[x264-devel] Re: quantiseation parameter

[Tuukka Toivonen]

On Tue, 28 Jun 2005, Tom Jacobs wrote:

> just a quick question as more of a sanity check for myself.
>
> what exactly does the quantisatio parameter -q value represent? i need a

It's a divisor.

See
http://www.rgu.ac.uk/files/h264_transform.pdf
(QP = Qstep)

A transform coefficient is divided by QP and rounded to integer.


Or that is what it originally was; in modern encoders it's purpose
is to control the image quality / bit rate. Thus, the quantized
transform coefficients could be derived with some more clever way
than just rounding to nearest integer. On the other hand, lambda
which controls how much bits are spent for motion vectors / mode
selection parameters, is derived from QP, so QP controls
motion estimation / mode selection also, indirectly.

> does it set the number of quantisation level allowed when quantising the

No... but the step between two successive levels.

> transformed data or has it got a cleverer rate control method attached to

See above.

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