Making a control more opaque will not make it render in front of another. Nor will it really make it appear to be in front of another.
Also, keep in mind that I said polygons, not objects. Even the polygons within an object need to be sorted with one another. And it is not possible to sort the polygons between objects, or even sort polygons perfeclty if any of them intersect.
The best you can do is to sort the polygons within the individual objects with one another, and sort the objects.
Of course you might just leave sorting the polyogns within the object up to the user.
In my opinion, there is no best way to allow someone to add 3d objects to an interface. I didn't support that out of the box with my own GUI system because if someone wants to do something like Metroid Prime, in my opinion, the best way might be to maybe hack a gui system like mine to render to an image and then use that image on a model of a helmet which you place over the camera. The 3d buttons you see in the interface might as well be sprites that just look 3D.
Or the best way might be to create a single object with a single surface for the face of the helmet, and texture all the elements in that surface with bits of the main texture, and actually deform the shape of the object to fit the curve of the helmet.
It's not possible to cover all situations, and corvering lots of them might result in a lot of messy code. So I just kept things simple in my own system since I could not really predict what the user might want.
Oh and another thing you could do...
Another way to render an interface like that would be to do what my GUI system does and render the interface with a second camera... But in this case you'd set to FOV of that second camera to be wider to create the fish eye effect to make it LOOK like the helmet is curved. It might be hard to get that to look right though if you have a helmet you want to match up wuth the curved glass.
Also, keep in mind that I said polygons, not objects. Even the polygons within an object need to be sorted with one another. And it is not possible to sort the polygons between objects, or even sort polygons perfeclty if any of them intersect.
The best you can do is to sort the polygons within the individual objects with one another, and sort the objects.
Of course you might just leave sorting the polyogns within the object up to the user.
In my opinion, there is no best way to allow someone to add 3d objects to an interface. I didn't support that out of the box with my own GUI system because if someone wants to do something like Metroid Prime, in my opinion, the best way might be to maybe hack a gui system like mine to render to an image and then use that image on a model of a helmet which you place over the camera. The 3d buttons you see in the interface might as well be sprites that just look 3D.
Or the best way might be to create a single object with a single surface for the face of the helmet, and texture all the elements in that surface with bits of the main texture, and actually deform the shape of the object to fit the curve of the helmet.
It's not possible to cover all situations, and corvering lots of them might result in a lot of messy code. So I just kept things simple in my own system since I could not really predict what the user might want.
Oh and another thing you could do...
Another way to render an interface like that would be to do what my GUI system does and render the interface with a second camera... But in this case you'd set to FOV of that second camera to be wider to create the fish eye effect to make it LOOK like the helmet is curved. It might be hard to get that to look right though if you have a helmet you want to match up wuth the curved glass.