Anti - Aliasing Halo

[vlado]

I always thought that alpha-compositing is only used if you want to mask out an existing background - in this case, the alpha is used to cut the object out of the background before adding the rgb object itself. However, if the object is already cut out, you just need to add the rgb object, without masking anything. In the the exampe posted by seraph, you will get a perfectly correct result if you simply add the rgb images, ignoring the alpha (if you do need a final alpha too, it is just the sum of the alpha of the two images). Of course, you need to render the different layers to a black background for the adding to work properly.

Best regards,
Vlado

[Seraph135]

Vlado is correct...Adding these two images together AND adding the sum of their alphas together to create a new alpha will create correct results.

A side note about this though. In my example the images were created with image motion blur and the RGB between the matted box and the teapot actually do not add together correctly. When the same thing is done using 3d motion blur in Vray the results are correct.

So it would seem that if your using image motion blur you can NOT make this method work...but with 3d blur you can. Can someone verify that?

Tim J