a mask and its invert are not creating the whole layer-after effects

[eb sh]

I'm new to after effects.

Suppose that we have a solid layer. create a rectangle mask with blending mode set to Add. duplicate this solid layer and invert the mask in this layer. now we have something like this:

we have transparent space/crack along the mask path. these is no mask feather that can cause this problm. I set preview resoloution to quarter so it becomes easier to see. I know that this problem could be solved by expanding one or both masks.

my question is why this is happening? and how to fix it without expanding masks?

[Mylenium · Accepted solution]

Thios is to be expected. AE's default transparency blending is not linear and semi-transparent values do not add up to 100%. You need to change the layer blending mode to Alpha Add or treat the transparency with Matte Choker or a channel effect.

Mylenium