[Mp4-tech] Luma Block Position

[gopi krishnaiah]

Hi Experts,
I am new to H.264. I have a brief over view of standard and looking
thru the code.
When the current frame is decoded following are the co-rodinates that are used:
 luma block position 
 luma pixel position 
 chroma pixel position 
I understood what  luma pixel position and chroma pixel position stands for.
Can any one explain what this  luma block position represents?
Regards
Gopi