ErMaC's Guide to All Things Video - Part 2
The Big Picture - Interlaced vs. Progressive, Fields vs. Frames, 3:2 pulldown and Inverse Telecine
In this guide we will expand on the information given out in the previous part, and introduce the concepts of Interlaced video, Fields, and Telecining along with their opposites.
1) Interlaced vs. Progressive
When you're watching your television, if you go up real close to it and watch carefully (but not too long, remember kids, it'll rot your eyes!) you'll notice that the picture sort of "shimmers." Old skool computer users will probably remember how back in the day, when resolutions higher than 800x600 were in the realm of super-highend workstations, you could sometimes get those higher resolutions if you really tried, but your picture would wind up flickering and shimmering, usually causing groans of disgust and a quick jump back down to a lower resolution. That's interlacing at work.
Progressive video means that every pixel on the screen is refreshed in order (in the case of a computer monitor), or simultaneously (in the case of film). Interlaced video is refreshed to the screen twice every frame - first every Even scanline is refreshed (the little gun at the back of your Cathode Ray Tube shoots all the correct phosphors on the even numbered rows of pixels) and then every Odd scanline. This means that while NTSC has a framerate of 29.97, the screen is actually being partially redrawn every 59.94 times a second. A half-frame is being drawn to the screen every 60th of a second, in other words. This leads to the notion of fields.
2) Fields vs. Frames
We already know that a Frame is a complete picture that is drawn onto the screen. But what is a field? A field is one of the two half-frames in interlaced video. In other words, NTSC has a refresh rates of 59.94 Fields per Second. So when you see console systems/games advertise "60 frames a second gameplay" that's actually only a half-truth (unless you hook your Dreamcast up to a computer monitor using the special cable). A TV can only display approximately 60 half frames a second, so your console is rendering 59.94 Frames, and then dropping half the scanlines to display on your TV.
This is why when you're normally watching Anime (traditionally produced on Film which is 24 Progressive Frames a second) and you see a pan that's done in 59.94 fields a second (such as in the new Sol Bianca) you immediately go "Whoa - that was smooth."
This has very important ramifications when it comes to working with digital video. When working on a computer, it's very easy to resize you video down from 704x480 to something like 576x384 (A simple 20% reduction in framesize). But if you're working with interlaced video, this is an extremely bad thing. What resizing video to a lower resolution basically does is it takes a sample of the pixels from the original source and blends them together to create the new pixels (again that's a gross simplification but it should suffice). This means that when you resize interlaced video you wind up blending scanlines together, which could be part of completely different images! For example:
Image in full 720x480 resolution
Enlarged portion - notice the interlaced scanlines are distinct.
Image after being resized to 576x384
Enlarged portion - notice some scanlines have been blended together!
This means that you're are seriously screwing up your video quality by doing this! If you do want to reduce the image size of your video, first you must either deinterlace it or reduce it by exactly 2 (i.e. 240 scanlines) by discarding 1/2 of the scanlines (as opposed to blending them).
Deinterlacing is the process of interpolating the pixels inbetween scanlines of the same field (in other words, reconstructing a semi-accurate companion field which would turn the field into a full frame). This has several drawbacks. Not only does it take a lot of processing power to do, it's inherently error prone.
What's my recommendation? Always work in 480 or 240 scanlines. If you need to lower your resolution, do so only in the horizontal direction. This can be done without the fear of scrweing up your image. For instance, decreasing your framesize from 720x480 to 360x480 maintains the full vertical resolution of the picture, but halves the framesize and provides decent image quality. Of course, once you are done you would want to resize it back to normal resolution (since at that point everyone would look really skinny), or you would output it to video using some device which automatically resized the video to NTSC resolution (such as a Real Hollywood+ DVD Decoder card).
Page 2 - 3:2 Pulldown and Inverse Telecine