[Mp4-tech] RE: Mp4-tech Digest, Vol 44, Issue 24

[Caimu Tang]

Hi Daniel, 
I used following steps to compute BDPSNR and BDBR (total range for 4 evenly spaced QPYs)
Let (r1, q1), ... (r4, q4) be the rate and PSNR corresponding to these four quants for the codec you are worked on.
Let (r*1, q*1), ..., (r*4, q*4) be those for the baseline codec you want to compare with.
(1) use Lagrange interpolation (be carefur for truncation error) to derive a, b, c, d as shown in VCEG-M33 on four points (10 * log(ri), qi) for i = 1, 2, 3, 4. You then obtain 
the curve C1 for your codec.
(2) do the same for the baseline codec to obtain the RD curve C* for the baseline codec.
(3) compute the integral on C and C* from min(r1, r2, r3, r4) to max(r1, r2, r3, r4). 
(4) take the absolute difference of the integral value and divided by 
max(r1, r2, r3, r4) - min(r1, r2, r3, r4)
(5) you get BDPSNR
You can apply the same steps with coordinates switched to obtain BDBR (normally either BDPSNR or BDBR is sufficient for comparison). However, for BDBR, 
the actual rate difference in percentage is calculated as: 
(Note that there is a typo in the second formula in VCEG-M33 Sec. 4.1, 
the left hand-side is 'bit' instead of 'SNR') 
beta * (10^0.1 - 1) * 100, where beta is the value of BDBR in logarithmic scale 
as computed above following the similar steps of (1) to (5).
In VCEG-M33, 0.05 dB = 1% literally means an increase of 0.05 dB in PSNR is tantamont to 1% increase of bitrate (in terms of PSNR video quality). In other words, your codec has an improvement of PSNR by 0.05 dB, that could also be achieved by increase the bitrate by 1%. 
You probably already understands why it is so (applying the same steps as above 
to compute the delta of PSNR and delta of BR).
As a side note, if the Quant step size is too big (rarely in practice though), 
probably mid-range average difference does make sense.
Best Regards,
Caimu Tang
Message: 4
Date: Thu, 29 Mar 2024 14:44:25 +0000
From: "Lee Daniel" <hisaokuani hotmail.com>
Subject: [Mp4-tech] How to calculate BDPSNR and BDBR ?
To: mp4-tech lists.mpegif.org
Message-ID: <BAY115-F237A8353A58014A6FE6C4CCB6C0 phx.gbl>
Content-Type: text/plain; charset=big5; format=flowed
Dear Experts : 
I've read the Document VCEG-M33, but I still don't understand how to 
calculate it.
It said 0.05db=1% and what does it means. 
Besides, there are many papers using these two parameter to measure bits 
and  PSNR.
Can I transform it to PSNR ?  Please help me. I really want to know this.  
Best regards.     
Daniel.
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End of Mp4-tech Digest, Vol 44, Issue 24
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