humble confession

[toonertime]

i confess to having difficulty with masking

say i have a black box 1 on a layer floating in an ocean, on another layer.
the ocean is like 65 percent transparency.

i have another box layer 2, distorted. what i wish to do, is
when the ocean wave rolls over the bottom of the box 1, only
the distorted box 2 will be revealed in the ocean layer. The box
that is untouched by water will reveal box 1, whenever the water
hits the box, reveal only box 2.

The particle tutorial 6.9 alluded to something like this, but I
can't get it right.

Masking just seems to elude me. I know this is probably simple
but so am I!

[slowtiger]

(edit)
OK, my first idea was wrong. Here's what worked:
http://www.slowtiger.de/examples/watertest/

Note that I first did the animation of box and water, then duplicated those layers. Water3 is a different mask than Water2: I just grabbed the two points at the bottom and translated them over the remaining points, thus creating a mask for the "sky" part of the image. You find the .anme file at http://www.slowtiger.de/examples/watert ... r.anme.zip.

[toonertime]

I don't know why you are called Slowtiger because
you solved this very quickly!

I doubt that I could have figured this out.

I have downloaded your tutorial and will analyze it.
Your box does exactly what I was interested in.
It seems to be a masking trick that would be
useful in many ways. Thank you so much for
your attention to this!

What a great site this is!

[slowtiger]

Actually this wasn't as hard as it may look. It's the very old trick of having two complementary masks: one masks A but not B, the other masks B but not A. It doesn't matter that the mask is moving, it's only important that one mask is the negative of the other.

An explanation is given at http://en.wikipedia.org/wiki/Matte_(filmmaking), unfortunately without illustrations.