|Version 8 (modified by slhck, 2 weeks ago) (diff)|
FFmpeg and H.265 Encoding Guide
H.265 (also known as HEVC) offers 50-75% more compression efficiency compared to H.264 video, while retaining the same visual quality. ffmpeg has support for HEVC encoding using the x265 encoder. Note: libx265 is under heavy development. The API may change.
Getting ffmpeg with libx265 support
In order to obtain a copy of ffmpeg with libx265 support, you need to build it yourself, adding the --enable-libx265 configuration flag, with x265 being installed on your system.
You can also download a static build, all of which bundle libx265.
Similar to x264, the x265 encoder has multiple rate control algorithms, including:
- 1-pass constant bitrate (by setting -b:v)
- 2-pass constant bitrate (see H.264#twopass)
- Constant Rate Factor (CRF)
In this guide we are going to focus on CRF and Two-Pass encoding.
Constant Rate Factor (CRF)
Use this mode if you want to retain good visual quality and don't care about the exact bitrate or filesize of the encoded file. The mode works exactly the same as in x264, so please read the H.264 guide for more info.
In this example, we will use the following settings:
- default CRF of 28. The CRF of 28 should visually correspond to libx264 video at CRF 23, but result in about half the file size.
- medium preset. The preset determines how fast the encoding process will be – at the expense of compression efficiency. Put differently, if you choose ultrafast, the encoding process is going to run fast, but the file size will be larger when compared to medium. The visual quality will be the same. Valid presets are ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow and placebo.
- AAC audio at 128 kBit/s. This uses the ffmpeg-internal encoder, but under AAC you will find info about more options.
ffmpeg -i input -c:v libx265 -preset medium -crf 28 -c:a aac -b:a 128k output.mp4
This method is generally used if you are targeting a specific output file size and output quality from frame to frame is of less importance. This is best explained with an example. Your video is 10 minutes (600 seconds) long and an output of 200 MiB is desired. Since bitrate = file size / duration:
(200 MiB * 8192 [converts MiB to kBit]) / 600 seconds = ~2730 kBit/s total bitrate 2730 - 128 kBit/s (desired audio bitrate) = 2602 kBit/s video bitrate
You can also forgo the bitrate calculation if you already know what final (average) bitrate you need.
For libx265, the -pass option (that you would use for libx264) is not applicable. Instead, use the private pass option with -x265-params (see below for more details):
ffmpeg -y -i input -c:v libx265 -preset medium -b:v 2600k -x265-params pass=1 -c:a aac -b:a 128k -f mp4 /dev/null && \ ffmpeg -i input -c:v libx265 -preset medium -b:v 2600k -x265-params pass=2 -c:a aac -b:a 128k output.mp4
As with CRF, choose the slowest preset you can tolerate.
In pass 1 specify a output format with -f that matches the output format in pass 2. Also in pass 1, specify the audio codec used in pass 2; in many cases -an in pass 1 will not work.
Generally, options are passed to x265 with the -x265-params argument. For fine-tuning the encoding process, you can therefore pass any option that is listed in the x265 documentation. Keep in mind that fine-tuning any of the options is generally not necessary, unless you absolutely know what you need to change.