Basically, before drawing X-Rays, we now bind a second depth buffer. After drawing XRays, we do an extra resolve pass where we overwrite the non-XRay depth buffer in pixels where the depth is not maximum (which means background pixel, since depth is cleared before drawing X-Ray objects). This ensures both scene and X-Rays keep their depth values and are ready for compositing. Well, the odd effect due to depth discontinuities can be expected, and X-Rays are a bit more expensive (extra buffer + resolve pass) but at least X-Rays won't invalidate depth values anymore. Whee!