This chapter is from the book
This chapter is from the book
This chapter is from the book
Temporal Preprocessing
Whatever the reason, sometimes you’ll need to adjust the frame rate of your video. Most of the time this is done to better match the source to the destination. Whatever the reason, temporal processing can be one of the most difficult steps to get right.
Easy Frame Rate Conversions
Certain frame rate conversions aren’t too hard and usually yield good-quality results. For example, when the source frame rate is a fractional frame rate, say 29.97 fps or 23.976 fps, converting to its corresponding integer frame rate—30 fps or 24 fps in this case—isn’t too hard. You’ll just change the amount of time each frame appears on the screen slightly. Audio will need to be adjusted to stay in sync, but most modern tools can do this compensation for you. Another simple conversion is when you want to go from a higher frame rate to one that is half as fast (or the other way around), such as 59.94 fps to 29.97 fps. In this conversion, you’ll simply drop (or duplicate) every other frame to get from the source rate to destination rate. Most modern nonlinear editing (NLE) systems can do these conversions in the timeline.
Telecine and Inverse Telecine
Telecine is the process of transferring motion-picture film into electronic form. The term is also used to describe the machine used in this process. Traditional motion-picture film runs at exactly 24 fps progressive scan, which doesn’t easily convert to 29.97 fps for NTSC.
Converting from film to PAL is easy. The video is sped up 4 percent to 25 fps and converted into progressive PAL.
The telecine process for NTSC is more complex: the film is first slowed down 0.1 percent to 23.976 fps and is then converted to 29.97 fps by using a process called 3:2 pulldown (Figure 4.4). In this process, the first frame of film becomes three fields of video, the next frame becomes two fields of video, the next becomes three fields, and so on, resulting in two out of five frames having fields that come from different source frames.
Figure 4.4 In the 3:2 pulldown process inherent to NTSC telecine, four frames of film become ten fields/five frames of video by using a three-then-two field pattern to maintain the smooth motion of the film in the newly created video.
To correctly compress video that has been telecined, it is crucial to be able to identify these duplicate frames and then remove them. Inverse telecine basically reconstructs the four frames from every five to turn the source back into progressive video.
The Difficult Conversion
There’s one particularly tricky conversion to be aware of. It occurs when your source is 30 fps (or 29.97) and your destination is 24 fps (or 23.976). Now when we say “tricky” or “difficult,” we mean that it’s difficult to get it to look good. Let’s dive in to see why.
When you want to get from 24 fps to 30 fps, you can add interlaced 3:2 pulldown to a clip to achieve the desired result with minimal unwanted motion artifacts. This is because you’re using fields created from a progressive image to create additional frames in a pattern that gives six additional frames per second that are distributed in an even cadence. You can reverse this process to get from 29.97i to 23.976p.
The problem arises when you start with a progressive source sequence and need to remove frames from it to lower the frame rate. To keep the dropped frames in an even cadence, you need to drop every fourth frame. The problem is that if you have smooth motion in your shot, like a car driving through the frame from left to right. The change in the car’s position between each frame is consistent, which means when you remove one of those frames, the car appears to move further between the third and fifth frames, which makes it look like it jumps. When this is repeated six times per second, the result is a really jumpy-looking video that is unpleasant to watch.
Just like with the upconversion process, the way to try to deal with this motion artifact is through interpolation. While there are a lot of standards converters on the market, there are a couple to take a look at for this specific conversion: Cinnafilm’s Tachyon and BlackMagic Design’s Teranex. But be warned, these tools can be fairly expensive and sometimes often yield subpar or unacceptable results depending on the content. Many times, the best way to deal with this situation is to manage expectations or try to find a solution that doesn’t involve this conversion.
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