Smoother Slow Motion

from interlaced footage with Premiere 6.5 and Photoshop

About Interlace

PAL, NTSC and even High Definition formats like 1080i or 720i use a technique called interlace to essentially get a 50 fps (frames per second) image from a video image recorded at 25 fps. They do this by storing two different images in the same frame. The images are called fields and the frame is said to be interlaced.
When your TV receives the video, it scans every second line in the frame from top to bottom and then starts again at the top of the same frame and scans the lines it missed the first time.
In order to illustrate how interlaced footage works, I've animated a circle travelling from left to right across the screen at 50 fps and then converted it to 25 fps interlaced.
In the picture on the left, you can see a still from the original image of the circle animation at 50 fps progressive. On the right, you can see what happens after the animation is converted to 25 fps interlaced, two fields are present in the one frame of video.
Below is an example of footage from a PAL DV camera of a girl playing softball, you can clearly see the two fields.
When you slow something down via the Speed setting, Premiere deinterlaces the footage down to 25 fps progressive, discarding half the fields and then makes duplicates of frames in order to make the clip slower. This is shown by the Field Options dialog box which has an option for "Deinterlace when speed is below 100%" (shown below).
If the checkbox is cleared, strange things start happening when Premiere repeats interlaced frames and they are interpreted by your TV with jittering. So how do we get slow motion from Premiere using all the information the camera captured?

How To Do It

We're going to use something called a track matte. This is a keying feature of Premiere where you have two pieces of footage which you want to merge and a greyscale image or video of the transparency you want called a matte. Areas of black in the matte are replaced with one of the clips and areas of white are replaced by the other. Grey areas are merged proportionally to both clips. We're also going to use Photoshop to deinterlace the footage so that we can split up the original clip into two clips which contain different fields of the original clip at 25p. I live in Australia where the video standard is PAL so this tutorial will use PAL footage to

Step 1. Place the clip which you want to slow down on the timeline and select File -> Export Clip -> Movie... When the Export dialog box appears, click on Settings... and then change the File Type to Filmstrip. Then save the Filmstrip on your disk.
Step 2. Open up Photoshop and open your Filmstrip. You'll see all the frames contained on one image map. Select Filter -> Video -> De-interlace... and set the radio buttons to eliminate Even Fields and to create new fields by Interpolation. Press OK and let it render. Since you've discarded the even fields, you're left with fields 1, 3, 5, 7 e.t.c. Select Save As... and save the Filmstrip as odd.flm or something meaningful.
Step 3. Open your Original, deinterlaced filmstrip again in Photoshop and repeat the step above, this time eliminating the Odd frames and saving as even.flm or something else meaningful. The image below shows the example still broken up into different fields.
Step 4. Now we're going to make the track matte. Open up Premiere again, right click on the empty bin and make a black colour matte. Drop this matte on to the timeline. Now look through the video effects and apply the effect Strobe Light to the black matte on the timeline. For PAL video, set the duration to be 1/25 second (0.04) and the period to be 2/25 second (0.08). For NTSC, choose 1/30 (0.333) and 2/30 (0.667). Now extend the length of the strobed black video until it is about twice as long as your original clip.

Step 5. Now add a new video track by right clicking on the timeline and clicking Add Video Track. Put your track matte on Video 3. Import your odd and even film strips, placing the odd one on Video 2 and the even one on Video 1A. Right click on both filmstrips and change their speeds to 50% with the Speed... option. Trim your track matte until it is the same length as the film strips. Now right click on odd.flm which should be on Video 2 and select Video Options -> Transparency... Change the drop down box to Track Matte and press OK.
Right click on the yellow work area indicator and click Render Work Area or press Enter to preview your slow motion scene. If the motion jerks backward and forward, you have the film strips in the wrong order. The easiest way to correct this is to go into the Transparency settings again and check the Reverse Key button. After rendering the shot should play properly.
So that's it, a method for getting a smoother slow motion shot from Premiere which uses every field your camera captures. You could now export the clip as DV to use in your movie. If you slow this clip down further it will look smoother than the original clip slowed down the same amount.
I hope you found this tutorial useful and easy to understand.


For more information on Interlace, try this Wikipedia article

Posted by Andrew at 10:39 pm, Sunday 20th January, 2008Permalink: http://productions.andrewgentle.com/tutorials/deinterlaced_slow_motion/

---

Comments