On Sat, 31 Mar 2001, Benedict Bridgwater wrote:
> FYI, thanks to Trent Piepho's note on JFIF using the Y'PbPr colorspace,
> not the Bt848's Y'CbCr, I've done some digging to find out what MJPG
> (AVI motion JPEG uses)...
>
> The bottom line is that it (MJPG) uses Y'CbCr.
I wouldn't trust just reading the docs, it seems like people have a hard time
agreeing on just what to call their colorspace.
I'd would look at some code that correctly decodes a MJPG from Y'CbCr to RGB
and see what equations they use. That should give you a good idea of what
they are really using.
For instance, in libjpeg, there is this comment:
* YCbCr is defined per CCIR 601-1, except that Cb and Cr are
* normalized to the range 0..MAXJSAMPLE rather than -0.5 .. 0.5.
* The conversion equations to be implemented are therefore
Which would make you think that they use rec 601 Y'CbCr, wouldn't it?
But then look at their equations:
* Y = 0.29900 * R + 0.58700 * G + 0.11400 * B
* Cb = -0.16874 * R - 0.33126 * G + 0.50000 * B + CENTERJSAMPLE
* Cr = 0.50000 * R - 0.41869 * G - 0.08131 * B + CENTERJSAMPLE
These look very similar to something from the color space faq...
Eq 2
[ Y' 601 ] [ 0.299 0.587 0.114 ] [ R' ]
[ PB 601 ]=[-0.168736 -0.331264 0.5 ]*[ G' ]
[ PR 601 ] [ 0.5 -0.418688 -0.081312 ] [ B' ]
Except now we have a different name!
> The other (nice to have a choice, isn't it?) MJPG specification, part of
> the OpenDML extended AVI specification, references Microsoft's
> specification, and says that OpenDML MJPG will always use 8 bit 4:2:2
> (i.e. H/VSubSampling = 2) Y'CbCr.
You sure about that? from the colorspace faq again..
In digital video, Rec. 601 standardizes subsampling denoted 4:2:2, where
CB and CR components are subsampled horizontally by a factor of two with
respect to luma. JPEG and MPEG conventionally subsample by a factor of
two in the vertical dimension as well, denoted 4:2:0.
