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. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
