Maya Python For Games and Film A Complete Reference For Maya Python and The Maya Python API 1st Edition Adam Mechtley PDF Download
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Chapter
CHAPTER OUTLINE
Interacting with Maya 4
Maya Embedded Language 5
Python 5
C++ Application Programming Interface 6
Python API 6
Executing Python in Maya 6
Command Line 6
Script Editor 8
Maya Shelf 10
Maya Commands and the Dependency Graph 11
Introduction to Python Commands 15
Flag Arguments and Python Core Object Types 19
Numbers 20
Strings 20
Lists 20
Tuples 21
Booleans 21
Flag = Object Type 21
Command Modes and Command Arguments 22
Create Mode 22
Edit Mode 23
Query Mode 23
Python Command Reference 24
Synopsis 25
Return Value 25
Related 25
Flags 25
Python Examples 26
Python Version 26
Python Online Documentation 26
Concluding Remarks 27
Maya Python for Games and Film. DOI: 10.1016/B978-0-12-378578-7.00001-6
© 2012 Elsevier Inc. All rights reserved. 3
4 CHAPTER 1 Maya Command Engine and User Interface
To fully understand what can be done with Python in Maya, we must first
discuss how Maya has been designed. There are several ways that users
can interact with or modify Maya. The standard method is to create content
using Maya’s graphical user interface (GUI). This interaction works like any
other software application: Users press buttons or select menu items that
create or modify their documents or workspaces. Despite how similar Maya
is to other software, however, its underlying design paradigm is unique in
many ways. Maya is an open product, built from the ground up to be capable
of supporting new features designed by users. Any Maya user can modify or
add new features, which can include a drastic redesign of the main interface
or one line of code that prints the name of the selected object.
In this chapter, we will explore these topics as you begin programming in
Python. First, we briefly describe Maya’s different programming options and
how they fit into Maya’s user interface. Next, we jump into Python by exploring
different means of executing Python code in Maya. Finally, we explore some
basic Maya commands, the primary means of modifying the Maya scene.
Python
Python is a scripting language that was formally introduced to Maya in
version 8.5. Python can execute the same Maya commands as MEL using
Maya’s Command Engine. However, Python is also more robust than MEL
because it is an object-oriented language. Moreover, Python has existed
since 1980 and has an extensive library of built-in features as well as a
large community outside of Maya users.
6 CHAPTER 1 Maya Command Engine and User Interface
Python API
When Autodesk introduced Python into Maya, they also created wrappers
for many of the classes in the Maya C++ API. As such, developers can use
much of the API functionality from Python. The total scope of classes acces-
sible to the Python API has grown and improved with each new version of
Maya. This powerful feature allows users to manipulate Maya API objects
in ordinary scripts, as well as to create plug-ins that add new features to Maya.
In this book, we focus on the different uses of Python in Maya, including
commands, user interfaces, and the Python API. Before we begin our
investigation, we will first look at the key tools that Maya Python program-
mers have at their disposal.
Command Line
The first tool of interest is the Command Line. It is located along the
bottom of the Maya GUI. You can see the Command Line highlighted in
Figure 1.2.
The Command Line should appear in the Maya GUI by default. If you
cannot see the Command Line, you can enable it from the Maya main
menu by selecting Display → UI Elements → Command Line.
The far left side of the Command Line has a toggle button, which says
“MEL” by default. If you press this button it will display “Python.”
Executing Python in Maya 7
The language displayed on this toggle button tells Maya which scripting
language to use when executing commands entered in the text field imme-
diately to the right of the button. The right half of the Command Line, a
gray bar, displays the results of the commands that were entered in the text
field. Let’s create a polygon sphere using the Command Line.
1. Switch the Command Line button to “Python.” The button is located on
the left side of the Command Line.
2. Click on the text field in the Command Line and enter the following
line of text.
import maya.cmds;
3. Press Enter.
4. Next enter the following line of code in the text field.
maya.cmds.polySphere();
5. Press Enter. The above command will create a polygon sphere object
in the viewport and will print the following results on the right side
of the Command Line.
# Result: [u'pSphere1', u'polySphere1']
8 CHAPTER 1 Maya Command Engine and User Interface
You can use the Command Line any time you need to quickly execute a
command. The Command Line will only let you enter one line of code at
a time though, which will not do you much good if you want to write a
complicated script. To perform more complex operations, you need the
Script Editor.
Script Editor
One of the most important tools for the Maya Python programmer is the Script
Editor. The Script Editor is an interface for creating short scripts to interact
with Maya. The Script Editor (shown on the right side in Figure 1.2) consists
of two panels. The top panel is called the History Panel and the bottom panel
is called the Input Panel. Let’s open the Script Editor and execute a command
to make a sphere.
1. Open a new scene by pressing Ctrl + N.
2. Open the Script Editor using either the button located near the bottom
right corner of Maya’s GUI, on the right side of the Command Line
(highlighted in Figure 1.2), or by navigating to Window → General
Editors → Script Editor in Maya’s main menu. By default the Script
Editor displays two tabs above the Input Panel. One tab says “MEL”
and the other tab says “Python.”
