The following tutorial requires Adobe After Effects. I’m not sure what versions this works on, but I’m using CS3 (8.0.2). A trial download of After Effects CS4 can be found here. I’m currently testing this conversion in Compressor and will post that tutorial later.
At TGR, slow motion is a huge part of the process. While I was there we used the Arri S 16mm camera extensively, which has a variable speed motor on it. We almost never shot 24fps. It was either slightly over-cranked, or really over-cranked. Ski and snowboard movies are now known for their slow motion look. Seeing professional skiers shred a line in real-time almost looks strange to me now. As a result, we never shot any action on video. I’m not a fan of the “True HD” ski films that have been coming out over the years because they just don’t look right to me. Another reason for the slow mo, was to get steadier shots from the helicopter. Most heli shots are handheld, and slowing to 48fps can really take the shake out. Well, the shake is still there, but it’s less apparent. Which brings me to my first point:
Spacial vs. Temporal Resolution
Your eyes see resolution in 2 ways – spacial, which you can think of as the raw number of pixels (or grains with film) of an image. HD is 1920×1080. This is its spacial resolution. Temporal resolution is based on time, in the case of video – the frames per second. Your eye and brain will think 2 different images of different resolutions will appear to have the same sharpness if the lower resolution image has a fast enough frame rate. This is why there are 2 HD formats: 1080i at 30 frames per second and 720p at 60 frames per second. These different formats were introduced at about the same time, both claiming to be HD. 720 is only half the resolution of 1080, but it runs at twice the frame rate. But I thought only 1080 is “true HD”? This is a myth perpetuated by marketers, satellite broadcasters and networks that want to reject your tape submissions. When 720p is played back at 60 frames per second, it will appear to have the same resolution as 1080 at 30 frames per second. But since TVs and broadcast capable of 60 frames per second never really happened, people think of 720p as lower resolution. With the prevalence of 24p this is even more the case because 720p24 doesn’t look anywhere near as sharp as 1080p24 because it’s not. Film takes a slightly different path on this. It is generally accepted that one frame of Super 16mm film has a resolution of 1920×1080, all else being equal. If you take a second Super 16mm film frame at 1920×1080, its grains will be in slightly different locations than on the previous frame. This is just part of the film emulsion manufacturing process. Grains will always be in different places. With video, there is a pixel grid that never moves. Each 1920×1080 frame will have the same pixel locations as the other 1920×1080 frames. But since film has grains constantly changing locations, its temporal resolution goes up as you add sequential frames. If you’re watching a film print in a theater, a sequence of Super 16mm images will appear to your eye as having a higher resolution than 1920×1080. This is why I find it interesting and annoying how some broadcasters will accept 720p sources such as Varicam, but not Super 16mm. Or accept XDCAM EX at 1080i from the Sony EX3, but not DVCPRO HD at 1080i from the HVX200, when the DVCPRO HD codec is in a higher color space (4:2:2 vs 4:2:0) and higher bitrate (100Mbps vs 35). This is also why as each new format is released (VHS, DVD, Blu-Ray, etc) you will see the studios rerelease their movies. Since most of them originated on 35mm or 65mm film, it will take a long time for the consumer formats to catch up to their superior resolution. The Wizard of Oz may have been made in 1939, but it’s still higher resolution than Blu-Ray. This is what I call the Resolution Myth.
Getting 60p from 60i
HDV cameras such as the Canon HV20 shoot 60 interlaced frames per second. This then gives you 30 frames per second. But that’s not entirely true. When shooting 60i, the camera is still capturing 60 moments in time. If you take 2 interlaced fields to make one frame, those 2 fields were captured at 2 different points in time. If you’re shooting 1920×1080 at 60i, you could think of it as 1920×540 at 60 frames per second since each field has alternating horizontal scan lines. Because these interlacing lines are alternating – field one is all odd scan lines and field two is all even scan lines, you can interpolate the missing lines by looking at the lines in alternate frames using both spacial interpolation (what the adjacent lines in the same frame look like) and temporal interpolation (what the lines in the previous and following frames look like). Did I lose anyone? The short of it is that the missing scan lines can be created very easily and with very low error in After Effects. What if my source is 30 frames (sometimes referred to as 30p, 29.97p or 29.97Psf)? This method will not work because your camera was capturing 30 moments in time vs. 60 moments with 60i. It is possible to create new frames with a programs such as Twixtor or Shake, and maybe with the speed control in the new Final Cut 7. But in creating a 60p result, it is better to have a 60i source than a 30p source.
The Method (60i to 60p to Slow Mo at 23.98)
For anything that you want in Slow Motion, shoot using a fast shutter, ideally 1/120th or faster. If your shutter is too slow (or wide for you film guys) you will get too much motion blur. For an approximation of the 180 degree shutter that most filmmakers use, use 1/120th since you’re really shooting 60fps even though your camera is set for 60i. Then capture using your NLE of choice using your normal settings.
The following assumes that you’re familiar with After Effects
1. Drop your clip into AE. Select the clip. Select File > Interpret Footage – Set to 23.976 and set “Separate Fields” to “Upper Field First” for HD sources.
2. Drop the clip on the New Composition droplet in the Project window.
3. Select Composition > Composition Settings and set the frame rate to 23.976.
4. Put the clip into your Composition. Select the clip in the timeline and select Layer > Time Stretch from the menu bar. Enter 200%. This will take each field (half of a frame) and turn it into a whole frame. Remember the number that appears as the new clip length. If playback seems to jump back and forth, double-check that you selected the correct field dominance in the Separate Fields box in Interpret Footage. This has turned your 60i footage into 60p. Since both your timeline and Interpret Footage are set for 23.976, the footage will now play in slow motion.
5. Go back to Composition > Composition Settings and double the duration of the Composition (enter the number that appeared in step 4).
6. Export through the Render Queue. Make sure Frame Blending is OFF, Field Render is OFF and your frame rate is 23.976. I prefer to use Apple ProRes as my export codec. If your source codec is HDV or DVCPRO, the sooner you can get out of those codecs, the better. If you export to ProRes, you will want to change the output resolution to 1920×1080 (HDV is 1440×1080 and DVCPRO HD is 1280×1080). In the Output Module Settings select Stretch and stretch to HDTV 1080 24.
That’s pretty much it. This method only works for sources that are 60i that you want to slow down to 23.976. If you have 60i footage that you want to play at 23.976 in realtime (the 24p film look), then you need something like Magic Bullet Frames. If you want to want to take a progressive source and change its frame rate or speed, your best bet is Twixtor. People who are currently using the Canon 5DMkII, which only shoots to 30p, are successfully using Twixtor to create 23.976 output.
After using this method on 60i footage from a Canon HV20, I have been pleasantly surprised. I never thought I would be able to get such usable slow motion from a 60i video camera. If shooting at a high frame rate produces a look that’s too stuttery to your eye, try adding some motion blur. ReVision Effects makes a great motion blur plug-in called ReelSmart Motion Blur. Using all of these different tools, it’s possible to shoot 60i all the time, and be able to create 23.976 output for both realtime and slow motion. Pretty cool.
EDIT: Added step 5. If you’re doubling the length of the clip, you also need to double the length of the Composition or else the clip will only play out halfway through. Unlike FCP, After Effects timelines are a set length.