Re:How Video Works
by Jackypot on Tuesday December 15, 2020 @03:55AM
(#25428368)
Perhaps someone here can answer a question I've had for a while: if you stream a video which is actually a static image (think a song on YouTube, the 'video' is just the album cover) is there any way to optimise that? Or must the server stream that same image constantly as though it was a regular video?
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That's optimized based on the video codec itself where you keep frames with major changes but surrounding frames can be optimized by encoding where certain pixels move. That's essentially what the bitrate tells us, how much of the bits are new and how many are copied from a previous frame. This is an oversimplification, but with static images, you would need a low bitrate after the initial few frames because it doesn't change. And bitrate is a proxy to how much data is transmitted while streaming, it's bits per second.
So a more concise answer, the codecs used for video by YouTube have the optimization you're thinking about built in. It would send a "pixel didn't change from last frame" and not need to send all the color information for all the pixels.
YouTube does not really do variable frame rate, and it's messy for editing, but it's another optimization that is possible and could be useful for the type of video you're describing.
The article describes DASH which you would need to send the full frame initially every time segment but per segment the previously described concept still applies. I don't believe YouTube uses DASH for anything outside live streams.
I do not know the answer, but I would absolutely shocked if it was streamed as a regular video. To the best of my knowledge, there are many, many optimizations which go into video.
This is a pretty good high-level video I came across - https://www.youtube.com/watch?v=r6Rp-uo6HmI
I mean, technically, it would be "streamed as a regular video", it's just that "regular video encoding" is pretty efficient at encoding a single unchanging image nowadays.
Re:How Video Works
by zinekeller on Tuesday December 15, 2020 @05:23AM
(#25428852)
The previous answers were speculative, but I have tried to download a random audio/video that was uploaded through YouTube's media programme (directly by media companies), and the answer is that they do re-send the video frame every 5 seconds for both H.264 and VP9, and it is encoded both at a rate of 25 frames per second and the width and height are the same. Here are the file details for both formats:
H.264/AAC4:
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : [email protected]
Format settings : CABAC / 2 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 2 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 3 min 29 s
Bit rate : 375 kb/s
Width : 1 080 pixels
Height : 1 080 pixels
Display aspect ratio : 1.000
Frame rate mode : Constant
Frame rate : 25.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.013
Stream size : 9.35 MiB (74%)
Writing library : x264 core 155 r2901 7d0ff22
Codec configuration box : avcC
Audio
ID : 2
Format : AAC LC
Format/Info : Advanced Audio Codec Low Complexity
Codec ID : mp4a-40-2
Duration : 3 min 29 s
Bit rate mode : Constant
Bit rate : 128 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 44.1 kHz
Frame rate : 43.066 FPS (1024 SPF)
Compression mode : Lossy
Stream size : 3.19 MiB (25%)
Default : Yes
Alternate group : 1
Video
ID : 1
Format : VP9
Codec ID : V_VP9
Duration : 3 min 29 s
Width : 1 080 pixels
Height : 1 080 pixels
Display aspect ratio : 1.000
Frame rate mode : Constant
Frame rate : 25.000 FPS
Language : English
Default : Yes
Forced : No
Color range : Limited
Audio
ID : 2
Format : Opus
Codec ID : A_OPUS
Duration : 3 min 29 s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 48.0 kHz
Bit depth : 32 bits
Compression mode : Lossy
Language : English
Default : Yes
Forced : No