transparency/opacity

Hi,


Probably what I'll ask here is a well known thing... But I don't get the
picture:

I get the felling that some magick++ function handles opacity as expected,
that is that the opacity increase (i.e. the transparency decreases) when
opacity values increases while others handles the opacity as a transparency
value (i.e. the image gets more transparent when the so-caleed "opacity"
increases). Does "opacity" means transparency then?. There is very little
information around this, but when I use pixel_cache[cache_index].opacity I
clearly get the opposite of want I expected: Transparent parts seems to get
MaxRGB value while "opaque" part gets 0...

Did I mess any simple thing expalining how this is used?

Thanks to anyone able the clarify this!

C. Milard