Games Development 2 Maya Embedded Language MEL

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Games Development 2Maya Embedded Language (MEL)

• CO3301
• Week 14

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Contents

• MEL Introduction
• Alternatives
• MEL Language Overview
• Objects, Nodes Attributes

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Introduction X-files

• So far we have imported meshes almost exclusively
  using Microsoft .X files
• However, we have seen the limitations of the
  format
• Only a single texture per material
• No normal maps, gloss maps etc.
• No implicit support for advanced vertex data
• E.g. tangents, binormals
• Also no support for custom data
• Specialised vertex, polygon or material
  information
• For unusual shaders

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Introduction Scripted Export

• In the 2nd year we noted these limitations and
  those of other file formats
• Concluded that no existing file format is
  flexible enough for the majority of game needs
• Suggested the possibility of writing custom
  exporters
• In the modelling tool - Maya or 3ds Max
• As well as exporting mesh data, could export
  other game data
• Lights, cameras, paths
• Physics info, object stats such as HP, etc.
• AI waypoints, game logic triggers etc.

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Maya MEL Scripting

• In this session we introduce the Maya Embedded
  Language (MEL)
• This scripting language is at the core of the
  Maya user interface
• The entire GUI is written in the language
• All Maya content and features can be accessed /
  controlled with the language
• Artists use MEL to create shortcuts, access
  advanced features and add custom controls
• But we are interested mainly in accessing
  exporting the scene contents

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Alternatives to MEL

• Maya also has a C API
• Internal access to Maya
• High performance, but more complex than MEL
• Latest versions also support Python scripting
• Similar interface and commands to MEL
• Most MEL commands have Python versions
• Further information in Maya documentation
• Using Maya-gtGeneral-gtMEL and Expressions
• Using Maya-gtGeneral-gtPython
• Developer Resources-gtAPI Guide

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MEL Language Overview

• MEL is similar to the language Perl
• With a similar command line style at times
• As such it traces its roots to C
• So it should feel reasonably familiar
• MEL scripts / functions act as new Maya commands
• Can be run from the Maya command line
• Or attached to new buttons or menus in the
  interface
• Export scripts lend themselves well to this

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MEL Types and Variables

• All variable names must begin with the symbol
• Thereafter follow usual C conventions
• Including case-sensitivity
• E.g. value1 or New_Variable
• Have the usual data types
• Integer (int), floating point (float) and string
• Variable sized (1D) arrays
• A vector type - a triple of floats
• Matrices (matrix), a fixed-size 2D table of
  floats
• The latter two are particularly useful for 3D
  data

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MEL Using Variables

• Variable declarations
• int a 5
• float b 3.456
• vector v ltlt1.2, 3.4, 6.5gtgt
• float ar 1.2, 3.4, 4.5 // Array of floats
• matrix mtx32 // A 3x2 matrix of floats
• No bool type, although true 1 and false 0
• Note the usual C comment style
• Assignment variations as C
• , , -, /, , , --, etc.
• Conditions same too
• , gt, gt, lt, lt, !, etc.

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MEL Arrays

• Arrays automatically expand as needed
• int scores // Declared as a zero element
  array scores150 3 // Now a 151 element
  array
• scores200 5 // Now a 201 element array
• Memory is allocated as this happens, beware
• int askingForTrouble
• askingForTrouble123456789 2
• Can find size of arrays and clear them
• string hats "blue", "red", "black
• print( size(hats) ) // 3
• clear( hats )
• print( size(hats) ) // 0

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MEL Control

• Blocks and most control structures also as C,
  e.g
• if (a b)
• ...
• else
• ...
• while (a lt size(arr))
• ...
• for loops the same, plus a for-in version
• string arr3 "red","green","blue"
• for (k in arr)
• print(k)

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MEL Procedures (functions)

• Create procedures using the following syntax
• global proc float SquareAndAdd(float x, float
  y)
• return x x y
• Without global the procedure is only available in
  the script file in which it is defined
• Not from Maya command line / interface
• No void, just leave out the return value if you
  dont want one

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MEL Vectors Matrices

• Vector reading
• vector test ltlt3.0, 7.7, 9.1gtgt
• print(test.y) // 7.7
• Cant write to individual vector elements
• (test.y) 5.5 // Error
• vector test ltlt3.0, 7.7, 9.1gtgt // Instead...
• test ltlttest.x, 5.5, test.zgtgt // OK
• Matrix reading and writing straightforward
• matrix a322 ltlt2.5, 4.5 1.12, 1.3gtgt
• a310 9.2
• print(a311 )