Dynamic Paint; invisible object is acting as brush anyhow

Question

I model a concrete structure in which water is rising and falling and would like to create wet walls using dynamic paint (Blender 2.81). Rising and falling water are modelled by two (or more) meshes whose visibility is animated (e.g. one on - the other off, under "Object properties - Visibility"). The "water" objects are defined simultaneously as brush, the concrete parts as canvas. The problem is, that all brush objects act as brush when baking, even the non-visible ones. Is there any trick to convince Blender that only visible objects should act as a brush while invisible ones shouldn't?

EDIT: There is a workaround by replacing disappearing of an object by sudden movement out of sight and vice-versa while the object stays visible all the time.

Answer 1

There are a few ways to do this, but both require new properties to be animated in sync with when you are animating the object's visibility.

The first way is to animate the visibility options on the Dynamic Paint modifier (on the brush object):

You can do this either by right-clicking on the icon and choosing 'Insert keyframe' or by pressing I when the cursor is over the icon. Note: You have to keyframe both the 'Show Viewport' (screen icon) and 'Show Render' (camera icon) settings to get this to work. Animate these values to be on when the object is visible and off when it isn't.

The second method is to animate the 'Alpha' value on the brush object to be '0' when you want it to have no effect and '1' when you do:

You may also have to animate the 'Wetness' value, depending on your setup.

Note that you should be able to copy your existing keyframes onto these properties by using the 'Dope Sheet' editor (but you must add at least one keyframe to these properties before they will appear in the Dope Sheet) by using the Key> Copy Keyframes and Key> Paste Keyframes menu options.