Digital Video - July 2008 - (Page 26) ROTOSCOPING UNMASKED BY REV. JOHN JACKMAN R otoscoping is one of the tedious necessities of many visual effects today. Any time a CGI element is placed in a live scene and an actor walks in front of it, a mask or matte must be created to overlay the actor over the CGI element. There are numerous tools to do this with, ranging from the basic After Effects animated masks to plug-ins such as Silhouette. They all have one thing in common: hours of frame-by-frame tweaking. Digital Film Tools Power Matte ($195, Windows or Mac OS X 10.4 or above) does not eliminate the time-consuming element of animating masks, but it does solve some issues of creating complex edges (such as hair) in a matte overlay where the subject has not been shot in front of a green- or bluescreen — though it can be used with blue- or greenscreen shots as well. And it does so with animated masks that are much simpler (and thus much less time-consuming to animate) than would be necessary to produce the same results on a complex foreground shape. Power Matte operates as a plug-in for Adobe After Effects. It is quite similar to the AE Inner/Outer Key effect, except that it works much better for many images. Two AE animated masks are used, one drawn just inside the foreground image and one outside to define the background. The area in between the masks is examined for edge values, and a transparency matte is created. To use Power Matte, set up an AE project and import the scene that you need to rotoscope, as well as other background footage or CGI elements that must be included in a final composite. I’ve intentionally chosen a shot that will demonstrate what Power Matte does well and where it will struggle. Now, starting dv july 2008 The final output (main image, above) shows the strengths of Power Matte in the clean wisps of hair, but also shows that where the difference between foreground and background is muddled or noisy, Power Mask can’t read your mind! Inset: Actor Burgess Jenkins (Remember the Titans, The Reaping) portrays John Wesley in the feature film Wesley. Masks are drawn just inside the foreground area and just outside. just before the needed in-point, draw the two masks that will define the foreground and the background. Unlike the AE Inner/Outer Key, these can be either open masks (as we’ll see on our footage) or closed masks. The Inner/Outer Key effect can only utilize closed masks. To create masks in After Effects, select the Pen tool and leftclick to add mask points to the image in the Composition window. Make sure the mask extends to the edge of the frame if it will be an open mask. To close the mask, click on the first point that you selected. To create the next mask, you have to select another tool and click on the frame before re-selecting the Pen tool to start the next mask. The easiest way to do this while the Pen tool is selected is to use Ctrl-click (Win) or Cmd-click (Mac) on the image in the Composition window. This temporarily activates the Convert Point tool. The next time you left-click, a new mask is created. Add the fewest points possible to create an effective mask! The more points you add, the more tedious the animation; but adding too few points will also created problems following a changing shape. Be sure to fully exploit the Bezier curve option whenever possible; this permits the use of fewer points, yet the shape of the curve can be animated easily. NonBezier points should only be used at fairly sharp corners. All www.dv.com 26 http://www.dv.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.