Sun Dec 20, 2009 01:41
It's been brought up that some dont know what proper frame rates and such are. I wanted to post some basic info.
Standard Film: Produced for theatrical display at 24fps
PAL video: Recorded with a PAL video camera at 50 fields per second. A field consists of just the odd- or even-numbered lines of a frame. Television was designed to refresh these in alternation as a cheap form of analog compression. The human eye supposedly compensates for this, but once you understand interlacing you will learn to see it on TV too and never enjoy TV again. Two fields do not make a complete frame, because they are captured 1/50 of a second apart in time, and thus they do not line up unless there is no motion.
NTSC Video: Recorded with an NTSC video camera at 60000/1001 fields per second, or 60 fields per second in the pre-color era. Otherwise similar to PAL.
Animation: Usually drawn at 24fps, but also comes in mixed-framerate varieties.
Computer Graphics (CG): Can be any framerate, but some are more common than others; 24 and 30 frames per second are typical for NTSC, and 25fps is typical for PAL.
Types of pulldown:
PAL 2:2 pulldown: The nicest of them all. Each frame is shown for the duration of two fields, by extracting the even and odd lines and showing them in alternation. If the original material is 24fps, this process speeds up the movie by 4%.
PAL 2:2:2:2:2:2:2:2:2:2:2:3 pulldown: Every 12th frame is shown for the duration of three fields, instead of just two. This avoids the 4% speedup issue, but makes the process much more difficult to reverse. It is usually seen in musical productions where adjusting the speed by 4% would seriously damage the musical score.
NTSC 3:2 telecine: Frames are shown alternately for the duration of 3 fields or 2 fields. This gives a fieldrate 2.5 times the original framerate. The result is also slowed down very slightly from 60 fields per second to 60000/1001 fields per second to maintain NTSC fieldrate.
NTSC 2:2 pulldown: Used for showing 30fps material on NTSC. Nice, just like 2:2 PAL pulldown.
When video is stored on DVD, consecutive pairs of fields are grouped as a frame, even though they are not intended to be shown at the same moment in time. The MPEG-2 standard used on DVD and digital TV provides a way both to encode the original progressive frames and to store the number of fields for which a frame should be shown in the header of that frame. If this method has been used, the movie will often be described as "soft-telecined", since the process only directs the DVD player to apply pulldown to the movie rather than altering the movie itself. This case is highly preferable since it can easily be reversed (actually ignored) by the encoder, and since it preserves maximal quality. However, many DVD and broadcast production studios do not use proper encoding techniques but instead produce movies with "hard telecine", where fields are actually duplicated in the encoded MPEG-2.
Due to the nature of MPEG-type compression, there are various constraints you should follow for maximal quality. MPEG splits the video up into 16x16 squares called macroblocks, each composed of 4 8x8 blocks of luma (intensity) information and two half-resolution 8x8 chroma (color) blocks (one for red-cyan axis and the other for the blue-yellow axis). Even if your movie width and height are not multiples of 16, the encoder will use enough 16x16 macroblocks to cover the whole picture area, and the extra space will go to waste. So in the interests of maximizing quality at a fixed file size, it is a bad idea to use dimensions that are not multiples of 16.
Most DVDs also have some degree of black borders at the edges. Leaving these in place will hurt quality a lot in several ways.
1.MPEG-type compression is highly dependent on frequency domain transformations, in particular the Discrete Cosine Transform (DCT), which is similar to the Fourier transform. This sort of encoding is efficient for representing patterns and smooth transitions, but it has a hard time with sharp edges. In order to encode them it must use many more bits, or else an artifact known as ringing will appear.
The frequency transform (DCT) takes place separately on each macroblock (actually each block), so this problem only applies when the sharp edge is inside a block. If your black borders begin exactly at multiple-of-16 pixel boundaries, this is not a problem. However, the black borders on DVDs rarely come nicely aligned, so in practice you will always need to crop to avoid this penalty.
In addition to frequency domain transforms, MPEG-type compression uses motion vectors to represent the change from one frame to the next. Motion vectors naturally work much less efficiently for new content coming in from the edges of the picture, because it is not present in the previous frame. As long as the picture extends all the way to the edge of the encoded region, motion vectors have no problem with content moving out the edges of the picture. However, in the presence of black borders, there can be trouble:
2.For each macroblock, MPEG-type compression stores a vector identifying which part of the previous frame should be copied into this macroblock as a base for predicting the next frame. Only the remaining differences need to be encoded. If a macroblock spans the edge of the picture and contains part of the black border, then motion vectors from other parts of the picture will overwrite the black border. This means that lots of bits must be spent either re-blackening the border that was overwritten, or (more likely) a motion vector will not be used at all and all the changes in this macroblock will have to be coded explicitly. Either way, encoding efficiency is greatly reduced.
Again, this problem only applies if black borders do not line up on multiple-of-16 boundaries.
3.Finally, suppose we have a macroblock in the interior of the picture, and an object is moving into this block from near the edge of the image. MPEG-type coding cannot say "copy the part that is inside the picture but not the black border." So the black border will get copied inside too, and lots of bits will have to be spent encoding the part of the picture that is supposed to be there.
If the picture runs all the way to the edge of the encoded area, MPEG has special optimizations to repeatedly copy the pixels at the edge of the picture when a motion vector comes from outside the encoded area. This feature becomes useless when the movie has black borders. Unlike problems 1 and 2, aligning the borders at multiples of 16 does not help here.
4. Despite the borders being entirely black and never changing, there is at least a minimal amount of overhead involved in having more macroblocks.
For all of these reasons, it is recommended to fully crop black borders. Further, if there is an area of noise/distortion at the edge of the picture, cropping this will improve encoding efficiency as well. Videophile purists who want to preserve the original as close as possible may object to this cropping, but unless you plan to encode at constant quantizer, the quality you gain from cropping will considerably exceed the amount of information lost at the edges.