Hey,

If i render a clip 2;1, then mix it down, does it recompress again  at 2:1 or does it use the media already created at that resolution to make the mixdown?

thanks

I'm pretty sure it 'copies' it.  I did some difference matte tests in Photoshop a while ago to try and answer this very question, and the mixed-down and unmixed-down frames were identical (at least visually)

Anecdotally, the speed of mixing down rendered vs. unrendered effects would also support this conclusion - but I guess we'd need an Avid developer to see this thread before we get a definitive answer.