VQM will compare the VMAF, SSIM and PSNR achieved with different values of the specified encoder parameter.
- What does VQM produce?
- Requirements
- Usage
- Overview Mode
- Combination Mode
- Available Arguments
- FFmpeg Builds
- About the model files
VQM produces a table to show the metrics, and graphs that show the per-frame VMAF, SSIM and PSNR.
The table is written to a file named metrics_table.txt and it contains the following for each value of the specified encoder parameter:
- Parameter value
- Time taken to transcode the video (seconds)
- Filesize (MB)
- Bitrate (Mbps)
- Video Multimethod Assessment Fusion (VMAF)
- Structural Similarity Index (SSIM)
- Peak Signal-to-Noise-Ratio (PSNR)
Here's an example:
VMAF/PSNR/SSIM values are in the format: Min | Standard Deviation | Mean
+-----------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
| preset | Encoding Time (s) | Size | Bitrate | VMAF | PSNR | SSIM |
+-----------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
| veryslow | 20.800 | 11.670 MB | 1.554 Mbps | 89.514 | 2.488 | 97.371 | 40.715 | 4.063 | 47.691 | 0.992 | 0.001 | 0.997 |
| slower | 12.712 | 12.531 MB | 1.669 Mbps | 88.464 | 2.463 | 97.401 | 40.644 | 4.041 | 47.844 | 0.993 | 0.001 | 0.998 |
| slow | 5.536 | 12.409 MB | 1.653 Mbps | 89.348 | 2.506 | 97.342 | 40.871 | 4.041 | 47.800 | 0.993 | 0.001 | 0.998 |
| medium | 4.109 | 12.817 MB | 1.707 Mbps | 89.529 | 2.414 | 97.490 | 40.927 | 4.023 | 47.958 | 0.993 | 0.001 | 0.998 |
| fast | 3.535 | 13.286 MB | 1.769 Mbps | 89.882 | 2.465 | 97.460 | 40.954 | 3.983 | 48.016 | 0.994 | 0.001 | 0.998 |
| faster | 2.620 | 13.141 MB | 1.750 Mbps | 88.592 | 2.603 | 97.257 | 40.877 | 3.989 | 48.006 | 0.994 | 0.001 | 0.998 |
| veryfast | 1.987 | 11.761 MB | 1.566 Mbps | 83.313 | 3.698 | 95.400 | 39.779 | 4.071 | 46.612 | 0.992 | 0.002 | 0.997 |
| superfast | 1.465 | 16.621 MB | 2.213 Mbps | 86.385 | 2.687 | 96.859 | 40.207 | 4.335 | 47.502 | 0.992 | 0.001 | 0.997 |
| ultrafast | 0.774 | 24.623 MB | 3.279 Mbps | 88.579 | 2.067 | 97.982 | 40.093 | 4.811 | 46.288 | 0.989 | 0.002 | 0.996 |
+-----------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
Original File: ForBiggerFun.mp4
Original Bitrate: 1.720 Mbps
VQM transcoded the file with the libx264 encoder
Encoder options: None
libvmaf n_subsample: 1
Command used: python main.py -i ForBiggerFun.mp4 -e libx264 -p preset -v veryslow slower slow medium fast faster veryfast superfast ultrafast
In addition to the table, two types of graphs are created:
- A graph (type 1) for each encoder parameter value, showing the per-frame VMAF, SSIM and PSNR.
- A graph (type 2) where the average VMAF is plotted against the value of the encoder parameter.
Here's an example of graph type 1:
This particular graph shows the per-frame VMAF score. An example of the per-frame SSIM graph and per-frame PSNR graph can be found in the example_graphs folder.
Here's an example of graph type 2 if you opt to compare CRF values:
You must specify an encoder (using the -e argument), an FFmpeg encoder parameter (using the -p argument, e.g. -p preset, -p crf or -p quality) and the values you want to compare (using the -v argument).
If you would like to test VQM without using your own video(s), you can use ForBiggerFun.mp4.
Examples:
python main.py -i ForBiggerFun.mp4 -e libx264 -p preset -v veryfast superfast ultrafast
python main.py -i ForBiggerFun.mp4 -e libx264 -p crf -v 22 23 24
python main.py -i ForBiggerFun.mp4 -e h264_amf -p quality -v balanced speed quality
Overview Mode can be activated by specifying the --interval and --clip-length arguments. The benefit of this mode is especially apparent with long videos, such as movies. What this mode does is create an overview video by grabbing a <clip length> seconds long segment every <interval> seconds from the original video. As the overview video can be much shorter than the original, the process of transcoding and calculating the quality metrics is quicker.
