[Mp4-tech] [AVC] On scanning luma DC block

[Shevach Riabtsev]

Dear experts 
Unless I am mistaken in AVC standard  luma DC block shall be scanned
before VLC encoding. There are two scanning maps: zigzag and alternate,
where alternate map is applied provided that a picture is field,
otherwise the zigzag scan is executed (perhaps there is a nuance for
MBAFF case).  
The reasoning why zigzag scan is used for luma DC block is clear. Indeed
Hadamard transform is a "first-order" approximation of DCT transform,
consequently Hadamard-transformed block is expected to have a
non-uniform energy distribution and so on. 
I don't understand why alternate scan shall be performed for luma DC
block if a picture is field? Indeed 4x4 luma DC block is luma AC one!!!
What's gain is achieved (if ever) if we reorder 4x4 luma DC block in
alternate scan instead of zigzag one? Does the vertical sub-sampling of
field pictures impacts on luma DC block coefficients?  
Regards,  Shevach 
Broadcom  Israel 
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