I'm rendering (in Karma) a cloud of very dense smoke whose center should obviously be completely opaque. Some of the pixels end up with an Alpha value that isn't exactly 1, however, more like 0.99124. All pixels that do not have an Alpha of 1 have that same exact value. Composited on a bright sky, the smoke cloud ends up looking noisy. Any idea what's happening (or how to fix it)?

Try increasing the opacity samples. The default is very low. The default number of primary samples is also extremely low for Karma. 9 works okay with Mantra but Karma usually needs like 32 or more to have a chance at anti-aliasing. You could do a test with opacity sample set to 0, this disables the stochastic sampling, it will be very slow but you will see if the volume becomes opaque or not.

Thanks jsmack, this seems to fix it. I can't afford rendering everything with more samples, but doing it on a much faster unlit render, and shuffling its Alpha in my previous, lower sample, lit render seems to do the trick.

Speaking of which, any good resources on optimizing Karma volume renders?