Suggested Compression Ratio with H.264?

Question

Note bene: I realize this is an immensely complicated question with about a million levels of nuance that I'm trying to reduce to a single number...

I'm about to undertake a large video encoding project using H.264 encoding. We are trying to create multiple bitrate profiles in order to accommodate streaming across internet connections, processors, devices, etc.

Generally speaking, what kind of compression ratio should I be expecting to see (while staying within a reasonable level of quality)?

For example, a 640x360 (16:9) pixel video file @ 24 frames per second and 16-bit color should yield an uncompressed file that is approximately 33 MB/s.

I've been told that, for that file, 500 Kbits/second (or 62 KB/s) is not an unreasonable video bitrate. That seems insane - more than 530:1 compression? That's 99.8% compression. Is my math wrong?

I'm just looking for a rough outer guide for quality, like "more than 500x compression is crazy" or "less than 400x is a waste of bandwidth". I've looked everywhere, and nothing gives me any kind of expected compression...

Answer 1

In a quite interesting document called H.264 Primer, a simple formula is given as an hint to compute the `ideal' output file bitrate, based on the video's characteristics:

[image width] x [image height] x [framerate] x [motion rank] x 0.07 = [desired bitrate]

where the image width and height is expressed in pixels, and the motion rank is an integer between 1 and 4, 1 being low motion, 2 being medium motion, and 4 being high motion (motion being the amount of image data that is changing between frames, see the linked document for more information).

So for instance, if we take a 1280x720 video at 24 FPS, with medium motion (movie with slow camera movements, not many scene changes...), the expected ideal bitrate would be:

1280 x 720 x 24 x 2 x 0.07 = 3,096,576 bps => approximatively 3000 kbps

This is purely a hint, and in my opinion, the only way to accurately find the ideal bitrate is trial by error :)

Answer 2

It will vary dramatically depending on the content of the source videos. I'll get to that in a bit.

640x360 is not that large. 512kbps is very reasonable and arguably standard. Maybe 768kbps if you are really interested in quality.

How is this possible? A simplified answer: There are a couple of techniques and facts about video compression that make this possible:

1. Not every video frame data structure in an common H.264 (or other CODECs for that matter) is a full image. Instead there are two types which are colloquially referred to as
   1. Key Frames: a full rendering of the entire video image
   2. Intra-frames: a description of changes to the previous frame. These frames generally make up the vast majority (80%-99%) of frames in a video.
2. H.264 is "lossy", as are many other CODECs. They do not reproduce a pixel-by-pixel, frame-by-frame exact duplicate of the original source video. Example: Lossy blocks: If all but one pixel in an area is the same color, the CODEC 'loses' the one pixel. So, instead of having to store information about every single pixel in a frame, the CODEC just says "x1, y1 to x2, y2 are all color x". Very efficient.

It is all wildly more complex than that, with zillions of different approaches, techniques and algorithms within specific CODECs and between CODECs to make this happen.

So, back to the "It will vary dramatically depending on the content of the source videos" comment: The compression ratio you'll see, and the resulting quality, will depend significantly upon:

• the contents of the video
• your tolerance for artifacts (blocks, loss of color, loss of definition)
• the CODEC parameters you set, and how you set them

Example: A video of a door in a room (like a security camera) with one key frame every ten minutes is going to have an amazingly high compression ratio. My back-of-the-napkin calculations put that scenario at 15,000:1 compression.

Since you are starting on a large video encoding project, I would recommend a couple of things to determine what your compression ratio is going to be:

• take a sample of the source videos you are going to encode. 100 or more being statistically relevant.
• encode them at various bit rates, with various parameters, to determine what resulting characteristics meets your needs

Changing the parameters of the encoder to make the videos smaller can have other impacts too:

• higher play back CPU requirements
• player CODEC expectations. Not all H.264 encoded videos can be played back by all players
• longer encoding times
• various degrading of quality

It's a big complicated subject. Good luck. My experienced "thumb-to-the-wind" test says you'll be more than happy with 512-768kbps for your project.