Twixtor is a powerful plug-in that 'intelligently slows down, speeds up or changes the frame rate of your image sequences' as described by its creator Re:Vision Effects, Inc.
Twixtor achieves its well-known slow motion effect by inferring previously non-existent frames between pre-existing 'real' frames of your video footage, a process known as interpolation. Because the algorithms underlying Twixtor are so good at tracking motion between discrete frames it is able to fabricate fairly accurate 'filler' frames that fit between the pre-existing frames of your video and thereby represent what is probably occurring between one time point and another. Because a heap of new frames are created after applying the Twixtor effect the time taken to play the effected footage from start to finish at any given frame rate increases and the illusion of slow motion is created. A key feature of Twixtor is the ability to re-time the playback of particular frames within a video clip, otherwise termed 'time remapping' or 'keyframing'. This can be used to both slow down and speed up a given section of footage. In this blog post I provide a step-by-step guide to utilising the time remap feature of Twixtor in Sony Vegas/Movie Studio to achieve both a fast motion and slow motion effect using a clip of a racecar as a case study.
TIME REMAPPING IN TWIXTOR: A HOW-TO GUIDE
Here is the short clip I want to time remap:
And here is what I want to end up with after using Twixtor's time remap feature:
The time remapped clip is actually part of a larger promotional video I produced for Curtin Motorsport Team, an engineering team at Curtin University in Western Australia. You can view the full video here if you want to see how the fast-slow-fast motion transition within this clip was used in the context of the whole video (the above clip appears at approximately 0:28). The idea with the original clip was to modify it such that it slowed down for the duration of the racecar's turn to capture the sliding motion of the vehicle and to inject a bit more drama and 'cool' factor into the video. A previously unintentional side effect of this was a speeding up of the clip both before and after the turn, but we'll get to that. Here's how to go about time remapping your own video clip:
1. Import the clip you want to time remap into the Sony Vegas timeline, and cut it to size. For the racecar clip, all I want is a clip of the racecar's entire sliding turn, plus a second or two of footage either side of that. All in all, I end up with a clip 78 frames in duration. I recommend changing the time format at this point to Absolute Frames (as I've done in the image below), as you'll find it will make it easier to time remap the clip. To do this, right-click on the large time display at the top left of the Timeline window, select 'Time Format' then 'Absolute Frames'.
2. Now you need to decide where exactly in your clip you want the slow motion effect to kick in. Before you do this, zoom in to your clip by clicking the 'plus' (+) or 'Zoom In Time' symbol situated at the bottom right of the Timeline window enough times such that when you press the left right and arrow keys on your keyboard to scroll through your clip, each press of a key equates to moving back or forward by a single frame. You can confirm this by referring to the Absolute Frames time display you just enabled. For the racecar clip, I want the slow motion effect to start at frame 35, when the car has pretty much just begun its sliding turn (see below). At this point all you want to do is simply note the frame number down.
3. Now find where you want the slow motion effect to end. For my clip, this is at frame 60, when the car has just come out of its turn and started to straighten up (see below). Again, note down the frame number.
4. Apply the Twixtor effect to your clip by finding the plug-in in the Video FX toolbar on the left side of the Sony Vegas interface, then click and drag the Default option onto your video clip in the Timeline window.
5. In the Twixtor options panel, leave Display set to Source and set Time Remap to Frame Number. For this clip I will set all other options as follows but please note these settings may not work best for what you are trying to achieve (I suspect I will address optimal Twixtor settings in a future post):
6. Click on the small clock or 'Animate' icon to the far right of the Frame slider panel and a small timeline window will drop down within the Twixtor options panel. Ensure 'Sync Cursor to Media Timeline' (it looks like a padlock beside a cursor line) is selected. Enabling this means that when you change the position of the cursor in the Vegas Timeline window the cursor in the newly appeared Twixtor timeline window will reflect that change and vice versa. Give it a try to make sure you understand how it works.
Now it is time to describe the Twixtor timeline window in more detail so you know what exactly you are dealing with. There are two lanes labelled 'Twixtor' and 'Frame'. Think of the Frame lane as a direct representation of the normal Sony Vegas Timeline window. Think of the Twixtor lane as a representation of how the original frames of your video clip as they now exist in time are being remapped to earlier or later points in time. Here's what I mean by that. At the moment you can see two 'nibs' or shape dots, one in each lane of the Twixtor timeline. These nibs represent keyframes, or key points in your video clip which signal the start or end of a slow or fast motion section of your video clip. At this point, these nibs are positioned at the '0' frame timepoint of the clip i.e. the very beginning of the clip (see below). If you hover your mouse cursor over the bottom nib (the one in the Frame lane) two numbers will appear. The bottom number refers to a particular frame from your original clip. This number is always the same as the frame number indicated by the cursor in the Twixtor timeline window (and, if your cursors are synced, the Sony Vegas Timeline window). At the moment this number is '0', because that's where the nibs are currently positioned. The top number, however, represents the new frame number to which the original frame has been reassigned in time. At the moment this number also reads '0', which means no reassignment has occurred and the clip is as it has always been. The positioning of these nibs means your 'Twixtored' clip will start where the original clip started, which is probably a good thing. Once you've reassigned a frame from your original clip you will understand the above a little better. So let's do that now.
You're now going to specify where exactly you want the slow motion effect to begin in the yet-to-be-created clip. Please note the difference between what I've just stated and what you did previously, which is figure out where exactly in the original clip you want the slow motion effect to begin. Remember, we are reassigning frames in time which means any given frame from the original clip that we want to signify the start of a slow or fast motion section will be remapped to a new timepoint in the Twixtored clip. Key to understanding this idea is the fact that your new clip will not actually be longer in duration than your original clip, even though you are applying a slow motion effect to a good portion of it. Check the two clips at the beginning of this post for confirmation. They are as long as each other, except the frames have been remapped in the second clip so that the middle section (where the car is turning) takes up a much larger portion of the clip than it previously did, whilst the beginning and end sections of the clip have been squeezed into much shorter time spaces.