3. Select the Python tab in the Script Editor.
4. Click somewhere inside the Input Panel and type the following lines of
code.
import maya.cmds;
maya.cmds.polySphere();
5. When you are finished press the Enter key on your numeric keypad. If
you do not have a numeric keypad, press Ctrl + Return.
The Enter key on the numeric keypad and the Ctrl + Return shortcut are
used only for executing code when working in the Script Editor. The reg-
ular Return key simply moves the input cursor to the next line in the Input
Panel. This convention allows you to enter scripts that contain more than
one line without executing them prematurely.
Just as in the Command Line example, the code you just executed created a
generic polygon sphere. You can see the code you executed in the History
Panel, but you do not see the same result line that you saw when using the
Command Line. In the Script Editor, you will only see a result line printed
when you execute a single line of code at a time.
6. Enter the same lines from step 4 into the Input Panel, but do not execute
them.
Executing Python in Maya 9
7. Highlight the second line with your cursor by triple-clicking it and then
press Ctrl + Return. The results from the last command entered should
now be shown in the History Panel.
# Result: [u'pSphere2', u'polySphere2']
Apart from printing results, there are two important things worth noting
about the previous step. First, highlighting a portion of code and then press-
ing Ctrl + Return will execute only the highlighted code. Second, high-
lighting code in this way before executing it prevents the contents of the
Input Panel from emptying out.
Another useful feature of the Script Editor is that it has support for marking
menus. Marking menus are powerful, context-sensitive, gesture-based
menus that appear throughout the Maya application. If you are unfamiliar
with marking menus in general, we recommend consulting any basic Maya
user’s guide.
To access the Script Editor’s marking menu, click and hold the right mouse
button (RMB) anywhere in the Script Editor window. If you have nothing
selected inside the Script Editor, the marking menu will allow you to
quickly create new tabs (for either MEL or Python) as well as navigate
between the tabs. As you can see, clicking the RMB, quickly flicking to
the left or right, and releasing the RMB allows you to rapidly switch
between your active tabs, no matter where your cursor is in the Script Edi-
tor window. However, the marking menu can also supply you with context-
sensitive operations, as in the following brief example.
1. Type the following code into the Input Panel of the Script Editor, but
do not execute it.
maya.cmds.polySphere()
2. Use the left mouse button (LMB) to highlight the word polySphere in
the Input Panel.
3. Click and hold the RMB to open the Script Editor’s marking menu. You
should see a new set of options in the bottom part of the marking menu.
4. Move your mouse over the Command Documentation option in the
bottom of the marking menu and release the RMB. Maya should
now open a web browser displaying the help documentation for the
polySphere command.
As you can see, the Script Editor is a very useful tool not only for creating
and executing Python scripts in Maya, but also for quickly pulling up infor-
mation about commands in your script. We will look at the command
documentation later in this chapter.
10 CHAPTER 1 Maya Command Engine and User Interface
Maya Shelf
Now that you understand how to use the Command Line and the Script
Editor, it is worth examining one final tool in the Maya GUI that will be
valuable to you. Let’s say you write a few lines of code in the Script Editor
and you want to use that series of commands later. Maya has a location for
storing custom buttons at the top of the main interface, called the Shelf,
which you can see in Figure 1.3. If you do not see the Shelf in your GUI
layout, you can enable it from Maya’s main menu using the Display →
UI Elements → Shelf option.
You can highlight lines of code in the Script Editor or Command Line and
drag them onto the Shelf for later use with the middle mouse button
(MMB). In the following example, you will create a short script and save it
to the Shelf.
1. Type in the following code into the Script Editor, but do not execute
it (when executed, this script will create a polygon sphere and then
change the sphere’s vertex colors to red).
import maya.cmds;
maya.cmds.polySphere(radius=5);
maya.cmds.polyColorPerVertex(
colorRGB=[1,0,0],
colorDisplayOption=True
);
2. Click the Custom tab in the Shelf. You can add buttons to any shelf, but
the Custom shelf is a convenient place for users to store their own
group of buttons.
3. Click and drag the LMB over the script you typed into the Script Editor
to highlight all of its lines.
4. With your cursor positioned over the highlighted text, click and hold
the MMB to drag the contents of your script onto the Shelf.
5. If you are using Maya 2010 or an earlier version, a dialog box will
appear. If you see this dialog box, select “Python” to tell Maya that
the script you are pasting is written using Python rather than MEL.
6. You will now see a new button appear in your Custom tab. Left-click on
your new button and you should see a red sphere appear in your viewport
as in Figure 1.3. If you are in wireframe mode, make sure you enter shaded
mode by clicking anywhere in your viewport and pressing the number 5 key.
