Hour 9. Relationships and Making Nodes Work Together

Maya’s versatility stems mainly from the fact that Autodesk does not try to predict or control the way their users will take advantage of Maya’s tools. Instead, they create a wide array of tools that can be connected in a near infinite number of ways. Ingenious Maya users around the world discover new ways to utilize Maya’s features in new ways every day. We’re going to take a look at how some relationships and connections are created for you, and how to create them yourself.

In this Hour, you will see the many subtle ways Maya connects nodes, and you’ll connect some yourself. You will also see how the different nodes can work together to produce some pretty impressive results, and you will hopefully get ideas for new ways to elevate Maya’s functionality further.

As you can see, even creating something as simple as a polygonal sphere creates multiple nodes that are connected. The creation node (with the sphere’s creation attributes) outputs to the shape node, the shape node is a child of the transform node, and so on. Clicking on the initialShadingGroup node and then clicking on Windows, Hypergraph: Connections again will open a second Hypergraph, showing all of the connections that this shading group has created. Figure 9.2 shows just how many connections are present for default nodes! The number of connections that are created by default even for the most rudimentary objects and effects is quite staggering. Again, though, you need not concern yourself with 90% of the connections that are made; only how to make custom connections to achieve specific results.

The Hypergraph is one of the easiest ways to see connections, because you can graph all of the connections of a selected node with the click of a button. Lines that go into the left side of a node in the Hypergraph are inputs, and lines that go out the right side are outputs. You can see the direction of a connection by the arrows that are drawn along the lines (the direction meaning a connection goes from the output of one node to the input of another). The Hypershade also graphs connections similarly. I created a very simple shading network and graphed it for you to demonstrate the similarity in Figure 9.3.

This is a simple shading network, showing all of the connections Maya creates automatically. The Ramp texture was created by simply clicking on the texture button to the right of the color channel. Maya created the correct connections when I chose the Ramp.

To make a connection in the Connection Editor, click on any attribute in the left panel (Outputs) and click on one or more attributes in the right panel (Inputs). You can have a single output connect to more than one input, but you cannot connect multiple outputs to the same input. Let’s load some attributes into these panels so we can start creating connections.

Create a polygonal sphere by clicking Create, Polygon Primitives, Sphere. In the Connection Editor, click at the top left where it says “Reload Left.” You will see that all of the potential outputs have loaded into the left pane, as in Figure 9.5.

Click on “Reload Right” on the top right and see the attributes load in the Inputs pane, as in Figure 9.6.

Selecting an attribute on the left makes it so Maya will use the output of this attribute as the input of one or many attributes on the right. Remember what was said in about compatible data in Hour 7, “Creating Node Networks in the Hypershade”? You can only connect an attribute that outputs compatible data to your desired input. Maya gives you visual feedback by graying out any input that is incompatible with the selected output. Let’s see this in action. In Figure 9.7, you can see that if we select the “caching” attribute, Maya grays out most of the inputs on the right because they do not use the same type of data.

We will now connect an attribute that is compatible and see the result. On the left pane, click on the + sign next to Translate and then click on translateZ. On the right pane, click on the + sign next to Scale and then click on scaleY. Maya has connected these two attributes. You will notice that like in Figure 9.8, the sphere has completely flattened. This is due to the fact that the current translateZ value is 0.

Open the Channel Box on the right of the screen and look at the channels for the sphere. The translate Y attribute is highlighted, which is Maya’s way of showing you that an attribute has a connection. As you see in Figure 9.9, Maya does not show you the output connection on Translate Z (the attribute that is controlling Scale Y); only the input channel is highlighted.

Now we’re going to test the connection. In the Persp panel, select and move the sphere along the Z axis. Notice as you move the sphere, the height (scaleY) is changing to match the translateZ values, as in Figure 9.10. Also notice that the connection is in real time, and does not require any calculation. Later on you will see that certain types of Maya tools and commands update at different times, such as expressions and constraints.

This connection is interesting, but it does not have much real-world application. Let’s make a connection that might be made in a real-world application, one that will help us in Hour 10, “Basic Rigging: Preparing Objects for Animation,” when we start creating basic rigs.

With the objects scaled nicely, we will create a connection between a custom attribute on the NURBS curve and an attribute on the sphere. Select the circle and open the Channel Box. At the top of the Channel Box, click on Edit, Add Attribute, as shown in Figure 9.12.

Now in the resulting menu that opens, we are going to create a new attribute that will later control the sphere material’s transparency. In the Long Name attribute, type in “transparency.” Leave the Data Type on “float” and make the Minimum 0, the Maximum 1, and the Default 0, like I did in Figure 9.13.

Click Ok, and you will see a new attribute called “Transparency” has been added to the Channel Box. Now we will connect this attribute to lambert1’s transparency channel. With the circle still selected, open the Connection Editor. Its attributes should be loaded on the left. Your custom attribute of “transparency” should be all the way on the bottom of the list on the left. Now open the Hypershade (click Windows, Rendering Editors, Hypershade). Select lambert1 and in the Connection Editor click on Reload Right. The attributes of lambert1 should all load into the right pane. Now we will make the connection. Scroll down all the way in the left pane and find “transparency.” Click on it and then find “transparency” in the right column as well. Click on the + symbol to expand its attributes and then click on transparency R, G, and B. If done correctly, it should look like Figure 9.14. The circle’s custom attribute is now doing what is commonly called “driving” the transparency of lambert1.

