I know this question might sound a bit easy to answer but It's driving me crazy. There are too many possible situations that a good alpha blending mechanism should handle, and for each Algorithm I can think of there is something missing.

these are the methods I've though about so far:

• first of I though about object sorting by depth, this one simply fails because Objects are not simple shapes, they might have curves and might loop inside each other. so I can't always tell which one is closer to camera.

• then I thought about sorting triangles but this one also might fail, thought I'm not sure how to implement it there is a rare case that might again cause problem, in which two triangle pass through each other. again no one can tell which one is nearer.

• the next thing was using depth buffer, at least the main reason we have depth buffer is because of the problems with sorting that I mentioned but now we get another problem. Since objects might be transparent, in a single pixel there might be more than one object visible. So for which Object should I store pixel depth?

• I then thought maybe I can only store the most front Object depth, and using that determine how should I blend next draw calls at that pixel. But again there was a problem, think about 2 semi transparent planes with a solid plane in middle of them. I was going to render the solid plane at the end, one can see the most distant plane. note that I was going to merge every two planes until there is only one color left for that pixel. Obviously I can use sorting methods too because of the same reasons I've explained above.

• Finally the only thing I imagine being able to work is to render all objects into different render targets and then sort those layers and display the final output. But this time I don't know how can I implement this algorithm.