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HM

HM (HEVC Test Model) is the original H.265 reference encoder, predating alternatives like x265. In the modern day, it joins other MPEG reference encoders such as JM & VTM in their reputations for being highly niche offerings that are used rarely due to their usage complexity & speed disadvantages.

HM is capable of producing higher quality streams than highly tuned x265, even at excruciatingly slow speeds. This is only a theoretical advantage, though, as HM is incapable of placing keyframes automatically with scene detection & would need a chunking tool reminiscent of Av1an to do this. For videos containing few enough frames where keyframe placement isn't a concern, HM is better in practice than x265 at the expense of a massive dropoff in speed. The encoder is entirely single-threaded (HM links OpenMP but never uses it) & is much slower than even x265 placebo.

Installation​

Build with GCC, not Clang

HM's CMake configuration turns compiler warnings into errors, & its suppression list predates current Clang versions. Modern Clang (tested with Clang 23) fails the build with -Wbitwise-instead-of-logical errors in TComDataCU.h & -Wmissing-braces errors in SEIFilmGrainSynthesizer.cpp. GCC is not affected: build with GCC, & everything compiles cleanly. (VTM, by contrast, builds fine with Clang.)

These build instructions are valid for Linux & macOS & require CMake.

git clone https://vcgit.hhi.fraunhofer.de/jvet/HM
cd HM/
mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++
make -j$(nproc)

The binary TAppEncoderStatic can be found within the bin/ directory & can be copied to /usr/local/bin for encoding. The matching decoder is TAppDecoderStatic. Decoding & other functions of the reference codec implementation aren't covered in this entry.

Usage​

Like VTM, HM is configuration-file driven: you start from one of the cfg files in the repository's cfg/ directory & override what you need on the command line.

Random access encode from a raw YUV file
TAppEncoderStatic -c cfg/encoder_randomaccess_main.cfg -i input.yuv -b output.265 -o /dev/null -wdt 1280 -hgt 720 -fr 50 -f 500 -q 27
Parameter order matters

Options are applied in the order they appear, & the last value wins. Every cfg file sets its own BitstreamFile (str.bin) & ReconFile (rec.yuv), so if -b/-o come before -c, the cfg silently overrides them & the bitstream lands in str.bin in the working directory. Always put -c first, then your overrides, & pass -o /dev/null to skip dumping the reconstructed YUV.

Make sure only to use raw planar YUV input when encoding with HM: Y4M is not supported & input cannot be piped from stdin. Use FFmpeg to dump a raw file first (ffmpeg -i in.mkv -pix_fmt yuv420p input.yuv). Each parameter does the following:

  • -i input.yuv -b output.265

Specifies the raw YUV input file & the raw HEVC bitstream output.

  • -c [path/to/config]

Specifies the desired HM configuration file. The main choices:

Config fileGOP sizeUse case
encoder_randomaccess_main.cfg16Highest efficiency, default choice
encoder_lowdelay_main.cfg8No future references
encoder_lowdelay_P_main.cfg8Low delay, P slices only
encoder_intra_main.cfg1All-intra

Variants with the same structure exist for 10-bit (*_main10.cfg) & range extensions (*_rext.cfg, covering 4:2:2/4:4:4 & higher bit depths). Lossless configs live in cfg/lossless/.

  • -wdt 1280 -hgt 720

Sets the input width & height. Unlike VTM there is no Y4M autodetection, so these are always required.

  • -fr 50 -f 500

Sets the framerate (integer only, no fractional framerates) & the number of frames to encode. In this case, we are encoding 500 frames of a video that is to be played back at 50fps.

  • -q 27

Sets the QP (0–51) for fixed-QP encoding. HM has no CRF-like mode; the only other option is single-pass bitrate targeting with RateControl=1 TargetBitrate=1000000 (bits per second).

  • -xPS 0

Enables xPSNR, a cross-component PSNR metric HM can compute alongside encoding. Leaving it disabled doesn't affect the encoded bitstream at all, only the metrics printed to the console.

Keyframe placement​

IntraPeriod controls intra period placement: -1 (the low delay default) puts a keyframe only at the very start, while the random access config uses -32, a fixed period aligned to the GOP size. There is no scene detection, so HM cannot adaptively place keyframes. For content with scene changes, either accept the fixed period or chunk the encode externally & stitch the bitstreams afterwards.

Muxing​

The output is a raw (Annex B) HEVC bitstream. Muxing with FFmpeg works:

Mux to MP4
ffmpeg -f hevc -i output.265 -c copy output.mp4

Note the explicit -f hevc; auto-detection of raw HEVC input sometimes fails. mp4box is a common alternative muxer.