DAVE CERF's GUIDE TO SLOMO IN FINAL CUT PRO
Slow motion cinematography is the result of playing back frames for a longer duration than they were exposed. For example, if you expose 240 frames of film in one second, then play them back at 24 fps, the resulting movie is 10 times longer (slower) than the original filmed event. Film cameras are relatively simple mechanical devices that allow you to crank up the speed to whatever rate the shutter and pull-down mechanism allow. Some film cameras can operate at 2,500 fps or higher (although film shot in these cameras often needs some readjustment in postproduction). Video, on the other hand, is always captured, recorded, and played back at a fixed rate, with a current limit around 60fps. This makes extreme slow motion effects harder to achieve (and less elegant) on video, because slowing down the video results in each frame held still on the screen for a long time, whereas with high-frame-rate film there are plenty of frames to fill the longer durations of time. On video, the slow motion effect is more like a slide show than smooth, continuous motion.
One obvious solution is to shoot film at high speed, then transfer it to video (a case where film still has a clear advantage, sorry George). Another possibility is to cross dissolve or blur from one frame to the next. This adds a smooth transition from one still frame to the next. The blur reduces the sharpness of the image, and compared to slowing down images shot at a high frame rate, this is somewhat of a cheat. However, there isn't much you can do about it until video can be recorded at much higher rates. Of course, many film cameras can't shoot at high frame rates either, so the whole super-slow-motion endeavor is somewhat specialized no matter what medium you are using. (There are some high speed digital cameras available now that allow you to capture lots of digital frames directly to your computer, so technology is starting to catch up with film. However, this feature isn't going to appear in consumer camcorders any time soon.)
Like almost all video applications, Final Cut Pro slows down video by playing each frame longer than originally intended. For example, if you set the speed of a 25fps clip to 25%, each frame plays back 4 times as long. Instead of each frame lasting 0.04 seconds, each frame lasts 0.16 seconds. This makes a one second clip last four seconds. If you slow down the video to 4%, each frame lasts for one second, making a clip that was originally one second long last for twenty-five seconds. However, watching each frame for an entire second looks more like a slide show of successive frames than slow motion created with a high speed film camera.
To solve the "slide-show" look of the long frames, a combination of motion blur and cross dissolving can be enabled. In Final Cut Pro, this is known as Frame Blending. Frame Blending causes the images to soften, but makes the static still images transition more slowly, creating the illusion that additional temporal information exists between frames.
Final Cut Pro can easily slow down video clips using the Modify Speed command (Modify > Speed, or Command-J). Constant speed changes are straightforward: enter a percentage higher than 100% for faster playback; enter a percentage lower than 100% for slower playback. You can enabled Frame Blending at this point, or any time you choose Modify > Speed.
Variable speed changes are more complex, and are the result of the Time Remapping feature (available since version 4). Although there are all sorts of things you can do with Time Remapping, I'm focusing on slow motion here. Time Remapping is simple, but poorly understood. One common misconception is that Time Remapping changes the speed of a clip. Technically, this isn't true. The frames of a clip always play back at the rate of its parent sequence. So, if you have a 30fps sequence, every clip in that sequence churns out frames at 30fps. The way that variable speed effects are created is by changing WHICH frames of the clip are played back, and how many times they are repeated (slow motion) or skipped (fast motion).
To better understand this feature, it might be helpful to give the feature a more accurate name, such as Choosing Which Frames of A Clip's Media File Play Back Within the Duration of the Clip and For How Long. While this is a mouthful, it accurately describes what Time Remapping allows you to do. For example, to create a still frame from the first frame of a clip, you use Time Remapping to make the clip only playback frame 1, no matter how long the clip lasts.
Think of a clip's media file as a container that holds a bunch of images, or frames, that are organized in chronological order (frame 1, frame 2, frame 3, and so on-as recorded by a camera). Normally, these frames play back in rapid succession at a given frame rate and in chronological order. For example, suppose you have a media file that contains 30 frames, and the media file has a fixed playback rate of 30fps. In one second, the clip in the Timeline outputs 30 frames. Under normal circumstances, when the clip plays back, it finds frame 1 and presents it. Then it grabs and presents frame 2, and so on, until all 30 frames have played back in chronological order. In other words, frame 1 in the media file MAPS to output frame 1 in the clip in the Timeline. Media file frame 2 maps to output frame 2 in the clip, and so on.
Time Remapping allows you to choose which media file frames play back in the clip in the Timeline, and when. For example, given the same 30 frames stored inside the clip's media file, you could choose to play back frame 1 over and over again, each time a frame is output. The effect would be a still frame (the ultimate slow motion: 0fps). Or, you could tell the clip to play back frame 30 first, then frame 29, and so on, until frame 1. The effect would be reversed speed. Remember that the clip always plays back at a fixed rate (the rate of its parent sequence), but you control which media file frames play back and when.
The Time Remapping feature presents information in a slightly unorthodox way to fit within the normal Final Cut Pro keyframe editor paradigm. As with all other parameters, the horizontal axis represents sequence and clip time. However, the vertical axis, which normally represents a scalar value (such as clip opacity or drop shadow offset) represents the frame numbers of the frames in the clip's media file. For example, if a clip's media file contains 300 frames (10 seconds at 30fps), the vertical axis ranges from 0 to 299. When you place a keyframe at a particular point in time (horizontal axis), the vertical value is the number of the media file frame that the clip will display at that moment.
