Questions about -crf vs. -qp (x264)

[bananacreamandpeca]

I'm not sure if I understand certain options correctly.
Can you tell me if I'm right or totally wrong?

How do the -crf and -qp options within x264 determine what bitrate
to use for your video?

1. Does -crf option determine the bitrate by a "quality-value" that you entered (ie. -crf 21),
And translate that to something like --> user wants approx. 85% Q per frame ???
Wich then let's the encoder determine wich bitrate will best match 85% quality on a per frame basis?

confusing sometimes.

2. And will setting up the -qp option let x264 behave more like "Constant Bitrate" with MP3 does?
When each frame was given a pre-determined amount of bits to use/frame-period.
Regardless if it could maintain transparancy or not.

Thanks for any useful info.
It gets confusing sometimes.

[chadamir]

http://forum.doom9.org/showthread.php?t=118358&highlight=quantizer

http://forum.doom9.org/showthread.php?t=119177&highlight=quantizer

[DarkZell666]

Lol it's all the wrong way round :

-qp means : give me the quality I asked for, no matter how many bits it needs (same quantizer for all frames).
-crf does the same, but instead of choosing a quantizer for each frame, it determines a "visual perception" factor (based on the initial QP you specified), and once again, it means "give me the visual quality I asked for, no matter how many bits it needs".

CBR is the complete opposite !! CBR is: I want the stream to use 500kbps, so destroy the picture as much as you need for it to fit into 500kbps.

But ehm ... did't you do some tests yet ?