Question

I’m on an OpenGL project.

I have some objects (just say 2) made of the same transparent material (alpha = 0.2, for example). The two objects intersect.

How can I make the intersection part look the same as other part (without border, no different color), so the too objects will look like as one?

Answer 1

I’m not sure if you would really want to do it. I will answer anyway, but first let me tell you why I think you don’t want that.

In real life, imagine a red stained glass and a blue one. If you look at them in a way that they partially overlap, the overlapping part clearly has a different color (purple). If you get 2 red glasses and look at them so that they have overlap, the overlapping part is more red. That’s exactly what is happening in your OpenGL program.

Now in general, when you have multiple transparent objects, you need to sort them based on their distance from your eye and the direction you are looking at. Then you draw them from farthest to closest. This is not a simple task by itself! Think of 2 objects that cross.

One way of achieving what you want is to sort the transparent objects, but draw from closest object to the farthest. This way, you practically don’t allow transparency on the same pixel to be done twice. Not a good idea.

Another way would be to do something very specific to these objects of special kind. I say special kind because apparently two of them overlapping doesn’t make any changes! You can do what you want by drawing to the stencil buffer instead of the draw buffer, then draw a rectangle with the color you want over the whole screen, but matching only that stencil.