[Xvid-devel] What is currentMV and predMV in SearchData struct

[Chien Tran]

Thanks for your answer.
So let's say I found the match_x and match_y of referenced Macroblock. How
could I get the exact Macroblock from the macroblock pointers (pMBs)?
Currently I see some places in the code do like this:
pMB = pMBs[x + y * mb_width]; //get Macroblock based on x, y location

But I am afraid I can not do like that if (x + y * mb_width) < 0, right?

2012/5/21 Michael Militzer <michael at xvid.org>

> Hi,
>
> your understanding is correct: currentMV stores the best candidate vector
> found so far during ME for a respective mode/partition (e.g. 16x16) and
> predMV holds the corresponding predictor vector that will be substracted
> before actual coding of the vector.
>
> Note that only the difference between best found vector and predictor
> vector is written to the bitstream (refer to e.g. get_pmv2() in
> mbprediction.c on how the predictor vector gets determined). Considering
> the predMV already during ME is important to estimate the motion vector
> cost (so the amount of bits needed to write the found vector into the
> bitstream). The more currentMV deviates from predMV, the higher the cost
> for coding the vector into the bitstream.
>
> Also note that motion vectors can indeed point outside the visible
> rectangle of a ref frame. So your understanding on how to determine the
> reference blocks for motion compensation is correct but you need to
> consider that there is a padded, "edge" area around reference frames.
> Therefore, "match_x < 0" is legal. You can take a closer look at how
> the edge padding is performed in image_setedges() (image/image.c).
>
> Best regards,
> Michael
>
>
> Quoting Chien Tran <chien.study at gmail.com>:
>
> > Hi everyone,
> > I am new to Xvid and I am doing a research about video compression. When
> I
> > look at the SearchData struct, I saw two important fields: currentMV and
> > predMV. I guessed it stored the information about the motion vector of
> > Macroblock, but I don't know what it really means to have 2 fields like
> > that. Its comment said currentMV is "best vectors found so far"
> > (currentMV[0] would be the MV for the 16x16 MB, I think) and the predMV
> is
> > "vector which predicts current vector". Could you guys please tell me
> what
> > currentMV and predMV are really used for?
> > Let's say I have the x, y of the current Macroblock. How can I know the
> > location of the matching Macroblock in previous Frame that were found in
> > Motion Estimation? Is it: match_x = x + currentMV[0].x; match_y = y +
> > currentMV[0].y? I did some tests and found that it sometimes has the
> value
> > that is out of the Macroblock array (more specifically, match_x < 0).
> >
> > --
> > Best wishes,
> >
> > Tran Xuan Chien
> > University of Information Technology
> > Phone: (+84) 1692 468 154
> > _______________________________________________
> > Xvid-devel mailing list
> > Xvid-devel at xvid.org
> > http://list.xvid.org/mailman/listinfo/xvid-devel
> >
> >
>
>
>
>
> _______________________________________________
> Xvid-devel mailing list
> Xvid-devel at xvid.org
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>



-- 
Best wishes,

Tran Xuan Chien
University of Information Technology
Phone: (+84) 1692 468 154