Example: python main.py -i ForBiggerFun.mp4 -p crf -v 17 18 19 --interval 5 --clip-length 2
In the example above, an overview video is created by concatenating 2-second-long* clips (--clip-length 2) every 5s* (--interval 5) from the original video. In this example, a one-minute-long video is turned into an overview video that is 20 seconds long. Transcoding and calculating the quality metrics of a video that is 20 seconds long is quicker than doing so with a one-minute-long video.
Sections marked with an asterisk will not be exact as FFmpeg will use the closest I-frames.
An alternative method of reducing the execution time of this program is by only using the first x seconds of the original video (you can do this with the -t argument), but Overview Mode provides a better representation of the whole video.
Instead of comparing the quality achieved with various values of one encoder parameter, Combination Mode allows you to compare the quality achieved with a combination of two or more parameters.
To activate Combination Mode, specify the -c or --combinations argument, followed by a list of combinations you wish to compare. The list of combinations must be surrounded in quotes, and each combination must be separated by a comma.
For example, if you want to compare the quality achieved with:
- The combination of preset
veryslowand a CRF value of18 - The combination of preset
slowerand CRF value of16
You would run something like:
python main.py -i "ForBiggerFun.mp4" -e libx265 -c "preset slow crf 24,preset medium crf 23"
The table produced will look something like this:
VMAF/PSNR/SSIM values are in the format: Min | Standard Deviation | Mean
+------------------------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
| Combination | Encoding Time (s) | Size | Bitrate | VMAF | PSNR | SSIM |
+------------------------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
| -preset slow -crf 24 | 32.700 | 9.768 MB | 1.301 Mbps | 90.631 | 2.206 | 97.704 | 41.371 | 4.015 | 47.687 | 0.991 | 0.001 | 0.997 |
| -preset medium -crf 23 | 12.680 | 10.018 MB | 1.334 Mbps | 89.330 | 2.635 | 97.069 | 41.001 | 4.017 | 47.380 | 0.991 | 0.002 | 0.997 |
+------------------------+-------------------+-----------+------------+-------------------------+-------------------------+-----------------------+
A graph will also be produced, comparing each combination:
- Combination Mode can be used alongside Overview Mode.
- You need to decide whether you want to use the regular mode, which compares the quality metrics achieved with various values of one particular encoder parameter (using the
-pand-varguments), OR Combination Mode. You cannot do both.
You can see a list of the available arguments with python main.py -h:
usage: main.py [-h] [--disable-psnr] [--disable-ssim] [-dp DECIMAL_PLACES] -i INPUT_VIDEO [-t TRANSCODE_LENGTH] [-o OUTPUT_FOLDER] [-vf VIDEO_FILTERS] [--av1-cpu-used <1-8>] [-e ENCODER] [-eo ENCODER_OPTIONS] [-p PARAMETER] [-v VALUES [VALUES ...]] [-c COMBINATIONS]
[-cl <1-60>] [--interval <1-600>] [-n <x>] [--n-threads N_THREADS] [--phone-model]
options:
-h, --help show this help message and exit
General Arguments:
--disable-psnr Disable PSNR calculation.
--disable-ssim Disable SSIM calculation.
-dp DECIMAL_PLACES, --decimal-places DECIMAL_PLACES
The number of decimal places to use for the data in the table
-i INPUT_VIDEO, --input-video INPUT_VIDEO
Input video. Can be a relative or absolute path, or a URL.
If the path contains a space, it must be surrounded in double quotes.
-t TRANSCODE_LENGTH, --transcode-length TRANSCODE_LENGTH
Create a lossless version of the original video that is just the first x seconds of the video.
This cut version of the original video is what will be transcoded and used as the reference video.
You cannot use this option in conjunction with the --interval or -cl argument.
-o OUTPUT_FOLDER, --output-folder OUTPUT_FOLDER
Use this argument if you want a specific name for the output folder. If you want the name of the output folder to contain a space, the string must be surrounded in double quotes
-vf VIDEO_FILTERS, --video-filters VIDEO_FILTERS
Apply video filter(s) to the original video before calculating quality metrics. Each filter must be separated by a comma.
Example: -vf bwdif=mode=0,crop=1920:800:0:140
Encoder Arguments:
--av1-cpu-used <1-8> Only applicable if the libaom-av1 (AV1) encoder is chosen. Set the quality/encoding speed tradeoff.
Lower values mean slower encoding but better quality, and vice-versa
-e ENCODER, --encoder ENCODER
Specify an ffmpeg video encoder.
Examples: libx265, h264_amf, libaom-av1
-eo ENCODER_OPTIONS, --encoder-options ENCODER_OPTIONS
Set general encoder options to use for all transcodes.
Use FFmpeg syntax. Must be surronded in quotes. Example:
--encoder-options='-crf 18 -x264-params keyint=123:min-keyint=20'
-p PARAMETER, --parameter PARAMETER
The encoder parameter to compare, e.g. preset, crf, quality.