7. In either the Twixtor timeline window or Sony Vegas Timeline window, move the cursor to that point in time where you want the frame from the original clip (i.e. the one you first noted down earlier) to appear in the Twixtored clip. Once you've done this, enter the frame number from the original clip into the text field of the Frame row in the Twixtor options panel and press Enter. A new pair of nibs will appear in the Twixtor timeline window signifying that you have successfully remapped a frame from your original clip to a new frame timepoint in the Twixtored clip.
For the racecar clip, I want that part of the clip where the car has just begun its sliding turn (i.e. frame 35) to start at a much earlier point in time (say, frame 10) in the new clip. Therefore, I will move my cursor to frame 10 in the Twixtor timeline window and type '35' into the aforementioned text field and press Enter. This has reassigned frame 35 from my original clip to frame 10 of the new clip. This means everything before frame 35 (i.e. frames 1-34) in my original clip will be squeezed into frames 1-9 in the modified clip, thereby significantly speeding this section up.
By hovering my mouse over the newly created nib in the Frame lane of the Twixtor timeline window, I can see that frame 35 (as indicated by the bottom number) from the original clip has now been remapped to frame 10 (as indicated by the top number) of the Twixtored clip. To visually confirm this you can change the Display option at the top of the Twixtor options panel to 'Twixtored output'. In my case, when the timeline cursor is positioned at frame 10 in either of the timeline windows the preview window at the top right of the Sony Vegas interface displays the frame that used to be placed at the 35th frame in my original clip.
You might be wondering how I arrived upon frame 10 as the timepoint at which frame 35 from the original racecar clip should be remapped. To be honest, this is probably what will you spend the most time figuring out as you develop your Twixtored clip and will depend on your preferences for timing, look and feel. By remapping frame 35 to frame 10 I feel that the slow motion section of the Twixtored racecar clip kicks in when the music in the Curtin Motorsport Team video complements it most. I also like the rate at which the 'squeezed' frames play before (and after) the slow motion effected section. Another thing to remember is that the earlier in time you remap the original frame, the stronger the slow motion effect is going to be, depending on the frame timepoint at which you specify the effect to stop. Let's go ahead and do that now.
8. This step is very similar to the last, in that you are going to reassign the second frame number you noted down earlier to a new position in the Twixtored clip. This is the last frame of the original clip that you want to apply the slow motion effect to. If you changed the Display option to 'Twixtored output' at any point, change it back to 'Source' now. Once again, navigate to that part of the clip where you would like the last frame of slow motion from the original clip to appear in the Twixtored clip. This is probably somewhere just prior to that point in time at which the original clip ends. Once your cursor is in position, type the second frame number you noted down earlier into the text field of the Frame slider panel in the Twixtor options panel and press Enter. A new pair of nibs will appear in the Twixtor timeline window, showing that a new frame has been remapped.
For the racecar clip, I want that part of the clip where the car emerges from its sliding turn (frame 60) to appear right near the end (around frame 68) of the new clip. Therefore, I will move my cursor to frame 68 in the Twixtor timeline window, type '60' into the Frame row text field and press Enter. As a result, frame 60 from my original clip is reassigned to frame 68 of the new clip. Because of this, everything after frame 60 (i.e. frames 61-78) in my original clip will be squeezed into frames 69-78 in the modified clip. Also note that whilst that part of the clip where the car is turning originally played for a duration of 25 frames (i.e. frame 35-60) at a rate of 29.97 frames per second (fps) this section will now play for a duration of 58 frames (i.e. frame 10-68) at a rate of 29.97 fps. This means this part of the clip will be more than twice as slow as before.
9. Now you have but one final keyframe to map; the one at the very end of the clip. Identify how many frames your original clip comprises by navigating to the end of the clip in the Sony Vegas Timeline window and make sure the vertical cursor is placed over the last frame. Remember, the Twixtor timeline cursor will mimic its position so don't worry about doing the same in the Twixtor timeline window. Type that frame number into the Frame row of the Twixtor options panel like before and press Enter. Your new clip's final keyframe has been mapped and your clip is ready to be previewed/rendered.
I know my racecar clip is 78 frames in length by left-clicking at the very end of the clip in the Timeline window and identifying the frame number shown on the timecode display. Therefore, I will type '78' into the Twixtor Frame row text field and press Enter. This creates my modified clip's final time remap, although in essence all I've done is simply tell Twixtor that frame 78 from the original clip should stay where it is (like frame 0 of the original clip, which is mapped to frame 0 of the modified clip as previously mentioned), therefore creating a Twixtored clip of identical duration.
I find the best way to go about making fine adjustments to my project and previewing it without without having to render the clip every time is by selecting/highlighting that part of the clip I want to watch in the Sony Vegas Timeline window and pressing Shift + B. This buffers the selected region for smooth playback (Twixtor will sap a fair portion of your computer's graphics processing power, making high-quality playback of your clip in real time virtually impossible in most cases). Once I'm happy with how the clip looks in the preview window I'll go ahead and render the project.
If you want more information on time remapping or any other Twixtor-related topic within the context of Sony Vegas, Sony Movie Studio or another non-linear editor I recommend taking a look at Re:Vision Effects, Inc.'s tutorials page here. Alternatively, throw me a line and I'll try to help you out!
This post, like all others I author, is effectively a work in progress; updated as needed. Was it informative? Easy to understand? Feel free to leave any comments or questions below!
Tim Szewczyk -