Quantization parameter

[koliva]

Hello all,

I have encoded my video by lencod.exe from JM software.
I have attached the encoder.cfg file that I used for encoding process. When I start encoding, it says,

Sequence type: IPPP (QP: I 28, P:28)

Does it mean that QP=28 was used for encoding?

In my own application, I am just simply dividing the values by QP and multiplying them if I want to get them back. So, could you please tell me what the corresponding value of 28 for that purpose?

Many thanks.

[Shevach]

QP means the Quantization Parameter, it is not the quantization scale factor. Actually the scale factors for both luma and chroma are derived from QP and several other parameters.

[Shevach]

Koliva,
I have forgotten to answer your question.
The following table is used for forward quantization in order to convert Qp to Qstep:

QP Qstep
0 0.625
................
4 1
5 1.125
..................
10 2
...............
28 16.25

In other words each increment of QP by 1 increases QStep by 12.5% .

[AVC_fan]

Hi,

What is use of QP 0-4 ? I feel we can have same effect with QP 1 as with 2 because they all expand data.

[nm]

What is use of QP 0-4 ? I feel we can have same effect with QP 1 as with 2 because they all expand data.

You are comparing to a compressed source. Even lossless QP 0 compresses ordinary raw video significantly unless it's very noisy.