Select the circle in the Persp panel, and in the Channel Box, click on the Transparency attribute and MMB drag back and forth to test out the connection. If working properly, the attribute will make it possible for you to control and animate the transparency of the material directly. In Figure 9.15, I have adjusted the attribute to .5, which is giving me a 50% transparent material.

Click here to view code image

pSphere1.scaleY = pSphere1.translateZ

This is simply telling Maya that when it evaluates the transformations of pSphere1, to make the Scale Y equal to the Translate Z value.

Although there is really no limit to the functionality you can achieve with expressions, it should be noted that expressions are very high level, and as such are evaluated late in the process of a scene update and can slow down a scene considerably depending on what the expression is doing. There are many low-level nodes that are very fast and can offer the same functionality as an expression in most cases, and are not affected by the same problems, such as scene updating and the order of operations. Still, we will talk more about creating expressions in Hour 13, “Animation: Adding Movement to Your Scene.”

Driven keys have the benefit of working very well and updating in real time, but have the drawback of being very high level and, as such, cause scene slowdown. They might give a lot of control but they are not the most computationally inexpensive way to create relationships between nodes. We will go over driven keys in more depth in Hour 13.

Figure 9.17 shows the Relationship Editor open, with the Sets menu set loaded within it. Note that we will always choose what type of node we want to connect in this editor by using the drop-down box in the top left instead of opening a new Relationship Editor window through Maya’s main menu.

We used the Connection Editor first so that you would have some familiarity with the paradigm of choosing attributes on the left and connecting them on the right. This is the same thing. On the left pane, Maya displays all of the possible nodes you can use to create a relationship, and in the right pane, all of the possible nodes that can be included in that relationship. To create a relationship, or to include a node in a set, simply select the correct node on the left and then select the node you want to add on the right. Maya has this special editor because the tasks to create the relationships manually would be very tedious. Suffice it to say, this editor makes it very easy to include or exclude objects from a set you’ve created, make lights affect certain objects and not affect others, or choose what UV set a texture uses when it is applied to a mesh.

Now, the same type of functionality is possible with expressions. If we wanted this functionality with an expression, we would type the following in the Expression Editor:

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pSphere1.scaleY = .5 * pSphere1.translateZ

As you might guess, the more complicated the relationship, the more difficult it can be to find the correct nodes to connect. Expressions might need to take over as the complexity goes too high to build the right node network for the result you desire. Finally, if control is really important, and you are going for an intuitive relationship, a driven key is a great idea. We will go over them in depth in Hour 13, but know that they offer an intuitive way to control the effect one node as on another—effects that could be extremely cumbersome to create with connected nodes, or require a very high level of math knowledge to achieve with expressions.

Depending on what you are looking for, choosing the right relationship to create normally starts with connections and then goes to expressions or driven keys as the complexity and/or control goes up.

A. They are not compatible! You must make sure that the data you want to drive another attribute comes in the same format that the attribute reads. For instance, a color swatch reads data in the format [r,g,b], with each value being between 0 and 255. You can’t simply connect a translation value to it; Maya doesn’t know what to do with data coming in a different format.

Q. What is the fastest way to delete a connection?

A. If you want speed, the fastest way is to right-click on the attribute in either the Channel Box or Attribute Editor and choose Break Connections. This is break all of the connections on that attribute, though. If you have more than one connection, you may instead want to select the connection in the Hypershade or Hypergraph and delete it, or deselect the node in the Connection Editor.

Q. Why can’t I change the value of a connected attribute?

A. You probably created the connection in the wrong direction. An attribute becomes basically locked when it has an input connection. You can only change the value of the attribute driving the connection, so check and see if you have the nodes switched in the Connection Editor.

Q. Why does the Relationship Editor not show any attributes?

A. The Relationship Editor connects nodes in predefined ways. For instance, it allows you to connect a UV set with a texture. This is a common task that in order to do manually through other editors would be cumbersome. Maya gives you this editor so that you can quickly create a small number of relationships that are common and essential to Maya. However, Maya can’t predict the other infinite connections you can create with the Connection Editor, which is why it is totally open.

2. What does it mean that an attribute is “driving” another?

3. Which type of connection allows you to create keyframes that act as grid points to control a connection?

4. What are the drawbacks of using expressions to connect attributes?

5. How many different Relationship Editors are there?

2. An attribute whose output is connected to the input of another node is “driving” that node.

3. Driven keys are relationships that are edited on a grid called the Graph Editor.

4. Expressions are calculated at a high level and therefore can be buggy and not evaluate correctly in a complex scene. They also can cause scene slowdown.

5. There is only one Relationship Editor. Maya breaks up the more common relationship types into different menus so that when you open the Relationship Editor, you are immediately presented with the correct menu set.