Re: Problem with Multipass Object Buffers

I like the way the motion blur in C4D looks better than using any of the post blurs I have used in After Effects. I too have found that you can't use object buffers to mask motion blurred renders.

Two ways you might approach it:

1. On a separate render, replace the texture on the blurred objects with 100 white in the Luminance channel of the texture, and make everything else in the scene black (0 in the Lum channel.) Don't include any other texture channels (except Alpha if needed.)

Render without an Alpha channel: this image itself will be used as one. The scene should render very quickly (although the amount of blur will affect this,) and you will get a black and white image, with blurred edges, that you can use like an object buffer. (This will work best if you checked "Straight Alpha" in the render settings of the original full-color render.)

2. Render each blurred object separately, with everything else in the scene tagged with a Compositing tag that has "Seen by camera" unchecked. Make sure you check "Alpha Channel" and "Straight Alpha" in the render settings. In a separate render, tag all the blurred objects with a Compositing tag that has "Seen by camera" unchecked, and have all the unblurred objects visible. Set appropriate objects buffers for the scene. Then comp the blurred renders over the unblurred render, using the object buffers to mask to taste.

I would love to hear other methods, but each has worked for me. And, by the way, if anyone has told you not to use motion blur with interlaced rendering, listen to them. Interlacing and motion blur don't work well in C4D.