Two Keys on One Layer
I am trying to layer water gushing up from a fountain with abstract animation. I think the easiest way is to use stock footage shot on a black background. I want to replace the black background with one abstract animation and the water gush with a different animation. I can do one or the other but can't figure out how to do both.
Here is a still image of example footage (it's watermarked because I haven't purchased anything yet; I wanted to embed the images but the link isn't working):
Another option is to use this footage:
I don't know if the gray and white checkerboard means it will be transparent but that has not been the case with the sample footage.
And here is an image of what the first footage looks like with the Color Difference key. I like this result as it preserves the details of the water:
Any assistance is greatly appreciated! Thanks.
I think I get it - you want to put something abstract within the water shapes right?
Couple of things off the top - that wide shot of the fountain should indeed have an alpha from the stock company - you're right that's what the checkerboard means - they usually don't supply the alpha in the low rez temp version just to save bandwidth. However, I don't think the alpha has been done properly. It should have some parts within the water that are blocking the alpha and other clear parts that let the background through - I only see alpha outside the body of the water. That shot looks like a CG shot that's just had an "outside alpha" rendered. So you could use that element to mask your abstract but it would be a fountain silhouette and would not look like the abstract is being distorted by the water. If that's the look you are after you are home free.
As for a luma or color key on the other water element, this can work but remember that while it keys out the black (or uses the black as a mask) it can also key out black "reflections" and distortions in the water itself that may be meant to be opaque. It can work if you don't look too close but often it just doesn't feel right.
Depending on the background you can often "screen" or "multiply "(the blend modes) the water on top and then use a distortion or displacement filter to warp the image being masked by the water. This is often a good enough illusion because parts of the water will step on the abstract and the background depending if they are brighter or darker. Water is usually clear but it usually isn't showing what's right behind it through the clear parts and when it does the background is distorted by the water drops. And you should be getting some specular highlights and reflections that will obliterate what you can see through it. Keep in mind, any added hilights should be "screen"ed on top so that any thing brighter behind will still penetrate through just the way real light works.
So the layer order would be from bottom to top.
Background, abstract art passing through a track matte of the water alpha, then the water again either screened or multiplied on top. For extra points you could add a displacement filter to the abstract, using the water element as the displacing layer and then mask all of that with the water alpha to contain the distortion, and that will give the abstract some refraction effects. You could also try the glass distortion effect or Mr Mercury effect (if its still around), compound blur might do the trick too if you use a blurred version of the water as the distorting element in that effect.
Thanks so much for the thorough and well-considered response. I’ve got some things to try. I’m sure I’ll have more questions as I keep working!