Hi Nikolay,
Steve Wright is a great instructor, I followed him since I had as his “student” a hands-on class in 2005 in San Francisco with him (GreenScreen Intensive, 10 days).
What he said is correct, and pretty much all he says, especially in his many books. I have read all, and every edition. So, I guess I know what he is talking about ;o). Since you have the visuals from him, I can leave it that way.
What you take away from it, “… but it turned out its just unpremultiplied alpha. …”, is not in general true.I don’t know even how to understand it that way from his content. If the RGB values were mixed before with other values, not straight, then no un-premultiply option with a gray map based matte/alpha will cure this. Not possible—this is important to understand!
A premultiplied image must have the RGBA information stored, as RGB has by itself no transparency information. BUT the RGB can be straight or mixed (on the edges or in semi transparent areas). If it is based on a mixture, again, it will not turn at any time into a straight RGB value for that object just by un-multiply it. It NEEDS (!) to be render separate and straight to be that way. No shortcut, no magical Nuke Node can do that. In his green-screen matte (later in the clip) you will not find the term “straight”, BTW.
Object Buffer, have no idea what they do in terms of Premultiplied or Straight. They just estimate the needed values to make the object transparent or semi transparent where it is given by the object. This is independent from Premultiplied or Straight, simply put, any alpha.
If the color values are very close of the Object1 and Object2 along the edge, then it works in any case to a certain degree and quality. Mostly this is used when time is of the essence or other reasons, missing knowledge.
The idea to “choke” the matte, would leave a black seam on your floor, or if done for both a gap in the compositing.
Often Buffers are rendered out for minor color corrections or in the same way to fade the colors of a back ground object, to simulate “atmosphere”. Those minor adjustments work great with a single pass RGB and Object Buffer.
But when it comes to heavy color corrections, the edge pixels are most-likely wrong after that. Again, it si about quality vs time.
In this Quality vs Time area, Cinema 4D has to deliver both options, as users request this. It doesn’t mean that when it is possible to do, that you get both, great quality and a short time (e.g., render)
(As a side note, if you follow NUKE and the idea of deep compositing, then we have to talk on a complete different level, since C4D has not that option, this forum is not the place for it.)
All the best
This is a screenshot from the tutorial, and clearly it is straight alpha image to begin with not mixture of any other object