MPGeometry renders image tiles in a multi-pass fashion. There is no Z-flighting because it is rendering the exact same geometry each pass (so the resulting depth values are identical) and the engine sets the depth testing function to GL_LEQUAL so that the Z test always passes. (This happens in MPTerrainEngineNode.) It then uses GL blending with pre-multiplied alpha to composite the layers.
Glenn Waldron / Pelican Mapping