I have footage of someone walking past a wall with graffiti along it, I used a tripod so the footage is stable and I have made a freeze-frame of the wall without the subject in it and taken out the graffiti and replaced it with brick wall using stamp tool.
Is there a way to have the subject walk past the wall with no graffiti and then have it slowly be painted in as he's walking past. I'm sure theres a way but cant figure out how to change the background whilst someone is walking past.
If I understand you correctly, what you'd want to do is put the clip with the graffiti underneath the clip where you've removed it, then use a mask on the upper layer which animates over time to make the upper layer disappear. The property to key frame is Mask Shape. I'd also suggest that you draw the mask in the first frame of the clip using the pen tool and create a number of points so that you can adjust the mask tightly to the figure as it moves along. If you are not as interested in making the reveal look like the figure is the cause of it, you might be able to get away with a standard square mask and a bit of feathering. -R
That's too bad. The current ways to do it are:
A) fast, but don't work very well.... and
B) God-awful slow and painstaking, but can yield better results in experienced hands.
A) Difference key -- compares the one layer (the clean plate of graffiti wall) to another (video of guy walking past graffiti wall) and deletes the portions that are the same. That's the theory, but in practice, it's not so good. There are too many variables, because the pixels have to be spot-on perfect in their match to disappear entirely.
B) Rotoscoping -- to isolate the subject from the background, the AE practitioner creates animated masks around the subject, establishing initial Mask Path keyframes, then adjusts mask vertices on subsequent video frames as necessary. This is VERY time-consuming and VERY painstaking work.
It won't help you, but AE 10 includes new features called Roto Roto Brush and Refine Matte, which work in conjunction to isolate subject from background in a semi-automatic way, greatly reducing the amount of hand work in rotoscoping.
So my not-so-good news for you is this: pick your poison. Sorry.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
It sounds like you have all the pieces you need.
The still with the wall with graffiti and no person in it is a "clean plate". You can use Difference Matte to create a alpha channel for the person walking by- that is to remove the wall and just have the person on a layer. This way you can put the clean wall behind and reveal the graffiti on it using animated masks.
Check out the AE basics forum for tips on stuff you do not know. http://forums.creativecow.net/adobeaftereffectsbasics
Also Adobe has a lot of material on their site.