[Gavino]

Originally Posted by ChrisBeringB

a: MT_Lutxyz(mask, frame1, frame2, yexpr="some expression f(x,y,z)")
b: MT_Lutxyz(frame2, mask, frame1, yexpr="some expression f(y,z,x)")
c: MT_Lutxyz(frame1, frame2, mask, yexpr="some expression f(z,x,y)")
...
I noticed x264 didn't give exactly the same output for options a,b,c.
This suggests a fundamental bug.

Perhaps just a typo, but you seem to have the expressions for b and c the wrong way round: b should be f(z,x,y) and c should be f(y,z,x). That might explain why you get different results.