Re: iBook and playing DVDs
Michel =?ISO-8859- writes:
> On Wed, 2002-05-15 at 11:02, Adrian Cox wrote:
>> Are you sure about this? I thought most DVDs carried either interlaced
>> NTSC or interlaced PAL. In the NTSC case the movie is converted to
>> 30fps interlaced by a 3:2 pulldown (3 fields from one frame, 2 fields
>> from the next). In the PAL case each frame is split into two fields,
>> which speeds the film up slightly. Adjusting the pitch of the audio is
>> optional in this case. I believe that in both cases this is normally
>> done when mastering the DVD, to allow for manual tweaking of the
>> process.
>
> You're right, forgot about this, but in the PAL case the two fields are
> basically the same as one frame, right?
Not really. PAL is like NTSC, except:
1. slower framerate
2. higher resolution
3. different color encoding (only in analog form?)
In both cases, you can pick any two adjacent fields
and call them a frame. There aren't any pixels in
the analog version, but if you chopped the lines
into pixels you'd have each pixel at a unique
moment in time. The last pixel shown is closer in
time to the first pixel of the _next_ field than
it is the the first pixel in the same field.
The problem is easier to visualize if you imagine
time to be the Z axis. Movie film has slices that
go perpendicular to time. Video has lines that
aren't quite perpendicular to any axis, and they
don't line up in adjacent fields. Video lines
aren't even horizontal, and some fields get a
half line at the top or bottom of the display.
Look for fringes on objects that are moving
horizontally, like this:
::::::..
:::::::..
:::::::..
::::..
::..
People cheat. To convert a movie, you're supposed
to do motion tracking of objects so that you can
generate data for intermediate points in time.
To convert back, you'd do the same thing, except
that it's to your advantage to guess what kind of
cheating happened with the previous conversion.
Consumer hardware has nowhere near the CPU power
needed to do motion tracking of objects, so you
also are forced to cheat.
So you start by pretending that a field is a
snapshot in time, or that it is an average over
some period of time. You ignore the half lines.
If fields N and N+2 look the same, you're looking
at 3:2 pulldown, so use this:
average(N,N+2), N+1, N+3, N+4
Note that NTSC is a wee bit less than 60 frames
per second.
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