Spotlightr accepts many different video and audio formats, but provides two main outputs which we will explain herein.
After upload every video is first compressed in its original resolution
videos are encoded as libx264/H.264/MPEG-4 AVC
videos are assigned a constant rate factor bit-rate
videos maintain the original key-framing
When starting the upload users have the option of securing (HLS encoding and segmenting) or not securing their content. If they choose to not secure it, then it will result in .mp4 output. Secured video results in .m3u8 video format
HLS - Secured Video Upload
The compressed file is separated into 2 second chunks encoded and compressed at a bit-rate for its resolution:
2160 : 12 Mbps
1080 : 5 Mbps
720 : 3 Mbps
360 : 1 Mbps
Bit-rate Constant Rate Factor (CRF) is set to 19
For x264, there are various encoding methods, with the Constant Rate Factor method being the most sophisticated. It results in variable bit-rate, but overall good quality in one single pass. CRF values range from 0 to 51, but standard values are somewhere between 19 and 26, depending on your source and what quality you want. 23 is default.
This option more or less doubles the transcoding process. Think of it as sort of as ironing a shirt once more once you deemed it good enough, just to be sure all the kinks are out of it. The process is of course slower, but in most cases produces a more optimal ratio of quality to size.
MP4 - Non Secured Video Upload
When converting to mp4 additional steps are implemented in an effort to optimize streaming, moving file metadata to the front etc.
The available resolutions and bit-rates are same as for HLS, but the file is not segmented or encoded.
Audio in both cases is compressed to 192k. The compressed file is stored on the server and the original gets deleted.
If original video resolution is over 360p then the compressed file is copied to lower resolution versions without changing any other settings of the initial compression aside from bit-rate.