I'm trying to use two layers of scrolling clouds going at different speeds and directions in a skybox using portals. I'd like to be able to use my cloud textures at full opacity, since I already have the alpha channels all set up for the clouds and is already very transparent to begin with. Unfortunately, when using portals, the alpha property seems to act like more of a threshold for how much of the image is shown, and flattens the existing image into not having any alpha, only solid or transparent. How can I fix this?
This is what it looks like in-game (with opacity of 128):
And this is what it's supposed to look like: