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Or, you can specify the end time instead of the duration:
Or, you can specify the end time instead of the duration:
:<code>-ss START -to END</code
:<code>-ss START -to END</code>
ffmpeg has two seeking modes: slow and accurate, and fast and accurate.
ffmpeg has two seeking modes: slow and accurate, and fast and accurate.

Revision as of 07:25, 15 April 2015

This guide assumes that you have installed gentoo.

The basic command

ffmpeg -i input.mkv output.webm

Wow! Is it really that easy? Yes. Yes it is. However, this will give you a really shitty webm that everyone will mock you for. You might as well be posting gifs. In order to make good quality webms, we need to add extra options to fine-tune it.

First, let's examine the structure of an ffmpeg command:

Starts the ffmpeg program you have on your computer. Pretty straightforward.
-i input.mkv
The file you want to convert. You always need an -i in front. If the filename has spaces you need to enclose it in quotes: -i "my file.mkv"
The name of the converted webm.

Basic options

Look at your webm. It is really shitty. How are you going to make it less shitty? By increasing the quality.

For webms, quality is controlled by 2 general settings:


Bitrate is simply the amount of data that goes into each second. Higher bitrates mean higher quality, but also with a bigger filesize.

"BITRATE" can be values such as 320K, 2M, and so on.


CRF is a setting that tries to make each second meet a certain amount of "quality." Some scenes need less data to look good, so the extra data can be used for other scenes.

"NUMBER" can be a value from 4 - 63, where lower is better quality.

Place your settings after your input. In general, this is where your options should always go. For example:

ffmpeg -i input.mkv -b:v 2M -crf 10 output.webm

You should always set both a bitrate and a CRF or else it will default to something really shitty. You will want to experiment with different values before settling on something you like. Personally, I set CRF to 4 and try to set a bitrate that hits just under the filesize limit. Optimizing these two settings will make your webm pretty decent already.

2 pass encoding

You should use 2 pass encoding whenever possible. During 2 pass encoding, ffmpeg scans the file once before encoding it. This makes the encoding faster and more efficient.

You need to use two commands to tell ffmpeg to use 2 pass encoding.

ffmpeg -i input.mkv -pass 1 output.webm
ffmpeg -i input.mkv -pass 2 output.webm
Enter the second after the first has finished. Answer "yes" when ffmpeg asks to overwrite. It will generate a log file that you can delete after your webm has finished encoding.

The rule for 2 pass encodings is that all of your options must be the same except for -pass. If you don't want to keep answering "yes," add -y to your options. This will stop ffmpeg from prompting you.


-threads 0

Simple! Makes encoding faster. You should always include this in your options.

Posting on 4chan

4chan doesn't allow sound. Any webms that still have sound cannot be uploaded.


will create a webm without sound.

4chan also limits files to 3MB. To find the maximum bitrate that will keep your webm within 3MB, you can do a simple calculation.

(3MB/seconds)*8 = bitrate.

For example, let's say you have a 10 second clip you want to post.

3MB/10 seconds = .3MB/s.

Since bitrate is measured in bits(b) and not bytes (B), multiply by 8 to get the final bitrate:


However, the encoder is not always exact, so you may end up going over or under. Just adjust your bitrate and try again.


ffmpeg -i input.mkv -b:v X -crf X -an -threads 0 -pass 1 -y output.webm
ffmpeg -i input.mkv -b:v X -crf X -an -threads 0 -pass 2 -y output.webm

Simply insert your filenames and choose a bitrate with a crf, and you will have a good quality webm to post.

Additional quality options

Combining bitrate, crf, and 2 pass encoding will already take you 90% of the way to the best you can get. Read on to squeeze out the extra 10%.



-auto-alt-ref 1

-quality good

-cpu-used 0


-bufsize 99M

-lag-in-frames 25

-arnr-maxframes 25

-arnr-strength 6

Ffmpeg Tools

In addition to encoding, ffmpeg has numerous other capabilities such as cutting, resizing, hardsubbing, and cropping.


To cut a clip from a long video, all you need to do is input the time the clip begins and the length.


Or, you can specify the end time instead of the duration:

-ss START -to END

ffmpeg has two seeking modes: slow and accurate, and fast and accurate.

ffmpeg -ss ROUGH -i "INPUT.mkv" -ss EXACT -t DURATION


-vf scale=-1:HEIGHT


First, extract the sub file.

ffmpeg -i "INPUT.mkv" -an -vn -c copy "SUB.ass"

Add this to your options

-vf subtitles="SUB.ass"

Unfortunately, if you use fast seeking it fucks up. Use slow seeking only.