Using HAP codecs v12 rendered from After Effects
HAP Q and HAP Q ALPHA are not currently supported.
If the composition background is black both straight and premultiplied HAP ALPHA function correctly and are transparent
If the composition background is white straight HAP ALPHA render transparent /premultiplied renders white.
Here's my current thinking on HAP with QLab in general
There are some misconceptions around HAP probably due to it's origins in the VJ community. Standard HAP is a very high quality codec, comparable to ProRes HQ rather than the lower quality versions of ProRes. In my testing there does not seem to be much difference in performance between it and ProRes HQ. If sequences are preloaded HAP has the edge if they are not then ProRes gives better results.
HAP alpha is more compact than Prores4444 and gives better performance in QLab.
Obviously the results might be different on other systems and with other test material but my current summary would be.
If you are primarily interested in maximum number of simultaneous clips then use ProRes proxy or LT
If you are primarily interested in video quality then try HAP or ProRes HQ
If you are doing clips with transparency then HAP alpha may have advantages over ProRes4444.
Preloading HAP clips seems more important than with other codecs to get best performance, and if not preloaded starts can be a bit raggedy.
It's also worth saying that your mileage may well vary with this, and if you do a lot of video work it is worth doing your own evaluations, to see if you agree with my findings or not!
Mic