Rotobrush reveals transparent area that it shouldn't

[ConsfusedUser]

Okay so.

The other day I filmed a video in front of a green screen. I'm am trying to create the effect made in this video https://www.youtube.com/watch?v=vqW2PBcAHHU. To do this you use the roto brush, but for some reason the brush is exceptionally slow to the point of it being unusable. I have a fairly good PC and I'm very frustrated that after all these years it's still a big problem, but whatever.

I decide to change tactic.

Part of the problem with the roto brush is that I kept having to make adjustments that would exponentially slow the process down. I'm working with clips that are a few second long but it still wasn't working. So my solution was to key out the greenscreen first, export it as a png sequence, then use the rotobrush on these fresh new png's with a higher accuracy, less changes and faster workflow.

HERE'S THE WEIRD THING!

When I use the rotobrush on the png's that have transparency, THE GREENSCREEN REAPPEARS!!! HUH?????

Now I'm back to square one and it's very frustrating

My only guess is that the png file saves everything and then tells the file where transparency should be such that the keyed out area is still in the png and After Effects can still pick it up. Does anyone have a solution to the bizarre occurence?

[Mylenium]

Transparency in computer graphics always means that there still is the RGB info, whether that's the origional data or a generic black fill. Alpha channels/ transparency are merely a means of telling programs that interpret that info to not display or process pixels or gradually blend them with the surroundings. so far that's perfectly normal. If you don't want your cloth, insert a colored solid behind your keyed work, but soemthing will always be there and, to get to the second point, Rotobrush will always make it visible again because it completely replaces transparency, not adds to it. As far as AE goes, everything is normal. In order to combine your data you therefore have to work with duplicate layers, pre,compositions, track mattes and/ or effects liek Channel Combiner to merge that info. In your case of course you would also simply use holdout mattes, i.e. an inner/ outer mask in Keylight to eradicate everything outside the screen that doesn't even need to be removed. so in essence you have several workflow issues here. To recap in short:

• Don't bother with keying if you want to use RB.
• If you key, use a mask and reference it in keylight to eradicate the furniture.
• Combine the result of both operations with mattes, pre-composing and effects.

Mylenium

[Rick Gerard]

1. Use a garbage matte (mask) to eliminate everything in the shot but your actor
2. Apply a keying effect to remove the solid-colored background (green screen)
3. If the actor's feet don't move or don't move very much, just animate another mask around the actor's feet to remove the remaining background
4. If the actor's feet move a lot, then duplicate the layer, draw a mask around the feet, leaving enough room for the feet to move, then Pre-compose the layer moving all attributes to the new composition
5. Open the pre-composed duplicate layer in the Layer Panel (easiest to do if you are in the paint workflow and have rotobrush selected), then eRotobrush just the feet
6. When rotobrush is complete and frozen, use the Rotobrushed layer as a track matte for the keyed copy of the footage, pre-compose both layers, and proceed with the composite.

No rendering is required, only using rotobrush or masking by hand on the part of the frame that can't be removed by keying, and a lot of time is saved. I almost never rotobrush footage that does not have a garbage mask or manual rotoscoping applied. I also color-correct footage and pre-composed it before using Rotobrush to improve edge contrast. 

I also almost never key footage that does not have at least one garbage matte. There is no reason to fiddle with parts of the image that you don't have to, and a mask is the easiest way to do that.

Hope this helps. 

[chucksing]

If you click the checkbox for 'invert foreground/background' it should maintain your original alpha from your key. Then you may need to do a few more clicks to get rid of the rest of your junk outside your greenscreen. I tested this with some solid colors, but in theory it should work for you.