Interlacing Progressive Footage
Interlacing Progressive Footage
A Creative COW Tutorial

 

Interlacing Progressive Footage

Marco Solorio Marco Solorio
OneRiver Media
Walnut Creek, CA, USA

©Marco Solorio and CreativeCOW.net



Article Focus:
We've all heard of de-interlacing footage, but what about re-interlacing? Marco Solorio of OneRiver Media shows us how to create interlaced footage from a progressive source. The results are picture perfect!

So why on Earth would you ever want to purposely interlace footage that was shot in a progressive format? Case in point: you’re working on a project that was shot entirely in interlaced format (60i) and you’ve been given some additional footage, maybe B-roll, or even motion graphics, that were given to you in progressive format (30p). When adding this 30p footage to the editing timeline, you quickly realize the flickering of the 30p footage just looks too out of place against the rest of the 60i footage in your timeline. Rather than keeping the 30p footage as is, this tutorial shows how you can turn your progressive footage (30p) into crystal clear interlaced footage (60i) with results so true that it will completely look as if it was shot on a 60i video camera!

Okay so let’s say you have some progressive footage that you need to apply to an interlaced video project. Typically, this may be a 30p source that needs to be 60i or a 24p source that needs to be 60i. Generally speaking, you can simply add the 30p clip right in the 60i timeline and you’re good to go. Likewise, if you have a 24p clip, you can just as well add 3:2 pulldown and boom, ready for a 60i timeline.

But if the progressive clip still just looks too jittery against the rest of the other 60i footage, something needs to be done. Adding interlaced fields from a progressive source isn’t too far-fetched as one might think. Like the de-interlacing process, "re-interlacing," for lack of a better term, is also done by means of pixel interpolation. Where de-interlacing may cut the vertical resolution in half, re-interlacing does something of the opposite by adding spatial interpolation. More specifically, it doubles the frame count.

In reality (and strictly speaking in terms of image quality), an interpolated re-interlaced clip can actually look better in quality than an interpolated de-interlaced clip. With that in mind, I've created examples you can download from this tutorial and drop in your DV timeline to see for yourself. I think you'll be surprised by the results.

For the sake of simplicity, I'll go over the methods that are relative for the same time-base. In other words, I'll first show you how to convert a 30p clip to 60i. The same method is used for converting a 25p clip to 50i. Later, I'll discuss converting 24p to 60i and 50i.

     

Frame Rate Naming Conventions

60i = 29.97 FPS, interlaced
60p = 59.94 FPS, progressive
50i = 25 FPS, interlaced (PAL)
50p = 50 FPS, progressive (PAL)
30p = 29.97 FPS, progressive
25p = 25 FPS, progressive (PAL)
24p = 24 or 23.976 FPS, progressive

The number represents the amount of images (not frames) to be seen in one second. E.g., in 60i video, there are 2 images for every frame of video (vertical resolution cut in half for each image... one image for even fields and another image for odd fields), thus a total of 59.94 "half" images are viewed every second.

The letter after the number says whether those images are interlaced (half resolution) or progressive (full resolution). Some people refer to "30i" to represent 29.97 interlaced frames, but this technically incorrect. In reality, 30i means 15 FPS, interlaced, which does not exist in any video standard.


The Tiki Test Subject

I've used one of my stock footage clips from my collection at RevoStock.com called, "Brady Bunch Tiki: V2" as a test clip to use as an example for this article. This original clip is 1080p24 (and is also supplied as 720p24, 576p25 PAL and 486p24 NTSC at RevoStock). For this tutorial, I created a master file by conforming it from 24p to 30p and reduced it down to SD letterbox 4x3. It's from this converted SD file on which we'll perform all our tests. I thought this clip would work well for this test since the tiki figure moves from frame-left to frame-right, ideal for interlacing.

Click to download
Figure 1 - A single frame from the 30p source. Click the image to download the clip.


Doubling Your Frame Count

The first step is to double up the literal frame count. This is accomplished in one of two methods.

  1. Double the TRT of the 30p clip so it's twice as long. Or...
  2. Double the frame-rate of the original file from 30p to 60p.

In order to do this properly, the new frames need to be interpolated by means of a vector-based pixel warping or morphing algorithm. This can be accomplished by a variety of different applications, including Motion 3 (by use of the Optical Flow feature), After Effects (by use of Layer > Frame Blending > Pixel Motion), Shake, the Twixtor plugin (which can be used in Final Cut Pro, After Effects and several other host applications), Boris FX, or any other application or plugin that can double the frame rate or frame count by means other than merely frame-blending the frames. You do NOT want to frame-blend this step or the entire process will not work.

The best way to tell if this step is working correctly is to look at the new frames that have been created. If they have an overlapping ghost look to them, then it's frame-blending, which you do not want. If the new frames literally look like new frames with no ghosting or overlapping, then you're on the right track.

