VR Video Bitrate Explained
Bitrate is the data rate for a particular video scene at a specific resolution. An indoor scene with a wall behind the action typically requires a lower bitrate than an outdoor scene with a lot of motion from, for example, changing shadows and rustling leaves.
Content producers can either set a constant bitrate, which will be the same throughout the scene, no matter how much bitrate is actually required for the best quality, or variable bitrate with constantly shifting data rate determined by the proportional motion change between group of frames: the smaller the change the lower bitrate required. GoPro6 output files are at 80Mbps but for indoor scenes with a constant background not all of that is required. Some outdoor scenes require more. At SLR we use constant quality with variable bitrate mode, making it interchangeable for a group of frames. With many tests we could not find constantly high bitrate, up to GoPro originals, making any beneficial effect on perceived video quality. It just takes up extra bandwidth and space on your hard drive. Using a high constant bitrate is the easy way to do things, but if the encoding is done correctly a smaller filesize can produce even better quality.
According to professionals from Endcoding.com: “Variable bitrate (VBR) encoding will produce better image quality than constant bitrate (CBR) in most applications. The primary benefit of VBR encoding is that it allocates a higher bitrate to the more complex segments of media files and lower bitrates to the simple segments. Adding up the bitrates and dividing by the duration (in seconds) gives the average bitrate for the file. This average bitrate usually compares favorably with the equivalent CBR bitrate. Although VBR encoding requires more processing time, for most content it produces superior visual quality. Constant bitrate (CBR) encoding maintains a set bitrate over the entire video clip, but limits the image quality in most cases—especially for complex video segments. CBR is often not an optimal choice for streaming since it does not allocate enough data for the complex segments: this results in lower quality overall and unused capacity on the simple segments".
A more technical explanation can be found at https://slhck.info/video/2017/03/01/rate-control.html and https://slhck.info/video/2017/02/24/crf-guide.html:
“You may ask if constant bitrate isn’t really better quality in the end? No, the perceived quality is the same, but essentially it wastes space by compressing less in areas you really won’t notice. The VBR copy will look equal or better to the CBR version. It least compresses the parts where you see details the most, and most compresses the parts where you see details the least.”
Using the same bitrate, videos using a newer codec, such as h.265, will look substantially better than an older codec like h.264. Customers capable of playing h.265 with nVidia cards and newer Android phones are advised to specifically download h.265 files. Also, h.264 hardware decoders (via GPU or phone chipsets) are limited to a maximum 4K resolution.
For each file at SexLikeReal we provide the original file (also called the “Master” file) which is obtained from the studio using constant bitrate, and a transcoded version with the same resolution, but with a constant quality variable bitrate. Please let me know if you see any difference.
Disclaimer:
It is much easier to encode videos with constant bitrate and this is the main reason why GoPro or similar cameras with hardware encoders produce constant bitrate video siles. Additionally, it is much easier to calculate how much space you need to shoot a specific video length and so it is much easier to calculate the remaining recording time for the camera software. Constant bitrate is very good and useful in situations like this.
H.264 and h.265 specifications don't cover how to encode video, it only covers how to decode it. This means that there might be completely different encoding algorithms and all of them will work as long as they produce video files that are compatible with decoding specifications. There are several encoders on the market, some of them work better than others, and some have different encoding options depending on what you need - fast encoding or good quality. There is a general rule - the more effort you put into encoding, the better the quality for the same bitrate/filesize. The same video can be encoded with different encoder settings to produce the same bitrate/file sizes but you will certainly see a difference in quality. Using a high constant bitrate is the easy way to do things, but if the encoding is done correctly a smaller filesize can produce even better quality.
You can find VR videos fundamentals explained at https://www.sexlikereal.com/blog/post/id/70-everything-you-need-to-know-about-vr-videos
Why some SLR encodings are bigger than original file?
SLR encodings maintain original quality. If the source video is provided in a "low bitrate" by algorithms measures then it's algorithmically encoded in higher bitrate to preserve the quality.
A simple example would be as follows. Save jpeg with 30% quality and it won't look good. Save it again with 30% quality and it's going to get worse. If we save it at 100% it won't look any worse than original, but might add up to the filesize.