NOTE: Remember that a clip and a media file are not the same thing. A clip is an object in Final Cut Pro that references a media file on disk. A clip's duration can be altered but the media file is unaffected. Therefore, even if you shorten a 10-second clip down to 2 seconds, Time Remapping still has access to all 300 of the frames in the media file. This means you can map any of those 300 frames anywhere you want within the 2 second clip.
When you create Time Remapping keyframes, you are saying "I want this particular media file frame to appear at this point in time in the sequence." When you place multiple keyframes, Final Cut Pro interpolates between them. To create slow motion effects, you place keyframes that are horizontally distant (a long time) but vertically close (a small range of frames). For example, when creating a 10 second slow motion effect from one second of video, you would do the following:
- Place the first keyframe at time 0 in the sequence, and set its value to 0. This means: "Starting at sequence time 0, play frame 0 from this clip's media file."
- Place a second keyframe at time 300 (10 seconds after the first keyframe), and set its value to 30. This means: "Ending at sequence time 300 (10 seconds later), play frame 30.
Final Cut Pro interpolates between frame 0 and frame 30 over the course of 10 seconds. In general, gentle Time Remapping ramps and curves (as opposed to steep ramps) result in slow motion. Upward ramps result in forward motion; downward ramps result in reverse motion. Adding bezier curves allow you to create gentle transitions from one speed to another.
The interface is slightly non-intuitive, so it takes some practice. The best way to get started is to do simple examples until you get the hang of it. Here's a simple one to get you started:
- Create an empty sequence.
- Double-click a clip in the Browser to open it in the Viewer.
- In the Viewer, press I to mark the first frame as the In Point.
- Click the duration field in the Viewer, then type 1000 to set a clip duration of ten seconds.
- Drag the clip from the Browser or Viewer into the beginning of the first video track in the Timeline. Make sure the clip and the Timeline have matching settings, including frame rates, to make sure no rendering is necessary. Also, audio is unnecessary for this example, so you can disable the audio source controls when you perform the edit or unlink the audio clip items in the Timeline and delete them.
- Control-click the Current Timecode field in the Timeline and choose Frames from the pop-up menu. Viewing a raw frame count, instead of timecode, is educational when you're first experimenting.
- Activate the Timeline, then press Shift-Z to expand the Timeline to the duration of the clip.
- Select the clip, then choose Modify > Speed (or press Command-J).
- In the Speed dialog that appears, choose Variable from the pop-up menu, choose Frame Blending, then click OK.
- In the lower left of the Timeline, click the Clip Keyframes control to display the Keyframe Graph area in the Timeline.
- Control-click on the same control and make sure that the Video > Keyframe Editor option is enabled.
- Control-click in the Keyframe Editor area beneath the clip, then choose Time Remap > Time Graph. The Time Remapping value graph line appears, showing two default keyframes that were added when you applied Variable Speed in the speed dialog.
- Double-click the clip so that it opens in the Viewer, then click the Motion tab.
- In the Motion tab, click the Time Remap disclosure triangle.
- Press Shift-Z to expand the ruler in the Viewer to the duration of the clip. In the Keyframe Graph area in the Viewer (the portion that looks like a Timeline), notice that the lower end of the vertical axis is labeled with 0 and the upper end is labeled with as well. Each point on the vertical axis represents a frame in the clip's media file. 0 is the first frame, and the high value (which depends on the duration of your media file) is the last frame. Assuming your clip is 30fps and set to 10 seconds long, the first keyframe is set to 0 and the last keyframe is set to 300. By default, when you applied Variable Speed to the clip, bezier handles were added to both keyframes. You can remove these by control-clicking each point and choosing Corner.
- Drag the second keyframe downward until the Tooltip displays 30. Tip: You can reduce how quickly the keyframe value changes by holding the Command key while you drag up or down. The clip now plays back at 10% speed (slow motion). Although the clip is 10 seconds long, only the first 30 frames are played back during this time.
You can now make further adjustments to in the Viewer or in the Timeline. You can add additional keyframes to continually modify the speed over time, and add bezier curves to subtly transition between speed values.
IN AFTER EFFECTS
Using the fields, cranking interlaced DV footage to 50% slow motion in After Effects is a piece of cake. Just right-click the footage file, choose "Interpret as", and choose deinterlacing. In the comp window, time-stretch the layer to 200%. Voila.
Move the slider TOWARDS the turtle and AWAY from the rabbit.
PLUGINS FOR AFTER EFFECTS & FINAL CUT PRO
Twixtor by RE:Vision Effects, Inc.
ReelSmart Twixtor enables you to speed up, slow down, or frame rate convert image sequences with visually stunning results. In order to achieve its unparalleled image quality, Twixtor synthesizes unique new frames by warping and interpolating frames of the original. Features include:
- Calculation of motion vectors at each pixel
- Warping and interpolation of frames to "time warp" input footage
- Smart processing of fields in the input and output as appropriate
- Stretching of a sequence using a single scaling parameter
- Keyframable retiming for complete control on a frame by frame basis
- Frame rate conversions made easy (Note: frame rate conversions are only supported in After Effects)
ReTimer Pro by REALVIZ
check out the amazing Phantom high speed digital cameras by Vision Research.
Achieve Natural Slow Motion with Inexpensive Camcorders.
by Thomas Worth
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