Example: -p preset
-v VALUES [VALUES ...], --values VALUES [VALUES ...]
The values of the specified encoder parameter to compare. Must be used alongside the -p option. Examples:
Compare presets: -p preset -v slow fast
Compare CRF values: -p crf -v 22 23
Compare h264_amf quality levels: -p quality -v balanced speed
-c COMBINATIONS, --combinations COMBINATIONS
Use this mode if you want to compare the quality achieved with a combination of two or more parameters.
The list of combinations must be surrounded in quotes, and each combination must be separated by a comma.
For example, if you want to compare the combination of preset veryslow and CRF 18, with the combination of preset slower and CRF 16:
-c 'preset veryslow crf 18,preset slower crf 16'
Overview Mode Arguments:
-cl <1-60>, --clip-length <1-60>
When using Overview Mode, a X seconds long segment is taken from the original video every --interval seconds and these segments are concatenated to create the overview video.
Specify a value for X (in the range 1-60)
--interval <1-600> To activate Overview Mode, this argument must be specified.
Overview Mode creates a lossless overview video by grabbing a --clip-length long segment every X seconds from the original video.
Specify a value for X (in the range 1-600)
VMAF Arguments:
-n <x>, --n-subsample <x>
Set a value for libvmaf's n_subsample option if you only want the VMAF/PSNR to be calculated for every nth frame.
Without this argument, VMAF/PSNR scores will be calculated for every frame.
--n-threads N_THREADS
Specify the number of threads to use when calculating VMAF
--phone-model Enable VMAF phone model
- Python 3.7+ installed and in your PATH.
pip install -r requirements.txt --upgrade- FFmpeg and FFprobe installed and in your PATH (or in the same directory as this program). Your build of FFmpeg must support v2.1.1 (or above) of the
libvmaffilter and the encoders you wish you test.
You can check what your build of FFmpeg supports by running ffmpeg -buildconf -hide_banner in the terminal. If --enable-libvmaf is not printed, your build of FFmpeg is not sufficient as VQM needs the libvmaf filter.
FFmpeg builds that support the libvmaf filter can be found in the FFmpeg Builds section.
For convenience, below are links to FFmpeg builds that support the libvmaf filter.
Windows: https://www.gyan.dev/ffmpeg/builds/ffmpeg-git-essentials.7z
macOS: https://evermeet.cx/ffmpeg - download both ffmpeg and ffprobe and add the binaries to your PATH.
Alternatively, you can install FFmpeg using Homebrew - brew install ffmpeg
Linux (kernels 3.2.0+): https://johnvansickle.com/ffmpeg.
Download the git master build. Installation instructions, as well as how to add FFmpeg and FFprobe to your PATH, can be found here.
Two model files are provided, vmaf_v0.6.1.json and vmaf_4k_v0.6.1.json. There is also the phone model that can be enabled by using the -pm argument.
This program uses the vmaf_v0.6.1.json model file by default, which is "based on the assumption that the viewers sit in front of a 1080p display in a living room-like environment with the viewing distance of 3x the screen height (3H)."
The phone model was created because the original model "did not accurately reflect how a viewer perceives quality on a phone. In particular, due to smaller screen size and longer viewing distance relative to the screen height (>3H), viewers perceive high-quality videos with smaller noticeable differences. For example, on a mobile phone, there is less distinction between 720p and 1080p videos compared to other devices. With this in mind, we trained and released a VMAF phone model."
The 4K model (vmaf_4k_v0.6.1.json) "predicts the subjective quality of video displayed on a 4K TV and viewed from a distance of 1.5H. A viewing distance of 1.5H is the maximum distance for the average viewer to appreciate the sharpness of 4K content. The 4K model is similar to the default model in the sense that both models capture quality at the critical angular frequency of 1/60 degree/pixel. However, the 4K model assumes a wider viewing angle, which affects the foveal vs peripheral vision that the subject uses."
The source of the quoted text, plus additional information about VMAF (such as the correct way to calculate VMAF), can be found here.
Notes:
-
If you are transcoding a video that will be viewed on a mobile phone, you can add the
-pmargument which will enable the phone model. -
If you are transcoding a video that will be viewed on a 4K display, the default model (
vmaf_v0.6.1.json) is fine if you are only interested in relative VMAF scores, i.e. the score differences between different encoder parameter values, but if you are interested in absolute scores, it may be better to use the 4K model file which predicts the subjective quality of video displayed on a 4K screen at a distance of 1.5x the height of the screen. To use the 4K model, replace the value of themodel_file_pathvariable in libvmaf.py with'vmaf_models/vmaf_4k_v0.6.1.json'.
