video

• x265 has the lowest filesize but visibly the lowest score, artifacts are worse than the slighty higher quality av1
• av1 has a pretty high score (59%) with the highest crf and only 3% of the filesize of the input

22min anime episode

• For anime, libx265 has a clear advantage with only 28% filesize compared to the input but only 2% quality loss compared to other codecs.
• The h264 animation tune is good for lowering filesize but still looses compared to av1
• The av1 flags for anime (the last entry) do not help much in reducing the filesize

Benchmark result interpretation

AV1 was consistently fast in encoding, low in filesize and high in quality compared to the original.
x265 has a very high compression, reaching the smallest filesizes in every comparison but also having a lower quality compared to the other codecs.
Denoising with atadenoise or hqdn3d helps with lowering filesize but also reduces quality.

I, personally, would recommend:

• If you need high availability and compatibility: x264
• If you want a good allrounder: AV1
• If you need very high compression or convert animation: x265

About GPU encoding

Modern GPUs support the option to encode and decode video streams directly on the GPU.
This is way faster than using the CPU, but also very limited in regards to compression.

Ffmpeg supports "libx265" for HEVC encoding via CPU and, for nvidia, the "hevc_nvenc" codec for GPU encoding.
Comparing these 2 is difficult, because hevc_nvenc does not support the crf flag.
The input file is 1.8MB in size, with libx265 it goes down to 1.1MB (-0.7MB) but with hevc_nvenc it goes up to 2.8MB (+1MB).
It is expected to have a lower filesize with the HEVC codec becaues it is more optimized for lower filesizes. GPU encoding prioritizes speed over filesize.

(OLD) video codec comparison

(This has been written and tested years ago, with ffmpeg v7. It is kept for historical purposes)

I am not an expert with video codecs.
I simply tested a 1 minute long clip with different encodings and input parameters. The table below shows my findings in "time taken for encoding" and "resulting filesize".

The input file is a 1 minute long clip from the "YuruCamp" movie (to be precise its from the 2nd to the 3rd minute of the movie).
The input video is h264 1920x1080 23.98fps and the audio is aac 128kb/s 48000Hz stereo.
The video has a few scenes with barely any movement and near the end firework explosions, which should make it a good clip to compare multiple codecs.

Compared codecs:

• AV1
• AVC/H264
• HEVC/H265
• VP8
• VP9

• AV1 looks very promising as it takes less time than HEVC but has a bit larger filesize (but still smaller than h264)
• H265 is not supported by firefox or the windows explorer (for preview image in file explorer)
• H264 with full constant-rate-factor looks unwatchable, while av1 looks very watchable while having a lower filesize too
• Lowering the resolution with av1 on full compression doesn't help much to lower the filesize (1.9MB -> 1.2MB if you lower from FullHD -> HD)
• The other av1 codecs (except libsvtav1) are really slow
• "Preset slow" doesn't help much if you compare the extra time taken to the lower filesize. It's better to just take a slower codec without a slow preset (e.g. h264 preset slow -> h265 default)
• The "tune"'s for the h264 codec help a lot for barely any extra time taken