Mirko, just a quick question/tip... why do you need to clear the depth buffer after drawing the skydome?
Wouldn''t it work exactly the same if you just disabled depth writes (ie glDepthMask) while drawing the skydome (which I assume you draw first, before drawing everything else) ...this way, you don''t get your landscape clipped by the dome, and you avoid a clear operation... its all good
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Another added bonus of this approach is that no matter how high some object may fly in the sky (like a plane overhead or sumfin), it will never go outside the dome... it reallt makes the sky seem infinitely far away...