Opacity of overlapping structures

[mizuchi3]

Hi Marco,
I was playing with the new opacity settings, they will be really useful indeed. However I find that if I set the opacity of an overlapping structure to 0, the structures which lie behind the now transparent object are not visible

[biasmv]

That's an inherent problem of the way opacity is implemented. When a transparent triangle gets rendered before something that is behind it, the triangle behind does not get rendered, because it fails the Z-test. To solve this with the current method, all the triangles would have to be sorted by projected Z-coordinates and that's relatively expensive in JavaScript. Another option is to implement per-object opacity as a post-processing pass.

I've added a small fix for the case where an the objects is completely transparent. That doesn't solve the visual artifacts when multiple objects are semi-transparent. To somewhat lessen the problems, we could order the objects such that the most opaque gets rendered first. When setting the opacity you could do the following(untested):

var alpha = 0.5;
obj.setOpacity(alpha);
obj.order(10 - alpha * 10);
viewer._objects.sort(function(lhs, rhs) { 
   return lhs.order() - rhs.order(); 
});

[mizuchi3]

Thanks, that works well, and is better than the binary on/off switch we were using. I notice that if option outline is true, then for 0 opacity, the object is drawn black. If outline is false, then the object is not visible at opacity 0 as expected.