Welcome to the community first of all. Don't worry, you'll be in good hands here. There's a lot of knowledge accumulated and lots of people willing and able to assist you.
It might be a good idea to go check out the videotutorials on the website and read some of the glossary, as you won't get around doing research on certain subjects and learn certain terms in order for us to help you properly.
To basically answer your questions:
That depends on the stock footage itself and the shot you want to composit it into, there is no "simple one way works every time" solution. I've made an example using my own stock footage clips and a random background.
There's three images. Two of stock footage, both shot on a black background, as you described, and one random pic for compositing.
Now you need to understand there's tons of different compositing methods and all they do is try to find out which pixels are transparent and which arent. Every single pixel of your image contains color information RGB ranging from 0 to 255, zero meaning no color, 255 meaning full. So a pixel with 0 / 0 / 0 is black, contains no color. A pixel with 255 / 255 / 255 is white. And all the others are some random numbers in between.
The "chroma" key picks a specific color and tries to erase only that from the image, so everything that isn't in that color stays visible. Most common used in bluescreen and greenscreen, where the background is made of one single color nowhere else to be found, so you can remove that and voila. Depending on your stock footage, different methods are more successful (smoke behaves differently then fire, or blood, or water, or an actor etc).
So a chroma key that keys out black on the smoke would look like this:
Every pixel that is pure black is removed, but all pixels that are even a shade of grey or different stay entirely visible. Useless for smoke, as obvious above.
Smoke is easiest done using a key called "add" or "multiply" which basically uses it's black and white information for transparency. Black is 100% transparent eg invisible, white is 0% transparent eg solid. All other shades of grey are shades of transparency. So 50% grey would be 50% transparent.
For smoke, which is black and white only, that gives quite good results, as that's already pretty close to how smoke looks:
Fire on the other hand, isn't quite as simple. Because fire does a lot of things in real life (not least of all distort the background by heat waves, which is often forgot).
So using the same technique we used successfully on the smoke leaves this:
Which doesn't look too real, as it's too transparent.
Now there's several ways around this. One of them is using some grading to change the contrast of the fire BEFORE it's keyed. So that there will be less black parts keyed out entirely:
Helps a bit and if used right, might actually do the trick especially on dark backgrounds. On this bright background, it simply doesn't work well either.
Another technique is to have TWO layers of the same fire with the same compositing method on top of each other, which will give this effect:
Better, as it leaves us with a clearer image of a fire. But still some things are missing that gives the effect away. First of all, fire is a light source, which smoke isn't. So that's already a whole angle that makes it more difficult to compose right. A bit of color changing helps a lot:
Here I simply darkened out some of the parts behind the fire with using the form of the explosion as a mask (you might actually have to do that manually in the worst case, on every frame, using a "garbage matte")
And I changed the colors of the background to fit in a bit better.
And as a last quick technique, you can add a third layer of the same explosion with the same compositing method and blur it out to give the fire that "glowy" hot feel. Like this:
That's pretty much the quickest way to get better looking fire and compositing. But as I said, compositing is an artform you have to learn step by step and by figuring out loads of techniques to get the right result. There's no fast way and every clib is different and has different challenges.
Another technique I sometimes use is this:
Make a layer with the fire black and white, then do a chroma key on the black, leaving roughly this:
If you add colored versions of the same clip with "add" and "multiply" modes on top of that like above, you have a fire which looks pretty solid and has a good mask:
And to put these "layers" into perspective. What you see is a combination of this to give the final impression when looked at from the front:
2. Yes, Stock footage is stock footage and works. There's loads of stock footage for all kinds of stuff out, some free, some low priced like mine above and some highly priced like artbeats.
Welcome to the world of visual effects.
And to your last question:
The fact you are in one shot and not in the other caused lighting differences wich will make the images brighter or darker. All you have to do is to "grade" the images until they match, change their colors and contrast. Compositelab can do that, effectslab isn't the right tool for that job.
And by "all you have to do" I again don't mean to suggest it's easy.
Last edited Wed, 25th Jun 2008, 2:44pm; edited 1 times in total.