Without knowing your exact render settings or what e.g. the matte after your Rotobrush looks like nobody can say anything specifically. Could be that you screwed your transparency with the RB already, could be that you simply used an unsuitable output CoDec, could be that you use a wrong footage interpretation when re-importing. Impossible to tell, since you have not told us anything about your procedures or even what version of AE you are referring to.
My apologies. I am using CS6 for AE. And as far as those other terms.... I'm afraid I am WAYYYYY beyond even coming close to understanding what you're saying haha. I literally just got this and started with some you tube tutorials to even be able to do that. Just that mine didn't turn out like the tutorials. Not sure why. Sorry I can't answer all those things you mentioned though because I don't even know what that stuff means. Thanks for the reply though.
Sorry. I didn't see before I posted that to get some assistance in the forums you had to already be be a semi pro with the program. I have spent a week of every waking moment trying to figure out why this keeps happening and all I keep getting is that I should start at the beginning (in one word formation or another). I will explain the steps I'm doing below and if I'm told after that to pretty much just figure it out, then I guess I just wasn't meant to use this program.
I have a video file of a woman walking, of which I want to cut her out, and use a different video as the back drop.
I use the roto brush tool (not using any settings or anything, just the default one) and make it so through the whole video it is just her and there is a black background (didn't make a solid, it's just the background that is there when I am done making it just her)
Then, I put that in render que and render it out. I have the setting as lossless. I didn't select anything else. It is exporting as AVI
Then I start a new project and have the video imported that I want for the background.
Then I import the video that I rendered of the woman walking and place it on the timeline. It is on the top layer.
And then... she is half transparent. Opacity is at 100. It is also that when I rendered it out. And if I double click the layer that she is on, there is the black background and she is not transparent.
This is where I am stuck. I followed dozens of tutorials and every time this is what I get.
*EDIT* I also select ADD for the layer of the woman walking too.
Any help is greatly appreciated.
EDIT: In case anyone else spends days on end searching the internet for this solution like I did.
Here is the solution: Select a transparent background on the video you want no background on, then in the Render Que, click on the LOSSLESS word (in yellow) Then choose RGB+Transparency from the list. THEN render it out. And you can export it.
*** Still don't know why when I import an image it disappears when I click on ADD, but I guess the more you spend on software, the more you have to just figure things out on your own. Funny.... I bought different software from Magix for $75 and they were prompt with support. Go figure.
After Effects is an incredibly powerful program and there are so many levels that you need to understand. There are very many resources at your disposal. One of the most powerful is the Search Help field in the top right corner of the app. Just type in your question and you will be pointed to many tutorials and articles on technique. In 20 years of using AE I have not known anyone that was able to just open up the app and produce even the most basic original product without several concentrated hours of training. There is a lot to learn. Your question is a very common one. Help was only a click away.
If you just want to poke around and try and find things the easiest way to render a project with an alpha channel is to simply use the Lossless with Alpha or the Tiff Sequence with Alpha presets from the output module. You wouldn't know to look for a rendering option with an alpha channel if you didn't know how transparency was achieved in video.
BTW, typing transparent background in AE's search help field brings up THIS list of resources. Click Adobe Content only and get this list. If you have a question on rendering, type rendering in the search field. I'm only pointing this out because in all the classes that I have taught since AE has had this search help feature, and in all the years that I have taught AE only about 5% of my students have ever searched the help files for help. Most of the time, most common questions can easily be answered there.