@davpe Thanks David, that's very informative. It's indeed a shame that there is no one actually teaching mantra online(only some pretty old ones from sideFX), which probably is one of the reasons that mantra was not very popular among regular users.
I have some basic knowledge of Arnold and ray tracing concept before switching to use Houdini, but still I found it confusing looking at all the tabs mantra has. (sampling and limits tab are all I have used for now)
So, I was doing a test render yesterday, but even such a simple scene took my computer 50 mins(10 core E5 2640 with k4200), don't know it's suppose to be this slow or I did anything wrong here( all light with default settings , 4X4 pxiel samples, diffuse and refelction quality setting to 2 , diffuse limit to 1). I thought this was a very low setting but still it take such a long time. Is mantra not good at handling simple scene?
also there are some issues with the indirect diffuse, dont know where those straight lines come from and the rectangle noise shape behind the figure.