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
[MPEG] Why initialize YUV = 128?
Started by ●October 7, 2005
Reply by ●October 7, 20052005-10-07
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, > DavyIf 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
Reply by ●October 8, 20052005-10-08
"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, > DavyIf 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
Reply by ●October 8, 20052005-10-08
Reply by ●October 8, 20052005-10-08
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
Reply by ●October 9, 20052005-10-09
"Robert" <zhushenli@gmail.com> a �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