[Mp4-tech] Q-pel interpolation

[Dickson Liang]

Satendra,
I guess for calculating the "j" pel, you need unsigned 20 (=8+6+6) bits at least for the intermediate values. The first (unsigned) 8 bits are for the signal itself, and each of the 6 bits are for the worst-case interpolation (20+20+1+1=42, so 6 bits).
For other pels such as f,i,q,k, I think 16 bits are definitely enough because you have to use the clipped values to calculate them. Clipped values are unsigned 8bits.
There are other cases in H264 video decoding that 16 bits are not enough. For example, scaling of the chroma DC (after inverse transform is done) would need more than 16 bits.
Dickson Liang    
----- Original Message ----
From: Satendra <spsatendra gmail.com>
To: mp4-tech lists.mpegif.org; jvt-experts lists.rwth-aachen.de
Sent: Wednesday, October 18, 2023 4:08:00 AM
Subject: [Mp4-tech] Q-pel interpolation
Hi,
I want to know the maximum size in bits, the intermediate values of Q-Pel interpolation may take.
Considering this figure
E         F           G   a     b    c    H               I                J
                        d    e     f     g
                        h    i      j     k    m
                        n    p     q    r  
K        L            M         s          N             P               Q
My concern is while interpolating for  j, f, q, i, k  locations.
Here finally we need to scale/shift the values by 10, does it mean that intermediate values can be of 18 bits. Hence all the operations of H264 decode can't be completed in 16 bit arithmetic.
Thanks
Satendra
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