There is a big difference between Rotoscoping and Rotobrush. Rotoscope is the process of drawing one or more masks or painting around a subject using as few keyframes as possible. It sounds like you are using Rotobrush. Try this and see if it helps.
Proper Rotobrush workflow:
- First Critical Step: Several intraframe (MPEG) compression formats have variable frame rates and extreme luminance and color compression schemes that predict frames. Decoding this kind of footage can be very problematic. If there is any suspicion that a problem clip is not a viable production format, it should be transcoded to a frame-based production format or even an image sequence.
- When you have made sure your footage is compatible, trim the footage to include only the frames that must be processed.
- When the footage has been trimmed, put it in a new comp by pre-composing or trimming the original shot in the Footage panel and creating a comp from trimmed footage. Name the comp "Roto" or something similar. Ensure the comp is the same length as the trimmed footage and that there are no other layers in the "Roto" comp.
- If there are any areas of the footage that need cleanup, apply those changes to the original trimmed footage. If the footage is difficult to analyze because of low contrast, motion blur, matching colors, or other problems, I add masks to rotoscope parts of the image by hand, add color correction, and even stabilize and crop footage that needs to be processed by rotobrush.
- When the "Roto" comp is ready to process, put it in a new comp and name it "Rotobrush." The optimized "Roto" comp should be the only composition in the Rotobrush comp. You should not have any other layers in the comp, and no effects should be applied to the nested Robrush comp.
- Select the Roto nested comp and apply Rotobrush
- Make sure you have carefully studied the Rotobrush workflow so you know how to optimize edges, propagate and freeze the matte created using the Rotobrush tool (effect).
- If you spent more than a few minutes generating and freezing your "Roto" layer (again, it should be the only layer in the main Rotobrush comp), the wisest thing to do is go to Composition/Pre-render. This will add the Rotobrush comp to the Render queue and set the Output Module to use High Quality With Alpha preset. Render the comp, and a professional quality Digital Intermediate (DI) will be created and added to the project. If the nested Rotobrush comp is included in any other comp, it will be automatically replaced with the rendered footage.
Rogobrush is one of the most resource-demanding effects you can use. Leaving a layer with Rotobrush applied in a comp with lots of other layers and effects is asking for trouble. The only time I ever do not Pre-render a Rotobrush layer is when Rotoberush will propagate and freeze in a couple of minutes.
Once you have the rendered footage in the comp, you can delete the Rotobrush Comp completely to remove any possibility of it fouling up the project, then use the rendered footage as a track matte or a source in your comp.
I forgot to mention one more critical detail. Only use Rotobrush on the part of the image that needs a matte. Sometimes you have to separate an entire actor from the background. Other times you only need transparency behind an actor's hand. Don't waste time creating transparency where it is not needed.
I hope this helps.
If you follow the suggested workflow, you should be able to use Rotobrush on any suitable footage successfully. If a simple, short section of uncorrected footage is causing problems with Rotobrush, check the footage format, check your version of AE, and make sure that no other apps are open and your system resources are correctly allocated. The problem could be anywhere.
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