You can edit your Shelf, including tabs and icons, by accessing the
Window → Settings/Preferences → Shelf Editor option from the main
Maya window. For more information on editing your Shelf, consult the
Maya documentation or a basic Maya user’s guide. Now that you have an
understanding of the different tools available in the Maya GUI, we can start
exploring Maya commands in greater detail.
data—creation, destruction, editing, and so on. All of the data in the core are
represented by a set of objects called nodes and a series of connections that
establish relationships among these nodes. Taken together, this set of relation-
ships among nodes is called the Dependency Graph (DG).
For example, the polygon sphere object you created earlier returned the
names of two nodes when you created it: a node that describes the geome-
try of the sphere and a transform node that determines the configuration of
the sphere shape in space. You can see information on nodes in an object’s
network using the Attribute Editor (Window → Attribute Editor in the
main menu) or as a visual representation in the Hypergraph (Window →
Hypergraph: Connections in the main menu). Because this point is so
important, it is worth looking at a brief example.
1. If you no longer have a polygon sphere in your scene, create one.
2. With your sphere object selected, open the Hypergraph displaying
connections by using the Window → Hypergraph: Connections option
from the main menu.
3. By default, the Hypergraph should display the connections for your
currently selected sphere as in Figure 1.4. If you do not see anything,
then select the option Graph → Input and Output Connections from
the Hypergraph window’s menu.
As you can see, a default polygon sphere consists of four basic nodes
connected by a sequence of arrows that show the flow of information. The
first node in the network is a polySphere node, which contains the para-
meters and functionality for outputting spherical geometry (e.g., the radius,
the number of subdivisions, and so on). In fact, if you highlight the arrow
showing the connection to the next node, a shape node, you can see what
data are being sent. In this case, the polySphere node’s output attribute is
piped into the inMesh attribute of the shape node.
If you were to delete the construction history of this polygonal sphere
(Edit → Delete by Type → History from the main menu), the polySphere
node would disappear and the sphere’s geometry would then be statically
stored in the shape node (pSphereShape1 in Figure 1.4). In short, if the
polySphere node were destroyed, its mesh information would be copied into
the pSphereShape node, and you would no longer be able to edit the radius
or number of subdivisions parametrically; you would have to use modeling
tools to do everything by hand.
While you can also see that information is piped from the shape node into a
shadingGroup node (to actually render the shape), there is a node that
appears to be floating on its own (pSphere1 in Figure 1.4). This separate
node is a special kind of object, a transform node, which describes the posi-
tion, scale, and orientation of the polygonal sphere’s geometry in space. The
reason why this node is not connected is because it belongs to a special part
of the DG, called the Directed Acyclic Graph (DAG). For right now, it suf-
fices to say that the DAG essentially describes the hierarchical relationship of
objects that have transform nodes, including what nodes are their parents
and what transformations they inherit from their parents.
The Maya DG is discussed in greater detail in Chapter 11 in the context of
the Maya API, yet this principle is critical for understanding how Maya
works. We strongly recommend consulting a Maya user guide if you feel
like you need further information in the meantime.
Although Maya is, as we pointed out, an open product, the data in the core
are closed to users at all times. Autodesk engineers may make changes
to the core from one version to another, but users may only communicate
with the application core through a defined set of interfaces that Autodesk
provides.
One such interface that can communicate with the core is the Command
Engine. In the past, Maya commands have often been conflated with
14 CHAPTER 1 Maya Command Engine and User Interface
documentation and are not directly available to Python. Again, this absence
is also not a limitation, as it is possible to execute MEL scripts with Python
when needed. Likewise, MEL can call Python commands and scripts when
required.1
Another important feature of the Maya Command Engine is how easy it is to
create commands that work for MEL and Python. Maya was designed so that
any new command added will be automatically available to both MEL and
Python. New commands can be created with the Maya C++ API or the Python
API. Now that you have a firmer understanding of how Maya commands fit
into the program’s architecture, we can go back to using some commands.
The first line shown is the polyCube MEL command, which is very similar
to the polySphere command we used earlier in this chapter. As you can see,
1
MEL can call Python code using the python command. Python can call MEL code
using the eval function in the maya.mel module. Note that using the python command
in MEL executes statements in the namespace of the __main__ module. For more infor-
mation on namespaces and modules, see Chapter 4.
16 CHAPTER 1 Maya Command Engine and User Interface
a MEL command was called when you selected the Cube option in the
Polygon Primitives menu. That MEL command was displayed in the
Script Editor’s History Panel.
Because Maya’s entire interface is written with MEL, the History Panel
always echoes MEL commands when using the default Maya interface.
Custom user interfaces could call the Python version of a command, in
which case the History Panel would display the Python command.
This problem is not terribly troublesome for Python users though. It does
not take much effort to convert a MEL command into Python syntax, so
this feature can still help you learn which commands to use. The following
example shows what the polyCube command looks like with Python.
import maya.cmds;
maya.cmds.polyCube(
w=1, h=1, d=1, sx=1, sy=1, sz=1,
ax=(0, 1, 0), cuv=4, ch=1
);
If you execute these lines of Python code they will produce the same result as
the MEL version. However, we need to break down the Python version of the
command so we can understand what is happening. Consider the first line:
import maya.cmds;
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