The Corner Pin Tracker in Boris Red

The newest addition to the RED family of motion filters is the Corner Pin Tracker. Like the other motion filters, it's very easy to use, and has similar controls. One difference is that its four trackers don't calculate averages, like the other motion filters do. In the Corner Pin Tracker, they follow four separate points on the source video.

The Corner Pin Tracker also designates the media whose four corners will follow those four trackers, distorting it as necessary to provide a convincing illusion.

To illustrate, I shot a very quick and dirty clip of a computer monitor.

One way to add a picture to the monitor would be to plug it in. That almost seems like cheating, though.

I was inspired by all those TV pop stars from the 1960s who were always filmed playing their instruments, none of which were ever actually plugged in. Most of the time, they didn't even have microphones. I was thinking of The Standells, in particular. Well before they went on to garage-band legend with their hit "Dirty Water" and "Riot on Sunset Strip," they cranked out a punky version of "I Want to Hold Your Hold" on The Munsters. (You can look it up.) As was the custom of the day, not a plug in sight.

So I asked myself, if they can make music with nothing plugged in, how hard could it be to add a picture to an unplugged monitor?

With Boris RED 2.1's new Corner Pin Tracker, it's very easy. If you'd like to play along to see for yourself, download the movie above to your hard drive, and set your Boris project preferences to 360 by 240. As with all of the Boris motion filters, your project size needs to match the size of the media.

Set-Up
Even though it has 4 trackers instead of one, the Corner Pin Tracker is every bit as easy to use as the others. I apply the filter to my clip, then click the disclosure triangle to see the Motion Tracker Source track. This is what provides the four corners that we'll track, which in this case is monitor.mov. The default is to V1 from your host video timeline, which makes sense in an NLE, of course. Instead, I set the Motion Tracker Source to monitor.mov.

Note that the Motion Tracker Source isn't visible in the final effect. It's sole function is to provide four corners' worth of position data, which will eventually be applied to another image. This has two implications. The first is that you can add any kind of filter you need to in order to get high contrast data to track. Applying the Invert/Solarize filter is one of my favorite methods, but the precise filter used will vary according to the footage.

It also means that you're by no means restricted to using the same clip as the background and as the Motion Tracker Source. For example, to make an image float across a scene as if a ghost were carrying it, shoot someone actually carrying a piece of brightly colored poster board. Then apply the Corner Pin Tracker to the four corners of the poster board, and apply a clip to the tracked data. You'll see a moving object, but you won't see what's making it move.

Targeting Assistance
As you know from the previous tutorials, the trackers are looking for an area of high contrast to follow. We have some obvious targets here, the four corners of the monitor. To see the trackers, I click on the Preview monitor on the Corner Pin Tracker track, which opens that layer in a separate window. This is where I can see the four trackers, and adjust them to the four corners.

Because I'm working with a small movie of not especially high resolution, the precise corners are a little difficult to see. Fortunately, the Corner Pin Tracker has built-in magnification, called Region Zoom. I've set the zoom to 3x, although the maximum is eight.

Select each of the four trackers on the tabs at the far left of the Controls window to adjust those as well.

Making Tracks
The method here is the same as with the other filters. With the CTI parked on the first keyframe of the effect, a default interpolation of anything but constant, and my keyframe mode set to Animate rather than Static, I click the Analyze Motion Path button. All four tracks are calculated.

I could, in fact, elect not to track all four corners, simply by turning those trackers off, and turning off the Analyze All checkbox. The most common use of this is if only one of the corner's tracks turns out to be inaccurate, and I want to readjust only that path without affecting the others. I'd select that tracker, Reset the controls, and re-analyze it. It's unlikely that this will happen, but the feature is there if you need it.

Advanced Tracking
That's actually the case with many of Boris RED's Advanced settings on the Corner Pin Tracker. For example, the default Sub-Pixel Accuracy setting of 1/16th has been sufficient for every track I've attempted, although accuracies up to 1/256th of a pixel are available.

As shown in the Motion Tracker tutorial, there are times when it's advantageous to search in a specific color channel, rather than use the RGB output. The red channel is especially useful, since it contains an image's contrast range.

If the object you want to track begins offscreen, Track in Reverse is extremely helpful. In that case, you'll begin with your CTI parked on the last keyframe, of course.

For more details on the other Advanced settings, be sure to check out the new and improved documentation for Boris RED, which is downloadable from our downloads page.

Fine Tuning
Once you have the corners tracked, I can of course assign any media I want, using the Corner Pin Source track. Once again, V1 is the default. Because I'm working in the standalone RED Engine, I have the default proxy image of a ball, but I could use any media I want: a spline object or primitive shape, imported EPS media, text, still images, imported movie files, or video from a host application timeline.

It doesn't even need to be a single-track clip that gets assigned as the Source. I could make the most complicated composition I can imagine, nest it inside a 3D container, then drag it and drop it on the Source track.

To keep things simple, and so that you can easily follow along, I'm going to use my favorite Natural Media Texture, Reptilian Skin. (I increased the scale of the scales just a bit.)

When I first apply my Corner Pin Source track, the sharp edges of the new video don't fit quite right with the rest of the image, which is rather soft. This is easily remedied on the Crop Source tab.

I slightly scaled the reptilian pattern, and softened the edges to make it blend more naturally into the scene.

There's one more thing I'd like to adjust. In the original movie, you can see the shadow I cast as I move around the monitor. After I've added the tracked media, you can still see the shadow I cast on the beige monitor box, although there's no shadow on the reptilian pattern mapped to the monitor.

If I wanted to spend a lot of time with it, I could rotoscope the shadow and reflection from the original movie, and composite it on the monitor, but I'd rather not. It's much easier to simply apply a Light Sweep filter to the Corner Pin Source track (the reptilian media), and color the light black instead of white.

It's a subtle difference, but ultimately much more convincing.

Motion Filters a-Go-Go
The Motion filters in Boris RED 3GL offer vastly more power than you'll likely ever need. The Corner Pin Tracker is a perfect example, offering easy access to a level of performance never before available inside the vast majority of nonlinear editing applications. And if your preference is standalone effects creation, the Boris RED Engine works exactly the same way, with the addition of handling the rendering itself.

Wherever you need your corners tracked and pinned, RED is ready.

Care to play along? Download the monitor movie at the beginning of the article, and download the RED setting by clicking here. Don't forget to set your project size to 320x240.

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