On Fri, 19 Oct 2001, Billy Biggs wrote:
> Mark Vojkovich ([EMAIL PROTECTED]):
>
> > > It's just something silly, I guess, but important for preserving the
> > > spacial position of the fields. Say you're scaling from 720x480 to
> > > 1024x768. 768/480 = 1.6, so, you want to scale up each field from
> > > 240 lines up to 766.4 lines. This gives a scaling of:
> > >
> > > [top field] y=0 to y=766.4
> > > [bot field] y=1.6 to y=768
> >
> > The *sources* are subpixel. All hardware is that way. You couldn't
> > bob video otherwise. You double the pitch and half the surface size
> > and you have a single field as a source (and bottom starts a scanline
> > later). You just adjust the offset into the texture appropriately to
> > get spatial alignment correct. It's much easier for the driver to do
> > than for you to do it.
>
> I've read this paragraph like 3 times and I still don't understand
> what you mean, sorry. I understand how doubling the pitch and halving
> the surface size gives a single field as a source, no problem, but when
> you scale it up you need to subpixel resize it.
>
> To clarify, say I'm blitting a top field from 720x480 to 1024x768:
>
> Top Field Bot Field
> source scale to source scale to
> x = 0 x = 0 x = 0 x = 0
> y = 0 y = 0 y = 1 y = 1.6
> width = 720 width = 720 width = 720 width = 720
> height = 240 height = 766.4 height = 240 height = 766.4
> pitch = 1440 pitch = 1440
>
> So, the source offsets for these fields are integers, but what I need
> them scaled to is not. Can hardware do this easily? I'd assume you'd
> have to round down from 1024x768 to something where all of these numbers
> are integers, or run at 1280x960.
It doesn't work that way. There's no such thing as subpixel
destinations so you have to stop thinking about it that way. You
have integer rectangles on the screen but you can align how the
source scales to that. You can essentially assign the corners
of that screen aligned rectangle to subpixel points within
the source. I can have the top line of the destination display
not the top line of the source but adjust the filter kernel so
that it starts displaying, for example, 5/16 of the way between the
first and second lines of the source, effectively shifting the
whole image up by 5/16 of a pixel. I have subpixel alignment
of the source within the destination rectangle.
I have not seen any hardware which cannot do this.
>
> > With write-only port attributes you can essentially extend the
> > Xv(Shm)PutImage function by specifying additional attributes before
> > the request. An attribute such as XV_DISPLAY_FIELD could be set to
> > indicate that the next Put request is to be a field. The value would
> > be 0=top, 1=bottom in MPEG fashion. It's a one-shot thing and only
> > applies to the next put. All drivers could add this without API or
> > protocol changes.
>
> Yes, this would be ideal.
>
Mark.
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