I have 4, 30 minute clips of a talking head shot on a pale greenish wall. I've tried keying the footage and his white shirt makes it all but impossible. I have CS5 After Effects and I have been playing with the rotobrush tool and it does a wonderful job. But, it seems to only to 20 frames at a time, then I have to start again and tell it what is the foreground. This is going to take forever! How can I tell rotobrush to use the settings I establish and to do the entire clip?
You can extend the spans just by dragging the edges of the analysis bar. It will not do all 30 minutes, though. Due to memory considerations and storing the baked solution, there certainly is a limit to the number of gfframes it will process in one sweep.
More infomation about Roto Brush spans is here:
It's especially important with Roto Brush that you read the instructions, and maybe watch a video tutorial or two. There are a few things about this new tool that are not what you might intuitively expect.