# MPEG skipped block and motion vector?

Started by September 10, 2005
```Hi all,

I am reading Fogg's MPEG FAQ.
(http://bmrc.berkeley.edu/frame/research/mpeg/mpeg2faq.html)
The FAQ have several diagrams about skipped block and motion vector.

How to determine the skipped block in B frame?
And what syntax in video stream describe it?

e.g. If I have a picture sequance I1 B2 B3 P4. The frame have frame
prediction.
If B2 have same microblock as I1 or P4, is the mb in B3 skipped?
And any other situations?

Any suggestions will be appreciated!
Best regards,
Davy

```
```"Davy" <zhushenli@gmail.com> a &#2013265929;crit dans le message de news: 1126406466.778822.64700@g14g2000cwa.googlegroups.com...
| Hi all,
|
| I am reading Fogg's MPEG FAQ.
| (http://bmrc.berkeley.edu/frame/research/mpeg/mpeg2faq.html)
| The FAQ have several diagrams about skipped block and motion vector.
|
| How to determine the skipped block in B frame?

Empty macroblocs with the same vector, prediction ... than the previous
one in the bitstream (something like that).

| And what syntax in video stream describe it?

Macroblocs are numbered. So if you have a macrobloc numbered with
23 and the next with 25, so there is a skipped macrobloc between the
two. You may mave many consecutive skipped macrobloc.

|
| e.g. If I have a picture sequance I1 B2 B3 P4. The frame have frame
| prediction.
| If B2 have same microblock as I1 or P4, is the mb in B3 skipped?

A macrobloc is skipped when it is empty and has nul vector in P picture or
empy with same vectors and prediction than previous macrobloc in B picture.
(roughtly)

| And any other situations?
|
| Any suggestions will be appreciated!
| Best regards,
| Davy
|

```
```Hi ast,

Thank you :-)

You said"
A macrobloc is skipped when it is empty
".
Is empty means the MB without any yuv details or y.u.v.=0?

Best regards,
Davy

```
```"Davy" <zhushenli@gmail.com> a &#2013265929;crit dans le message de news: 1126574117.619809.267930@f14g2000cwb.googlegroups.com...
| Hi ast,
|
| Thank you :-)
|
| You said"
| A macrobloc is skipped when it is empty
| ".
| Is empty means the MB without any yuv details or y.u.v.=0?
|

empty means empty in the bitstream. So no bit are used to code
the macrobloc.

So the prediction error (difference between the macrobloc and the
predictor) after DCT and Quantization is all zeros.

```