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Marco Solorio
OneRiver Media
Walnut Creek, CA, USA
©Marco Solorio and CreativeCOW.net
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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. |
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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. |
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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.
Figure 1 - A single frame from the 30p source. Click the image to download the clip.
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| Doubling Your Frame Count |
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The first step is to double up the literal frame count. This is accomplished in one of two methods.
- Double the TRT of the 30p clip so it's twice as long. Or...
- 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.
Figure 2 - Red circles show that this clip has been frame blended for doubling the TRT. We DON'T want that!
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| Convert to an Interlaced Format |
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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:
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.
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.
From there you'll want to select your format choice and render location. Click Render and you're done.
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.
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).
Figure 3 - A properly rendered clip will employ
true interlaced fields and run as smooth as silk in 60i. Click image to
download.
There are a few ways to turn a 24p clip into a 60i (or 50i) format.
- Conform 24p to 30p. Then convert to 60p, then 60i as outlined above. Note this will increase the speed of the clip.
- 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.
- Spatially interpolate 24p to 30p (or 60p) using pixel morphing/warping and then convert to 60i as outlined above.
- For
PAL, conform 24p to 25p. Then convert to 50p, then 50i as outlined
above. This will increase the speed but is hardly noticeable.
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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