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Its probably because your hardware is not quite up to scratch, which Mac do you have?
 
It may be dependent on video card.

On my office Mac, it was non-transparent. On my home Mac it's was semi-transparent. On my MBP, it was very much transparent.

Each successive Mac has a better video card.
 
I upgraded from tiger and it was transparent, now I just did a clean install and its not transparent?
 
If you're using a machine that doesn't support the full Core Image stuff then it's not going to be transparent.

but when i did a upgrade it was transparent
but today i did the clean install and its not transparent

should I do another clean install?
 
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