Will Head
Chroma keying just got a shot in the arm with the new Ultra Key tool built into Premiere Pro CS5, which creates great keys with minimal hassle.
We’ve looked at chroma keying – the process of digitally replacing the background behind your subject with something much more interesting – in previous columns, but since then a new green screen tool has arrived on the block in the form of Ultra Key.
This isn’t a standalone application: it comes as a standard effect inside Adobe’s Premier Pro CS5 video editor. Both Premiere and Final Cut have had their own built-in keyers for some time, but the big difference with Ultra Key is how easy it is to use to get good results.
With enough time on your hands and sufficient tweaking, you can achieve reasonable results with many built-in keyers. However, time isn’t always a luxury you have when you’re up against a deadline and the demanding client is breathing down your neck.
The big difference with Ultra Key is that it only requires minimal tweaking to get an extremely good key, even if the footage you’re working on isn’t the best to start with. This means you can get on with editing rather than messing around with settings. The other factor in its favour is that it’s built into Premiere Pro, so you don’t need to fire up another application and jump across just to do your green screen work. It can also be accelerated by a suitable Nvidia graphics card, so you get real-time performance straight on the timeline, which means you can preview the results without rendering.
Previously, to get a comparable key, you either had to shell out extra for the Keylight plug-in for Final Cut, or throw your footage into After Effects and use Keylight in there, before pulling everything back into your edit suite. Just being able to achieve a hassle-free key straight on the timeline is a huge timesaver. If you do a lot of chroma keying, then the integration of Ultra Key into Premiere Pro’s workflow could be worth the asking price alone. It really speeds up green screen work.
The technology behind Ultra Key has been around for a while. It was originally a standalone product called Ultra from Serious Magic that only ran on Windows. In addition to green screening, Ultra also came bundled with a number of virtual sets, so you could easily create professional-looking videos without having to shell out extra on physical items to make the backdrop look interesting.
Adobe bought Serious Magic in 2006 and Ultra made a brief appearance as a standalone application in the Windows versions of CS3, but it was completely dropped from CS4. Adobe has used the time in between to take the technology behind Ultra and build it right into Premiere Pro – on both the Mac and PC – and the wait has most definitely been worth it.
Like all keyers, better-quality footage will mean better final results. Ultra can still handle things such as unevenly lit footage pretty well, but if you have control over the shoot, the better you can make your source material, the easier it will be to achieve a good key (see MacUser, 4 June 2010, p80 for tips on how to achieve this).
Once you’ve got your footage, the first step is to trim out any large areas that you don’t need to key using one of Premiere’s Garbage Matte filters. Available in either four-, eight- or 16-point options, the Garbage Matte lets you easily mask off areas of the shot where no action occurs. Move the points on the matte close to the edge of your subject, allowing for any movement such as arms, for example, and anything outside the matte will become transparent by default. A matte is particularly useful at eliminating the areas of uneven lighting that occur at the far corners of the green screen. Since no action actually occurs there, it makes more sense to mask them out manually rather than try to tweak the keyer later. You can also easily eliminate items such as lighting rigs that might be in the edge of a shot, but have no action occurring directly in front of them.
With a matte tight enough on your subject, but wide enough to allow for any natural movement that will occur during the shot, you’re ready to start applying the magic.
The first stage of achieving a good key with Ultra Key is to use the dropper to tell it which colour you want to remove from the shot – the green background, for example. Despite your best efforts at lighting your screen evenly, it’s inevitable that some areas will be darker than others. As a general rule, it’s best to pick a darker part of the screen instead of a lighter bit, as this is easier to correct when it comes to tweaking.
The default setting will remove most of the screen, but it’s likely you’ll still see some shadows remaining. If you’re working with particularly good footage, with an evenly lit green screen, then Ultra Key’s Aggressive setting might be all you need. In just two clicks, you’ve achieved an acceptable key and you can get on with finishing other, more important aspects of your edit.
If you prefer to tweak things manually, or just want a little more control over the process, then your first port of call should be the Pedestal setting in the Matte Generation section. Increase this to remove any shadows from the green screen that remain.
The default view in Ultra Key is Composite, which shows you what the final result will look like. While this is fine for most purposes, if you switch to Alpha Channel view, you can see for definite how good your key is. The white areas show what will be included in the final image and the black areas show what will be cut out. If you see any grey, then those items will be semi-transparent, which will result in a poor effect. The Alpha Channel view should only show black and white areas for the best results.
Next, you’ll need to turn your attention to the edges of your subject, to avoid any green tinge around their profile. The Choke and Soften controls in the Matte Cleanup section let you slim down the outline, so any spill is removed. Be careful not to turn them up too high, though, otherwise your subject will end up looking like they’re suffering from a chronic wasting disease.
With the bulk of your keying already under control, Ultra Key has another couple of nifty features built in to help you get the right look. If your subject is looking a little washed out under all the studio lights, you can increase the saturation to put a bit of colour back into their cheeks. You’ll also find Hue and Luminance controls in the Color Correction section, so you can quickly tweak colour settings without having to resort to using a separate filter.
In the time it would take to achieve a quick-and-dirty key in some chroma key tools, you can have a finished result with Ultra Key. With well-shot footage, it could be as little as two clicks of the mouse. Add in the fact that it works directly on the timeline and can take advantage of Adobe’s new GPU acceleration to use the graphics processing might in your Mac (if you’ve got a compatible Nvidia card) to speed up real-time performance, and the power of Ultra Key quickly becomes apparent. If you do a lot of green screen work, the amount of time saved should be recouped in the purchase price of Premiere Pro.