Thanks for the reply. I figured that might be the case. It makes sense that it would be set up to work just on the 100% parallax layers, as that's going to be pretty much all the practical cases.
As for setting all layers to 50% parallax, I agree that that would be pointless, I was just using it as a simple example of paired layers with matching parallax.
I actually happened to be faced with a special case that is related to that example, but a little more complicated, in which I would have several layers that would have variable parallax rates, but the parallax rates would always all be the same value across all the layers, or at least all the layers for which collision detection matters.
Reason being, I figured out a way to scroll layers completely independently of each other, which I've been interested in for a while, but the method involves keeping the scroll coordinates fixed and adjusting the parallax scales of each layer instead. The parallax scales for each layer become the individual scroll coordinates for that layer. It works, but I was worried that the weird parallax values would break the collision optimization, and it sounds like they would. In this case the game world layers would all have the same parallax scales, and would stay lined up, but they wouldn't always be set to 100%.
The main reason I've been interested in scrolling layers independently is that I'd like to be able to have a "game" world that scrolls normally, and an overlay-able "menu" system that can scroll from page-to-page independently of the game world. Currently there's no way that I know of to decouple the scrolling of the menu layers from the scrolling of the game world layers.
I'm aware that it's technically possible to make a system that kind of approximates the en-mass coordinate shifting that layers can do, but it seems a bit tricky to make it play well across multiple object types, and with anything else that effects coordinates, like behaviors, or for that matter Particle object particles, which don't move with the source object.
I've seen some other threads talking about independently scrolling layers, and it sounds like it's not currently planned. For what it's worth, I think it would be a cool feature, but I can also definitely understand why it might not be a priority.
In any case, thanks again for the info.