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Slow Motion in 24p Filmmaking

The way to achieve natural slow motion with inexpensive camcorders.
by Thomas Worth (2-26-2003)

Recent Updates
3-5-2003: Experiment with RE:Vision Effects Twixtor (video clip example)

We budget filmmakers are totally stoked now that there is an inexpensive way to shoot true 24p footage on a DV camera. The Panasonic AG-DVX100 and the Canon XL2 are excellent tools for budget filmmakers going for the "Hollywood" look -- you can finally free yourself from the mediocre clutches of all those "film look" plug-ins and camcorder "movie" modes.

But the purpose of this article is not to point out the benefits of true progressive scan 24 frames per second. It's to talk about something I haven't seen a lot of on the web, which is how to achieve great slow-motion photography while shooting a 24p project.

Some of you may be thinking, "it hasn't been talked about because it's really not that big of a deal." Well, the truth is that many filmmakers put great emphasis on visual style, and the limitations of cheap cameras (even 24p cameras) have restricted the creative potential of visual filmmakers. The ability to shoot slow motion sequences is part of that potential, and is an issue that needs addressing.

Let me just break it down for you:

With a real film camera, the cinematographer can mechanically vary the speed that the film runs through it. This has nothing to do with exposure or shutter speed or any of that stuff -- this is purely the "frame rate." All right, you're now thinking, "But dude, film runs at 24 frames per second, like, why would anyone change it? All the projectors run at 24, so don't you like, have to stick to that?" Yes, you do, if you want your footage to stay within the realm of "real time." This means that 1 second of real life corresponds to 1 second of footage on your camera. In this case, you shoot at 24. However, when you creatively want to alter time, you change the rate that the camera "samples the scene," or captures pictures. If you set the camera to shoot at 100 frames per second but play the film back at 24, time gets really slow because you're playing back what you originally captured real time at 24% of its original speed. The great thing about all this is that even though you're playing the footage back at 24% of its original speed, it still looks nice and smooth because it is being played at the standard rate of 24 frames/sec. Alternatively, if you shoot at 8 frames per second, time gets really fast -- it's the same concept as I mentioned before, just in the other direction. All this frame rate manipulation is not something camcorders have had an incredible amount of flexibility with, as they always have to be running at 29.97 frames per second so their footage will play back properly on televisions (regular TVs always run at 29.97 fps).

And now for my next point: Both the DVX100 and the XL2 are still camcorders. While you can vary the frame rate between 24 and 30, you can't go any higher. So, if you want to capture slow-motion footage in progressive scan mode, you have to choose one of those. Slowing 30 progressive frames down to 24 gives a little slow motion (80% of the original speed), but not much.


The solution is 60i mode (the normal, non-24p shooting mode)

60i mode on any inexpensive camcorder (e.g. one costing less than $25K), captures the most information per second of any of the shooting modes. Although you end up with 30 whole frames, you in fact have 60 "half-frames." Using a tool like Adobe After Effects or a combination of After Effects and third-party plug-ins, you can interpolate or "fill-in" the extra fields to end up with 60 whole frames. That's 60 unique images in one second, which is a lot better than 30 if you're trying to achieve a dramatic slo-mo effect. That's 50% of the original speed when played at 30 frames/sec, and 41.6% when played at 24. The best thing about this is that no temporal interpolation is necessary -- that is, you don't have to use After Effects to do any "blending" between frames. The reason we want to stay away from frame blending is because of the introduction of motion blur, which I will elaborate on in a minute. The bottom line is that in the end, you see nothing but frames the camera really captured itself -- the same as a real film camera.

Here are a couple of QuickTime movies that demonstrate this technique:

Example 1 (60i, normal speed)
Example 2 (24p, slow motion)

Both of these clips use the exact same source footage. The first clip is running at the normal rate of 30 frames/sec. (remember, it was shot in 60i) The second clip has been processed with After Effects to utilize all the video information, and has been time-stretched and slowed to 24 frames/sec. Realize that these are all REAL frames. The only interpolation has been to smoothly "double" the fields of each 1/2 frame to make a whole frame. No frame blending was used in After Effects whatsoever! In fact, I disabled it when I rendered to be sure!

You can take the footage you've converted using this process and integrate it with the other footage you've shot in 24p. Since the footage comes out of After Effects as 24p, the integration should be seamless. If you intend to integrate the slow-mo footage with other footage that's been shot using a camera's "24p standard" mode, you can have After Effects insert a 3:2 pulldown into the video stream to make it a DV legal 29.97.

So how do I do this, man?!

Okay -- first, shoot your slow motion sequences in 60i and with a shutter speed of at least 1/120 (for an explanation of why to use a fast shutter, click here). Faster shutter speeds look better, but if you need more light you can get away with 1/120. Just switch your camera to that mode when you're ready. When you're done, capture the footage normally (as 29.97fps) and then bring it into After Effects. Here are the settings to use:

NOTE: This assumes you have general working knowledge of After Effects.

