Nuke | Myth Busting: Concept of Premultiplication

what's up guys Joey here from school of motion comm on behalf of the foundry thank you guys very much for watching this video if you're just getting into nuke then there's a few things you're going to have to wrap your head around and one of them is the concept of pre multiplication so that's what this video is about and I'm going to attempt to explain it to you in less than five minutes so let's hop into nuke so here we are a nuke and I've loaded in two TIFF files one is going to be my background and that's this nice image of some mountains with a lake and the other image is going to be my foreground and this is an image of a hawk and if I hit a you can see the alpha channel that came along with this tiff file alright so in theory we have an alpha channel that matches the contours of the bird and that should be all we need to composite the bird over this image so let's just try it we're going to use a merge node which is one of the most commonly used nodes in nuke and this is the node that composites one thing over another and the way it works is by compositing a over B alright so now I'm going to look through that merge node and we are going to see a big problem here the background has been kind of tinted blue the bird looks okay but the background got all messed up so once again if you look at the original you can see how nice and crisp this image looks and then once we've merged the bird over it it looks all wrong and and you know our client would be very unhappy if we gave this to them so what do we need to do to fix it well let's go back and look at this image for a second now we have an alpha Channel alright and one of the things that you may not know coming from After Effects is that there's a step that happens before a foreground get smolts composited over a background and that step is called pre multiplication what happens is all of these colored pixels get multiplied at times their alpha channel and the effect of that is all of the pixels on the outside of the bird are going to turn black and all of the pixels on the inside of the bird are going to stay the same and what's really important are these pixels along the edge here these gray ones they are actually going going to darken those pixels on the bird just by a little bit which is going to make the composite work alright and an easier way to think about it is to think of this is a giant sheet of cookie dough and we have this nice bird shaped cookie cutter and we're just going to use that to kind of stamp out the dough and turn all this stuff black because we don't need that okay so I'm going to use a pre-molded all right to do that step of pre multiplying and once we've done that you can see right away all of the outside pixels turn black we've gotten rid of the background okay and now when we look through the merge node voila we have a perfect composite well it's not perfect but now we're ready to color correct and do all the things we need to do to really make it feel like this bird is in this environment so that is the concept of pre multiplication now there's actually some math going on behind the scenes and if you're into that sort of thing you can head over to school of motion comm and check out the article called pre multiplication demystified and that will explain exactly what's going on you don't need to know that but some of you may want to and it might just help your understanding of compositing get a little bit better so thank you guys for watching and if you watch the next video in the series we'll talk about how to deal with grading pre multiplied images and how to manage them so thank you guys for watching and I will see you later

Channel: Foundry

Views: 15,850

Keywords: The Foundry, NUKE, NUKEX, compositing, premultiplication, vfx, tutorial, training

Id: NDwqTuig7yQ

Length: 3min 37sec (217 seconds)

Published: Fri Jan 31 2014