Why do the mbps fluctuate so much on blu-ray?

[Underworld54]

I am watching X-Files IWTB and I notice the mbps fluctuate quite a bit. I've seen it go from around 13.0mbps to nearly 33.0 mbps. Can someone give me a clear reason for there fluctuations?

[singhcr]

It is a variable bit rate encode. You do that to maximize the PQ for a given video file size. You have the highest bitrate on the fast action scenes, and low bitrate on slower scenes, etc.

[coolmilo]

Is it faster versus slower or Darker images versus lighter images?

[4K2K]

Originally Posted by coolmilo

Is it faster versus slower or Darker images versus lighter images?

Shouldn't it depend on how complex it is to encode?
eg.

A) Little or no changes per frame, or very predictable changes per frame = low bitrate (eg. scrolling white titles over black background where it's easy to track each line of text).

B) Many complex changes per frame, where it's harder to track the movement of each macroblock = higher bitrate needed (fast action scene with many different things happening in the frame).

A cartoon car moving across the screen fast might be easy to encode (as in A) if it's very predictable in it's movement, but a real car in a real scene moving fast through the scene, like in a car chase will probably need lots of bitrate, especially if the camera is moving a lot too. It will just depend on how complex things are and how easy or not things are to track using macroblocks I think.

I think you have a point though, I think the compressors might try to compress darker scenes more than lighter scenes, as they'll generally be less visible - but I'm not sure it that's what they do. But doing that could lead to crushed blacks, or no detail in the blacks or loads of blocking in the blacks if they're not careful, which is why they sometimes encode with an LCD set to elevated black levels to check for artefacts in the blacks that will be especially visible on consumer's TVs that are not set to the proper brightness levels.