Lesson #04 AE Premult in Nuke

Today´s Lesson is about the great possibility in After Effects to perform a Premultiplied – Matted with Color. Imagine you got a 3D Rendering not on black but for instance on white.

At this point I want to say thank you to Christian Zilliken an awesome Lead Shading Artist, I worked with on numerous shows. He supports this tutorial with a little 3D spider

Website  http://ardworx.de

IMDB:     http://www.imdb.com/name/nm3646381


The above rendering is how it should look like, and the below one is how you get it for Compositing. So what would your first steps be.


If you just do a Premult on the white Background Image you get something like this



Do you see the white Outline, that´s not good. So what´s your next step.

Please, NEVER EVER NEVER  run first to your Supervisor complain about the CG Artist and say you won´t start until your Renderings are perfect. Your CG Artist is your best friend on your shot. He will valuate it if you fix minor problems in comp without being a drama queen, he will support you with even more awesome CG Renderings, and he will ask for you to be the comper on his next shot.

Even because you can fix these problems within less than one minute. Try to find your supervisor within this time.

First we start with with creating a constant with your actual background color, in our case it´s white.

Next we will invert our CG Rendering Alpha Channel and multiply it with our Constant Color Channels. Image

It will look like this one. Keep the background color information and loose the foreground color information. The foreground gets a zero value, and semitransparent values will get darker upon their visibility.


The whole trick on this tutorial is just to subtract this “only background color image” with your CG Rendering


So you will subtract all the background color information by the same value it has in your Rendering. Your background color gets zero. Because your foreground object is black and has a value of zero. You won´t change values on your foreground object and it stays untouched


You´re almost done. Just your Alpha channel may get messed up after all the operations. Shuffle your original Alpha Channel back into the flow.


You see you would have lost more time searching the Supervisor than doing this fast fix.

In some cases you get these Layers with a colored background only this way technically correct. Remember a VrayBumpNormals Pass.Image

With this method you can face even this problem.