I am trying to implement a depth map in my 2D game, so that I can use deferred rendering. My problem is, that when sprites are drawn on top of eachother, I cannot get the last z-value stored in that pixel. I am not using the vertex part of the pipeline, instead relying on spritebatch to do layering for me. Is there a way I can easily retireve the previous depth value in the texture? I am using alpha transparency for my sprites, and am not sure if this will cause problems.
This picture is the depth map, where white -> closer to the front. Therefore the white sprite should appear on top of the darker sprite, but it doesn't (the darker sprite is drawn after the lighter one).