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Adrian Wong-Ken
by Adrian Wong-Ken
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Computer Monitor Green Screen Replacement with Foreground Obstructions and Cats

06/09/2014
Computer Monitor Green Screen Replacement with Foreground Obstructions and Cats

Fix It In Post...

Nobody wants to 'fix it in post', but sometimes you have to. Most commercial video producers will have to shoot a computer screen, tablet or phone at some point in their career—especially if the client happens to be a software or technology firm like Dirigo. Computer screens don’t translate well to video. They can look really bad—those pesky stripes that appear.

To a person's eye, the pixels on a standard cathode ray tube television (or LCD monitor which has a different frequency) screen glow for about 1/30 a second. Video cameras are less sensitive to light's persistence, so the pixels glow for a shorter duration when captured by a video camera. The flicker or rolling black bar on a recording of a TV's image is a group of pixels that have faded before the camera has imaged them.

The other issues is that the TV or monitor's scanning frequency is often out of sync with the scanning frequency of the recording camera or device. Both of these issues are difficult to fix. Modern video cameras attempt to make adjustment to remedy screen capture, but, the result is lackluster.

Thankfully, several methods are available to correct this. This blog post is not about 3D modeling. When 3D is not appropriate, green screen replacement should be performed in order to retain the highest image fidelity.

This video will show the entire process:

Shooting

The first thing you’ll need to pull-off a screen replacement is a green screen graphic to put on your computer monitor. If you intend to have camera motion with your screen replacement tracking markers as well as geometric guides are recommended. You can download an example of Dirgo's green screen graphics here (a 'virus-Free 724kb Zip file' with 4 .jpg images and one Adobe Illustrator .ai file). Once you’ve put this marker on your screen you’ll want to set your stage and light accordingly. Make sure you reduce as much green light spill from your monitor on your actor as possible, this can be done by balancing lights as well as your monitor’s brightness value. After recording the footage it’s time to bring your footage into Adobe After Effects.

Tracking - Setting The Spline

Choose the segment of footage you want to replace and bring it into After Effects CS Mocha for tacking. For those who have not used Mocha it is a planer tracking system that works as an assistant when doing things like creating and adjusting screens. In this example I have an obstructed screen from my actor, two elements come into play here; the obstruction will mess-up my tracking data and the right side of the screen is not visible. In order to have proper perspective the planar surface will still need to be the same size as the monitor behind the actor. To achieve this without a graphical reference it’s time to dust off some high school geometry. Use the spline tool to mark the first two points that are visible. Then, run the third vertical parallel with the bottom portion of the screen so that the hypotenuse of the spline line runs parallel with the top right triangle hypotenuse as well as crosses the center mark of the graphic. This will tell you the width the screen should be. Once you have these three lines the fourth spline line will run parallel with the grid markers as well as the top of the screen.

To track these lines make sure you have perspective checked on the motion box. You’ll also want to crank up the Min% Pixels Used, if I’m not in a rush I’ll go up to 90% and let the tracker do a detailed analysis.

Tracking - Removing the Foreground

The actor in the foreground needs to be masked-out so that the tracker doesn’t use this as part of the tracking data. The roto doesn’t need to be perfect, but, pay attention to potential trouble spots. Manually track this roto—it shouldn’t take too long in my situation because the motion isn’t extreme. Set the roto layer on top of the screen layer and set the blend mode to subtract.

Tracking - Setting the Surface

Now, go back to your screen layer and set up the planar surface so that it lines up with the tracking splines, its best to make your surface a little bigger than it should be than a little smaller. Now track the screen and bring the data back into After Effects. If you have trouble with your surface slipping try adjusting the points so the background and foreground intersect less.

Compositing and Blending

Once you’re back in After Effects paste the tracking data to the layer that you want to composite on the screen, it should stick where you want it. To remove the green screen use the keylight effect. Use the clip black to remove the markers and the softening values to blend the screen. Once you have a good key there are a few things to do to sell the effect. First, duplicate the replaced screen, blur it a lot, set the transfer mode to add and bring down the opacity. This will simulate the glow effect. Then, add the noise effect to simulate camera noise. Finally put a black solid beneath the entire composition as a safety net for any strange opacity errors, color to taste and then export.

And there you have it, our first use of a cat image on the Dirigo blog!

About Dirigo

If you're in need of video shooting or production services in Portland or Southern Maine, give Dirigo a shout. We're a full-service agency with a heavy digital/internet footprint. Our video production group runs independently from the rest of the Dirigo organization (frankly, just like web development, media buying, or bulk e-mail sending), providing agency breadth, experience, and service at a near freelance price-point. My name is Adrian Wong-Ken and you can reach me by phone at 207-347-7360 x284.

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