How to blend transparent shapes?

Question

On the left are 2 Sprite shadows with alpha of 0.5

I thought I could work around this by using draw_rect but no :<
draw_ functions also work like this

Answer 1

Put both boxes on a separate scene
Put the scene in a Viewport node
Set the Transparent Bg on on the Viewport
Put the Viewport in a ViewportContainer
Set the Viewport container's modulate property to 0.5

dee end

Comments

Interesting! I wonder how I could do this for hundreds of objects.

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You would put the objects in the scene. Dah. You can put all these objects below the Viewport node and have them interact with your main scene. The Viewport will output one texture that you can then set to transparent (don't forget the ViewportContainer).
The best solution would be for the Engine to have a node with the modulate property that automatically composes one texture of its children. Sadly, a node that merges its children like that is only available in the "Alabama" version of the engine.