Now we have some of the basics out of the way, we can move on to making more complex node networks. This is where the difference between layer-based compositing and node-based compositing really becomes apparent. Just to save some time, I have already read in two pieces of footage: one is a still image of a canyon and the other is an image sequence that features a CG spaceship.

Let’s build a more complex network. I am going to go ahead and connect the Viewer to the canyon. Now, one common task of compositing is to apply effects or filters to alter the image. So, for example, I can blur this canyon. If there are no nodes selected in the Node Graph, and I make a new node - for example, I can go to Filter > Blur with the right-mouse menu - the new node comes in by itself, it’s not connected. There are actually several ways to make these connections between nodes. Of course, one is simply to click+drag a pipe and drop it on top of another node by connecting the Viewer. If you have a pre-existing connection line, for example, Viewer1 pipe is connected to Read1, I can simply click+drag my new node and drop it on the current connection. Now, before we go further, I want to talk about what these pipes mean. You can look at the pipes and see the arrow heads and from that you can determine which direction the information is flowing. So, I can tell the information is flowing out of Read1, through Blur1, and down into Viewer1.

Now, there are other ways to connect these together too. If I disconnect these pipes, click+drag the ends, and break them, you can also simply connect the inputs and outputs. For example, the Blur node has an input pipe. I can tell it’s an input pipe because it’s flowing towards the node on the top here. The Viewer also has an input pipe. Some nodes also have outputs. There is an output right here on Blur1 and an output pipe right here on Read1 - that means the information is flowing out of those nodes. So, I can connect the input of Blur1 to the output of Read1. Now let’s grab the input pipe and drop it on Read1, which means that Read1 is now sending information to Blur1. You can also grab the output of Blur1 and drop it on Viewer1, which means the input of Viewer1 is coming from Blur1. There is a third way to do that, if I disconnect these one more time. Connect the Viewer back up to the canyon and then select canyon. If a node is selected, and I create a new filter like Blur, it is automatically inserted downstream. Let me delete this other Blur, as I don’t need it right now. So, there is a little network. There is no blur happening right now, and one thing that happens with some of the filter nodes is they are off by default. If I increase the size property on the Blur, it becomes blurry.

Another important task of compositing is combining multiple images or pieces of footage. Now, in a layer-based system, you would simply stack layers. In a program like Nuke, you would have to use a special node to merge inputs. Let’s give that a try. I am going to delete the Blur node out. I am going to bring in a Merge node, and that’s under Merge > Merge or the M key. The Merge node has an A and B input, plus an output. You can relate this to layer-based compositing by thinking of the B input as the lower layer, and you can think of the A input as the upper layer. So, what you can do is connect the Viewer to the output of the Merge1. In this case, connect the B to the canyon and connect the A to the spaceship. If I zoom in, I can see my spaceship is now composited over the top of my canyon. Now, the reason that works is there’s alpha transparency around the spaceship. This was rendered in Maya. Nuke automatically recognizes the alpha channel, so this black is converted to transparency. You can tell that Nuke has recognized the alpha because of this little white line right here that’s right beside the red, green, and blue lines. These are channel lines. So, here it recognizes the rgb+alpha, whereas over on the canyon it just sees rgb. In that case, here's your basic merge. Now, there is a few things to know about the merge that are useful. One thing is you can have more than one A input. In fact, as soon as you connect A, the A2 appears on the left. If you connect A2, you will have A3, and so on. The highest A number is equivalent to the highest layer, so A3 is on top of A2, and so on, with B on the very bottom.

Now, there is a mask input on the right side, however, we will talk about that later on when we get to rotoscoping. The Merge node also has a mix slider at the very bottom here. The mix slider controls the influence of the A input. If I reduce that, A starts to fade out. Another important aspect of the Merge node is the operation up here at the top. In layer-based compositing, you may have something like a blending mode. Now, a blending mode determines how the upper layers combine with the lower layers - some type of mathematical formula. In Nuke, it’s the operation and that defaults to over. Over is similar to normal inside of After Effects. You can change this menu though to other styles, for example plus. Plus adds A and B together. In this case, it gives me a semi-transparent, brighter result in that area. I will turn that to over for now. If you are curious what these operations do, let your mouse hover over the menu. You will see all of the different mathematical formulas for all of the different operations. In this case, a capital A and B represents the rgb values of the A inputs and the B inputs, whereas the lower-case a and b indicates the alpha values of A and B inputs.

If you are working with CG renders, you might be concerned with premultiplication. For example, Maya premultiplies automatically and that affects the quality of the alpha edges. If you want to interpret something that is premultiplied, open up the Read node and there is a premultiplied checkbox right here - click that on.

We have now created a more complex network. We have also merged together two inputs. Now, we are ready to move on to working with transforms, and we will do that in the next video.