"Robert" <zhushenli@gmail.com> a &#4294967295;crit dans le message de news: 1128696604.493332.136360@g47g2000cwa.googlegroups.com...
| Hi all,
|
| I am reading MPEG source code. I found all the Y U V are initialized to
| 128. Why?
|
| Before the I picture transform, the source picture was subtract from a
| "virtual" reference picture(Y U V are all 128). What's it mean?
|
| Any suggestions will be appreciated!
| Best regards,
| Davy
|

Doesn't the DCT need some input data whose values are centered around 0.

Source data are in range [0; 255]

After substracting 128, range become [-128; 127] suitable for DCT transform.

But i have not clear idea about that, maybe it's right or maybe not .

ast 

Robert wrote:
> Hi all,
> 
> Thank you for your help ;-)
> 
> But what's "apply a delta" mean?

Add a delta (change) which may be positive or negative.

Cheers,
Nicholas Sherlock

Hi all,

Thank you for your help ;-)

But what's "apply a delta" mean?

Davy

"Robert" <zhushenli@gmail.com> wrote in message 
news:1128696604.493332.136360@g47g2000cwa.googlegroups.com...
> Hi all,
>
> I am reading MPEG source code. I found all the Y U V are initialized to
> 128. Why?
>
> Before the I picture transform, the source picture was subtract from a
> "virtual" reference picture(Y U V are all 128). What's it mean?
>
> Any suggestions will be appreciated!
> Best regards,
> Davy


If you are starting from nothing, and going to apply a delta to an image - 
doesn't it make sense to start from in the middle?

-- 
"Wise men talk because they have something to
say; fools talk because they have to say something." - Plato 

Robert wrote:
> Hi all,
>
> I am reading MPEG source code. I found all the Y U V are initialized to
> 128. Why?
>
> Before the I picture transform, the source picture was subtract from a
> "virtual" reference picture(Y U V are all 128). What's it mean?
>
> Any suggestions will be appreciated!
> Best regards,
> Davy

If the pixels are 8 bits, then all 128 is a flat, gray image. Locking
the U and V components at 128, Y=0 would give all black and Y=255 would
give all white. The U and V components -- sometimes called red chroma
(or Cr) and blue chroma (or Cb) -- are neutral at the midpoint of their
range. With a Y>0, if they're both zero, the image is green, and if
they're both maxed (255), the image is magenta.

Cheers! --M

Hi all,

I am reading MPEG source code. I found all the Y U V are initialized to
128. Why?

Before the I picture transform, the source picture was subtract from a
"virtual" reference picture(Y U V are all 128). What's it mean?

Any suggestions will be appreciated!
Best regards,
Davy