What does VBV underflow mean?

[Chengbin]

I got a warning message saying VBV underflow (-9544 bits) 41kb/s

What does that mean?

I'm using vbv-maxrate 5000 --vbv-bufsize 1000

[Dark Shikari]

It means that x264 wasn't able to abide by your (in this case, rather absurd) VBV restrictions.

[Chengbin]

So it means at some point of my video, the bitrate is over 5000Kbps?

[LoRd_MuldeR]

So it means at some point of my video, the bitrate is over 5000Kbps?

No, it means that at some point in the video the VBV buffer will run out of data.

VBV-Maxrate doesn't restrict the maximum video bitrate. It restricts the maximum rate at which new data can enter the VBV buffer.

This video bitrate may exceed the VBV-Maxrate for a short period of time, as long as enough data is left in the buffer...

[Chengbin]

My use of VBV was to restrict the maximum bitrate for the 1 second scenes that go over 5000Kbps.

Can you explain what a buffer is in VBV?

[Guest]

Chengbin, instead of striking you a rule 1 for not searching for this readily available and often discussed information I'm just going to close the thread. Next time, though, it may be different.