Project Settings
Set your "Timecode Base" to 24 fps

Composition Settings
Set the "Frame Rate" to 24. It is not necessary to use 23.976. Slow motion footage is almost always shot "MOS," that is, there is no sound recorded and no need to alter the frame rate for sound sync. The rest of the settings in After Effects should be set to match the rest of your footage.

Interpret Footage
This is a very important step! Conform the frame rate of your clip to 24 frames per second, then set "Separate Fields" to "Lower Field First." NOV 2008 UPDATE: Many HD cameras shoot upper field first (HDV, for example). If your results look jittery, it's probably because this setting is wrong. Keep in mind also that some third-party plug-ins for After Effects handle the field interpolation themselves, in which case you would not have After Effects separate the fields. The plug-in would take care of this.

Stretching the Video
Now, drop the clip into the composition. Select it in the Timeline and choose "Time Stretch" from the "Layer" menu. Enter a value of 200% into the box. At 200%, After Effects will double and play each field as a whole frame, but this only works if you separate the fields using the "Interpret Footage" command. I haven't tried any other value but 200%, but I don't think there's any point to it anyway since you are already stretching the footage as far as it will go (e.g. there isn't any more video information). If you go over 200%, After Effects will attempt to interpolate frames, and that isn't what we want. At this point, you may be wondering why the video looks really blocky. The reason is because After Effects is "doubling" the fields to make one whole frame. To alleviate this unwanted side effect, choose the best quality rendering option in the timeline. It's the little thing between the (*) and the (f) that looks like a little blocky backslash (\). When you click it, it should change to a nice smooth forward slash (/) and your video should change to look much smoother. Oh and while you're down there, UN-check frame blending if it happens to be on.

Previewing
If you want to preview the slow motion effect, set the preview frame rate in the Time Console palette to 24 (or "Auto," as long as the composition's frame rate is 24). It should play back nicely if you do a RAM preview.

Finishing Up
When you're done with everything, add the comp to the render queue and make sure that Frame Blending is OFF, Field Render is OFF and your frame rate is 24 fps. As long as those settings are correct, it should render properly.


Closing Arguments

This technique will produce the most natural slow motion effect possible. If, for some reason, you want motion slower than what you get with this technique, you will need to have After Effects or a third-party plug-in like Twixtor blend frames together to achieve a time stretch over 200% (thanks to Marcus at dvfilm.com for bringing the existence of this plug-in to my attention). Marcus suggests using the plug-in with 30p footage instead of 60i to realize improved spatial quality since you don't have to interpolate fields. However, at 30p you will be giving up 30 half-frames. It may still be better to shoot at 60i because you will be capturing the most information you can per second, making blending artifacts less evident. You must decide whether to sacrifice spatial or temporal quality. Unfortunately, you can't have both.

I have done experiments with two After Effects plug-ins, FieldsKit and Twixtor. They are both by RE:Vision Effects. FieldsKit is a deinterlacing tool -- it takes interlaced video and converts it to progressive by interpolating the missing fields. It is better than After Effects at doing this because it looks at adjacent frames to figure out the best way to create the missing data. Twixtor does a number of things, but in this case I was interested in its ability to time-stretch my footage and still make it look natural. Twixtor differs from After Effects' method of temporal interpolation (frame blending) by employing a type of motion-tracking technology to create new frames instead of simply blending adjacent frames together (which creates the dreaded motion blur). Although under reasonable circumstances this works extremely well, you may experience some visual "gloopyness" if you try to time stretch the clip too much (over 300%).

As mentioned briefly before, an unfortunate product of frame blending is the introduction of unnatural motion blur. This blur would not exist in real high-speed photography since it would be impossible to expose the film at less than the frame rate, and at frame rates of 60-100 frames/sec, you would not get much motion blur. In fact, you wouldn't even be able to expose the film at either 1/60 or 1/100 respectively, since the rotating shutter in a film camera cannot be open 360 degrees. My advice is to shoot your footage with a shutter speed of at least 1/120 to eliminate motion blur that would not be present at frame rates over 60fps.

Below is a clip that I shot at 1/210 with a Canon GL1 in normal video mode and then manipulated with After Effects using the Twixtor plug-in. Although I used Twixtor to perform the variable timing, the same exact thing could be done using After Effects' built-in Time Remapping function:

Variable Speed Test (24p)

Here's another clip that I shot with a PAL XL1S at 50i, then converted to 24p:

Handstand Speed Test (24p)

I hope someone finds this information useful. I'm always experimenting with peculiar video techniques, so I figured I'd share this with my fellow filmmakers. Now go make something cool.

Thomas Worth
Rarevision

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