[Mp4-tech] [h264] Negative qp

[Gary Sullivan]

Hichem et al,
Yes, negative QP is possible in some profiles.  But do not confuse QP with the quantizer reconstruction step size.  QP is just a control parameter that controls what the step size is, it is not the step size itself.  There is nothing too magical about negative or positive values.  The relationship between QP and step size is approximately proportional to two raised to the power of QP/6.
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of h M
	Sent: Friday, February 16, 2024 8:02 AM
	To: mp4-tech lists.mpegif.org
	Subject: [Mp4-tech] [h264] Negative qp
	Dear experts,
	In 8.5.2 section, I have found the following note:
	NOTE 1 - QP quantisation parameter values QPY and QSY are always in the range of -QpBdOffset Y to 51, inclusive. QP quantisation parameter values QPC and QSC are always in the range of -QpBdOffsetC to 51, inclusive.
	So, could we really have a negative QP for either luma or chroma? And what does it means ? 
	Best regards
	Hichem
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