transcoding video with ffmpeg
Transcoding video with FFmpeg
In this chapter, we’ll be going to see how to transcode a video file into the two most common formats!
Converting to H.264
H264 is one of the most popular video codecs. Most devices, browsers and video players understand how to play it. It is efficient in storing video content, but as with most advanced video codecs, it is a resource intensive-process to encode and decode.
A complete command line for a high-quality H.264 transcoding with high-quality AAC audio is the following:
ffmpeg -y -i bbb_sunflower_1080p_60fps_normal.mp4 \
-c:v libx264 -preset slow -crf 22 \
-profile:v main -g 250 -pix_fmt yuv420p \
-map 0:0 -map 0:1 \
-acodec aac -ar 44100 -b:a 320k bbb_transcoded_h264_HQ.movMake sure to understand this command and to customize it to match your needs.
To help you do that, let’s dissect this command!
Global options:
- -y: Overwrite the output.
Input options:
- -i bbb_sunflower_1080p_60fps_normal.mp4: The input file.
Output options:
-c:v libx264: Set the codec to libx264.
-preset slow: libx264 has a lot of variables that you can be tune, and most of them balance the coding speed and the resulting file size. To make your life easier, there are presets by which you can easily declare what you need: small size or speed.
-crf 22: This is the constant rate factor, the main option for setting image quality. It is a number between 0-51, where 0 is lossless, and 51 is the worst quality. Generally, you want something between 17 and 28. This is the option to tune the balance between image quality and file size. Check my comparison video here.
-profile:v main -g 250 -pix_fmt yuv420p: These are advanced options, guaranteeing you a quite backward compatible result. (See this, this, and this.)
-map 0:0 -map 0:1: You might not need this: these options are selecting the correct video and audio streams. In our case, we have two audio streams, and we need the stereo one to avoid some issues with our aac stream.
-acodec aac: Select the AAC (Advanced Audio Coding) codec for the audio in the output. We need to be more specific than just -f for the format. We need to specify the audio codec here manually.
-ar 44100: Set the audio sampling rate (learn more about that in previous chapters of this article).
-b:a 320k: Set the audio bitrate (learn more about that in previous chapters of this article).
30seconds_of_bb.mkv: The output file name. All the options since the last -i (or the last output file) considered to be a modifier for this output.
Let’s see the output:
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'bbb_sunflower_1080p_60fps_normal.mp4':
[...]
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (mp3 (mp3float) -> aac (native))
[...]
Output #0, mov, to 'bbb_transcoded_h264_HQ.mov':
Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 60 fps, 15360 tbn, 60 tbc (default)
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, 5.1(side), fltp, 320 kb/s (default)
[...]
frame=38074 fps= 35 q=-1.0 Lsize= 324855kB time=00:10:34.51 bitrate=4194.1kbits/s dup=2 drop=0 speed=0.58x From this, we understand that FFmpeg chose the mp3 stream from the input file because we told it to do so. (Remember, it has two audio streams in it, a stereo mp3 and a 5.1 ac3.) We also see that my machine could transcode with 35fps (0.58 times the playback speed), and our settings resulted in an average video bitrate of 4200 kbit/s.
The video bitrate is an interesting question in this mode. With the CRF option, we specify the “constant visual quality” we want. To reach a constant visual quality, the encoder works hard to guess how much it can compress certain parts of every frame, and the result of that guess defines the final average video bitrate.
If you want even better results with H.264, and you can afford a bit more processing time and a bit more complicated process, check out the 2-pass encoding instead of the constant rate factor method introduced above.
To learn more about these two different rate control methods, read the awesome Understanding Rate Control Modes article. And to learn more about the intricacies of H.264 encoding, check out the H264 encoding guide.
Finally, later on, I will show you a comparison video that shows how different CRF values perform!
Converting to H.265
H.265 is the successor of H.264, according to the official FFmpeg manual, it offers 25-50% bitrate savings while retaining the same visual quality.
A complete command line for a high-quality H.265 transcoding with high-quality AAC audio is the following:
ffmpeg -y -i bbb_sunflower_1080p_60fps_normal.mp4 \
-c:v libx265 -preset slow -crf 27 \
-profile:v main -g 250 -pix_fmt yuv420p \
-map 0:0 -map 0:1 \
-acodec aac -ar 44100 -b:a 320k bbb_transcoded_h265_HQ.movAnd the result is:
...
encoded 38074 frames in 3384.84s (11.25 fps), 1720.32 kb/s, Avg QP:35.29H.265 also has multiple rate control algorithms, I used the CRF method here. If you want to use a different rate control algorithm, then you may check out the H.265 encoding guide. Also, check out the next section, where I’ll reveal how different CRF values perform!
This command is almost the same as what we used in the H.264 example above, so please refer to that section to understand the arguments.
If we compare H.264 and H.265 with our commands above, taking into account this 10-minute long video on my system, these are the results:
- H.264 is 3 times faster (35 fps vs 11 fps)
- H.264 produces a 2 times larger file (318 mb vs 156 mb)
Comparing CRF values with H.264 and H.265
I have created a video for your convenience, that shows the different crf values in action. The selected frame had some movement on it with the leaves in the bunny’s hand. Movement is important with video codecs, as usually that’s where quality losses are first visible.
This video shows how the different CRF values perform, from 0-51 with the H.264 and H.265 formats!
H.264 & H.265 CRF comparison video
(Can you guess which program I was using to make this?:))