I have been working in 3dsmax for six years now, so I feel like an idiot asking this question now. But I am a compositor primarily, so go easy on me. Anyway, my question is regarding "matte lines" when I render objects with motion blurs with alpha channels. Example: I render a blue sphere moving with a big motion blur across the screen. The environment is black. I render it with an Alpha pass. Now when I go into After Effects and use the alpha pass as a luma matte for the sphere, and lets say I want to put it on a redish background, there are black matte lines that appear. It makes sense of course that this happens. The blue sphere with a motion blur rendered against black will show a blue to black gradient at the edges. And the alpha will reveal whatever you see in the main pass of the sphere. What I have been doing for the past many years, is render the object against the bakground that it appears against in the final shot. But this is dumb. I always add so many other layers to the bg during compositing, so it ends up not being the same as whatever my 3d object was rendred out on. So my big question: How do people really do this? How do the pros do it? They obviously don't render it out against what the final background of the shot will be?