I’ve downloaded gmax from Discreet and found it supports OpenGL, Direct3D and HEIDI renderer? What is it HEIDI?

heidi is a proprietary 3D-interface, made by discreet; on some graphic-boards you would get better perfmance than with GL; but only, as you can guess, if they are optimized for heidi or if thei bring optimized drivers for heidi with them - but in normal cases, with consumer boards, you will get better performance by using OpenGL.
yes, most people will claim that the performance with D3D is the same, or better; but i have to deny this, because each test i made with D3D in comparison to Gl, was that D3D looses under 3DSMAX - apart from that, D3D seems to be very buggy if you run 3DSMAX for several hours a day, the app crashes very often.
but yes, there is a little chance of D3D: i hat no possiblity to test this issue under 3DSMAX6.0, until now…