Tutorial of the Month – Nuke Expand Edges or how to get rid of outlines
Ok, here we go, another Keying Tip, which I have to use sometimes to enhance Edges. This is a nice trick especially when dealing with lower quality material, like DSRL greenscreens, where Edges often tend get jaggy or you end up having black or white outlines. So let’s start
I build this tutorial based on the previous one, where we prepared the plate using the clean screen technique. I pulled a quick key using the IBK Keyer and the standard Greenscreen Keyer for the inner Hard matte, which looks alright for a start and as this is not about Keyin0g, I can’t be bothered to make a fantasti(s)c(h) mask. But when we AddMix (and I always use an AddMix) the FG over our Background we will end up having a nasty black Outline, especially on the right shoulder.
So let’s fix that. Most compositors , when having outlines start to try to use an Edge Detect and ColorGrade the Edge, or some even Erode the heck out of their masks. Color Grading can work, depending on your background, Eroding rarely works or better say should be avoided, as you don’t want to lose any detail.
So what can we do ?? Well, as we have a nice edge, and we have a nice foreground image, we can try to expand the color information into this nasty area, so that it looks more like a pleasing edge. If you have read the Even Out Tutorial you know, that an UNPREMULT Operation can expand Color Information to fill in certain areas, and this is exactly what we need. To begin with, we premultiply our foreground, and start by adding a BLUR node. We will start with small values here , like 2, and after that add an UNPREMULT operation. You will see that the foreground image extends a little bit further out now. Ok, good for a start, so now add a MERGE and put that on top of our Original, and do the same again. PREMULT ->BLUR->UNPREMULT - this time the BLUR value can be increased slightly, to like 4.
We do this again and again …let’s say 4 or 5 times- by always increasing the blur amount slightly….you can play around with that. We want to go in small steps just because we do not want to expand too much out, as this will look like a warped edge and we want to save as much detail as possible, so therefore rather increase the number of iterations than using a too high blur value
Ok, now we have to bring back the original, and to do that, we need to shrink the alpha slightly and put the original on top of our extended FG…which looks like the image below. Sometimes you will need an EDGEBLUR to hide the transition a little bit, and NEVER forget to add GRAIN to the extended Edges.
Finally we copy the original Alpha Channel into the new FG Plate and AddMIX that on top of the Background. As you can see, all the black outlines are gone, and we were able to keep almost all of the detail in the FG. Below is a before and after…so judge on your own..it sometimes is amazing how easy it can be
Again you can find a sample script, which also includes the Even Out part here. Play around with it and enhance your edges guys !!