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[MPEG] Why initialize YUV = 128?

Started by Robert October 7, 2005
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

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
"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
Hi all,

Thank you for your help ;-)

But what's "apply a delta" mean?

Davy

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
"Robert" <zhushenli@gmail.com> a &#2013265929;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