Interlacing
Figure 2 - Red circles show that this clip has been frame blended for doubling the TRT. We DON'T want that!


Convert to an Interlaced Format

Once your clip is rendered with twice the frame count, you'll now be ready to interlace it, which can be done in After Effects, Final Cut Pro and pretty much any other video application. I found that After Effects renders out a cleaner interlace (actually, a perfect interlace) than does Final Cut Pro, so you may want to keep that in mind. If your clip is now 30p at twice the TRT, you'll want to turn it into a 60p clip at normal speed. Here's how:

Adobe After Effects
Select the rendered clip in the Project window and right-click it and select Interpret Footage > Main. In the Frame Rate section, conform the frame-rate to 59.94 FPS.

Frame Rate

The clip will now be 60p. Create a new 60i comp in After Effects and place the 60p clip in the timeline. Even though your timeline is only 29.97 FPS and you can't see the extra frames when scrubbing frame by frame, don't fear; when you render the final clip, it will use the extra frames in the 60p clip to create the new fields.

Let's go ahead and render this as a final clip by selecting Composition > Make Movie from the menu bar. This should open up the Render Queue window with a new comp in the queue. You'll need to change the Render Settings either by selecting a pulldown option next to it or by clicking the name next to the pulldown option.

The thing to absolutely keep in mind is to make certain you render this clip with Field Rendering turned on. You'll need to select either Upper Field First (UFF) or Lower Field First (LFF), depending on your editing hardware and format of choice. For the samples provided in this tutorial, I rendered to NTSC DV, so LFF it will be.

Time Sampling

From there you'll want to select your format choice and render location. Click Render and you're done.


Final Cut Pro
Bring your rendered 30p clip (that is now twice the TRT) into your FCP timeline. Right-click and select, "Speed..." Change the Speed setting to exactly 200% and make sure Frame Blending is turned on. This will not interpolate new motion as one may think with the "Frame Blending" nomenclature. Rather it's telling FCP to turn on field-rendering on anything that can accept it. Since we shrunk the TRT of the clip by half, it's essentially a 60p clip now. If the clip you brought to the timeline was 60p, then you're already one step ahead.

Now when we render the clip in the FCP timeline, you have a finished 60i clip with smooth motion.


The Finished Clip

When everything is rendered correctly, you'll have an interlaced clip that's nothing short of amazing. For my testing, I used Twixtor in After Effects to perform the initial frame doubling. I then rendered the final clip using After Effects as described above and below is the result of that render. Place the clip in your DV timeline and compare it to the original 30p clip. It's next to impossible to tell that this clip was once progressive! It's easy to think that the clip was shot using an interlaced video camera (when in reality, the original clip was shot using a Canon 20D DSLR still camera, using a 60mm prime lens with a combination of time-lapse and a motorized motion rig).

Click to download
Figure 3 - A properly rendered clip will employ true interlaced fields and run as smooth as silk in 60i. Click image to download.


Working with 24p

There are a few ways to turn a 24p clip into a 60i (or 50i) format.

  1. Conform 24p to 30p. Then convert to 60p, then 60i as outlined above. Note this will increase the speed of the clip.
  2. Add 3:2 pulldown so the clip works in 60i. No interpolation is required and no speed increases are incurred, but it may look a little jittery as compared to surrounding 60i clips.
  3. Spatially interpolate 24p to 30p (or 60p) using pixel morphing/warping and then convert to 60i as outlined above.
  4. For PAL, conform 24p to 25p. Then convert to 50p, then 50i as outlined above. This will increase the speed but is hardly noticeable.

Conclusion

The key to this entire process lies in the frame-doubling step. The best frame-doubler will give you the best results in the end. The actual interlacing part is easy but it's the frame-doubling part where the real magic happens. Once properly interlaced, the clip will be so clean it's basically impossible to know it came from a progressive source and not an interlaced one. The results can be quite amazing to say the least.

Some plugins will allow you to re-interlace with fewer steps as listed above (e.g., RE:Vision Effects Twixtor). For the best quality, I recommend rendering the final video clip out of After Effects as opposed to your NLE if the choice is between the two.

Feel free to discuss this tutorial in Creative Cow's After Effects forum.


Downloadable Files:

Links:


Marco Solorio began his career in media production at the start of the 1990's and has been accredited with many industry achievements. He's the recipient of numerous creative awards ranging from visual effects to editorial to music composition. He's a published writer for both print and web mediums and has appeared on and off-camera for both creative and technical topics. Marco owns OneRiver Media, a successful post-production facility in the San Francisco Bay Area. He also owns Cinesoft, a software company whose flagship product, Media Batch was developed as proprietary server software but is now available to the entire industry. He's also known for the internationally recognized OneRiver Media Codec Resource Site, a resource that compares various codecs for the benefit of end-users and developers alike.

: : : Page design, layout and images by Marco Solorio
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