interlaced video question for you XV experts

[Benjamin Herrenschmidt]

On Thu, 2005-03-17 at 21:36 -0600, Billy Biggs wrote:
> Benjamin Herrenschmidt (benh at kernel.crashing.org):
> 
> > > In MPEG-2 there is a flag called "top_field_first" which indicates
> > > the parity for each encoded frame.
> > 
> > Well, from memory, there is something about the fact that the "top"
> > line is either on the first or the second field of a frame that is
> > different between PAL and NTSC, no ? That is, on one of them, the
> > second field starts in the middle of the screen above the first field
> > since scanning isn't exactly horizontal, or something like that ?
> > 
> > I don't remember much of this, I wasn't on the HW side though :)
> 
>   Yeah, NTSC has the funny half-scanline.
> 
>   In most digital video standards though, you only have like 720x480
> images instead of the "full" 720x486 which would include the half
> scanline.  But, I don't see how which one starts the frame determines
> the order that you should play them back.  It's really orthogonal and
> something you'd decide when you're storing the media, not playing it
> back.  I believe you of course that there are storage formats which
> store NTSC and PAL differently like that.

Well, the second one beeing vertically "above" the first one with the
funny half scanline thing, you end up wanting the image to contain the
second field first (on top) ... well, that's how we did back then at
least.

Ben.