VDK offers a number of trained VMAF models to be used in different scenarios. Besides the default VMAF model which predicts the quality of a video displayed on a HDTV in a living-room viewing condition, VDK also includes a number of additional models, covering mobile phone and 4KTV viewing conditions.
The default VMAF model (model/vmaf_v0.6.1.pkl) is trained to predict the quality of videos displayed on a 1080p HDTV in a living-room-like environment. All the subjective data were collected in such a way that the distorted videos (with native resolutions of 1080p, 720p, 480p etc.) get rescaled to 1080 resolution and shown on the 1080p display with a viewing distance of three times the screen height (3H). Note that 3H is the critical distance for a viewer to appreciate 1080p resolution sharpness (see recommendation).
This model is trained using subjective data collected in a lab experiment, based on the absolute categorical rating (ACR) methodology, with the exception that after viewing a video sequence, a subject votes on a continuous scale (from "bad" to "excellent", with evenly spaced markers of "poor", "fair" and "good" in between), instead of the more conventional five-level discrete scale. The test content are video clips selected from the Netflix catalog, each 10 seconds long. For each clip, a combination of 6 resolutions and 3 encoding parameters are used to generate the processed video sequences, resulting 18 impairment conditions for testing.
The raw subjective scores collected are then cleaned up using the MLE methodology, as described in SUREAL. The aggregate subjective scores after clean-up are mapped to a score in the VMAF score, where "bad" is mapped to roughly score 20, and "excellent" to 100.
The default VMAF model (model/vmaf_v0.6.1.pkl) also offers a custom model for cellular phone screen viewing. This model can be invoked by adding --phone-model option in the commands run_vmaf, run_vmaf_in_batch, run_testing and vmafossexec, e.g.:
./run_vmaf yuv420p 576 324 \
python/test/resource/yuv/src01_hrc00_576x324.yuv \
python/test/resource/yuv/src01_hrc01_576x324.yuv \
--phone-model
The subjective experiment uses similar video sequences as the default 1080p HDTV model, except that they were watched on a cellular phone screen (Samsung S5 with resolution 1920x1080). Instead of fixating the viewing distance, each subject is instructed to view the video at a distance he/she feels comfortable with. In the trained model, the score ranges from 0 to 100, which is linear with the subjective voting scale, where roughly "bad" is mapped to score 20, and "excellent" is mapped to score 100.
Invoking the phone model will generate VMAF scores higher than in the regular model, which is more suitable for laptop, TV, etc. viewing conditions. An example VMAF–bitrate relationship for the two models is shown below:
From the figure it can be interpreted that due to the factors of screen size and viewing distance, the same distorted video would be perceived as having a higher quality when viewed on a phone screen than on a laptop/TV screen, and when the quality score reaches its maximum (100), further increasing the encoding bitrate would not result in any perceptual improvement in quality.
As of VDK v1.3.7 (June 2018), we have added a new 4K VMAF model at model/vmaf_4k_v0.6.1.pkl, which predicts the subjective quality of video displayed on a 4KTV and viewed from the distance of 1.5 times the height of the display device (1.5H). Again, this model is trained with subjective data collected in a lab experiment, using the ACR methodology (notice that it uses the original 5-level discrete scale instead of the continuous scale). The viewing distance of 1.5H is the critical distance for a human subject to appreciate the quality of 4K content (see recommendation). More details can be found in this slide deck.
To invoke this model, specify the model path using the --model option. For example:
./run_vmaf yuv420p 3840 2160 ref_path dis_path --model model/vmaf_4k_v0.6.1.pkl