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Game Scripting

Game Scripting. By: Nicholas Haines. Aurora Neverwinter Toolset . What is Scripting?. Interpreted Language As the game runs. Advantages. Ease of use Makes the game more data driven Instead of hard coding into game engine Allows for in-game “tweaking” Quick results

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Game Scripting

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  1. Game Scripting By: Nicholas Haines

  2. Aurora Neverwinter Toolset

  3. What is Scripting? • Interpreted Language • As the game runs

  4. Advantages • Ease of use • Makes the game more data driven • Instead of hard coding into game engine • Allows for in-game “tweaking” • Quick results • Does not require a recompile • Allows user modability • Game can be patched at a later date • Many publicly available scripting languages

  5. Disadvantages • Performance • Not noticeable most of the time in real world performance • Automatic memory management • Can cause problems if it interrupts a command or takes awhile to complete • Poor debugging tools • Some languages don’t give warnings • Tends to be very hard to find the errors

  6. Lua In The Industry • Ease of Use • for non-programmers • Speed • don’t slow down the game • Size • Lua executable is ~ 160kb • Well Documented • Lua documentation is lacking • Flexible • Clean and flexible interface Falko Poiker

  7. Jobs

  8. Optimizing Script Execution

  9. Using Patterns • Object: Reduce the number of instructions • Look for patterns in the opcode output • Replace with optimized version • Check for addresses calculated multiple times and save to a spare register • Replace matching with reference to register • May shorten code by up to 50% assuming decoding script is the bottleneck of the virtual machine [VM]

  10. Non-Branched, Constant Scripts • Native function calls can be intercepted inside the VM • Store as a function pointer along its arguments • Big list of stored functions is called an Execute List • Will now traverse execute list instead of having to interpret the opcode again

  11. Conditional jumps cannot be predicted A Jump Table can be used to decrease decode time during execution Contains the offset of native opcode executing specific script opcode Use pure jump table opcodes for most common instructions Combine jump table/parse for less common instructions to reduce the table size Note: jump table values change whenever the VM’s code is changed Example to be decoded mov [vm_reg0], vm_reg1 (using x86 asm) ;get contents of vm register 0 into eax mov eax, [ecx+0] ;get contents of vm register 1 into ebx mov ebx, [ecx+4] ;solve indirection mov [eax], ebx ;increment vm instruction pointer add edx, 4 ;transfer program control to next opcode jump [edx] Branched Scripts ~3 cycles ~3 cycles ~3 cycles ~1 cycles ~3 cycles

  12. Monster::PlayerSpotted(bool hear_player, bool see_player, float* pos) { if(!see_player) { if(hear_player) { //we have heard something..check out //situation..keep an eye on the place FocusAttention(pos); //native function } else { //someone told us about the //player (monster friend or so) //let's help our friends WalkTo(pos); //native function } } else { //we can see the enemy..let's kill him! ShootAt(pos); //native function } } Generates each possibility and removes branching Code to the left becomes: Monster::PlayerSpotted(bool hear_player=true, bool see_player=false, float* pos) { FocusAttention(pos); } This can be optimized using execution lists Branched into Non-Branched

  13. Advanced Script Debugging

  14. Exchanging Debug Information • Debugger creates a new processes that uses the VM to execute scripts • VM sends string messages to Debugger • VM needs memory buffers for the Debugger to write to breakpoint hit Virtual Machine Debugger access violation update breakpoints

  15. Debug information Handling Breakpoints Variable Watch Call Stack Step-by-Step Execution Step-by-Step Assembly Execution Register Watch Memory Watch Step-by-Step High-Level Execution Step Over Execution Useful Features

  16. Adding Error Reporting

  17. Create A Parser For Your Script • Use tools like Lex and Yacc • Since mostly nonprogrammers use the scripting language, make meaningful error messages.

  18. Example code: int x; int y; y = 0; x = SomeFunction() - 5; if( x > OtherFunction() ) y = 3; Possible Errors Semicolon missing Parentheses missing Unrecognized characters Unrecognized keyword Function misspelled Undeclared variable Variable declared more than once A Simple Language

  19. Reporting with yyerror() • Prints “syntax error” to standard output • Overwrite yyerror() with a useful version • Derive a custom scanner function • Print formatted error messages to a specified error file • Include line number and copy of the line • This can be stored as a log or displayed in another application

  20. Identifying Specific Errors • Unknown Character Errors . yyerror(“Error: Unknown character ‘%c’ “, *yytext); • Append grammar to trap missing semicolon • Do the same to catch unrecognized functions • Undeclared and Redeclared Variables • Parser must check variable table to see if the variable has been previously declared

  21. References • AI Game Programming WISDOM 2 Sections 9.1-9.3 • Game Dev Articles: Scripting by Falko Poiker http://www.relic.com/industry/articles/scripting.php • The Secret Life of Game Scripting By Brian Hook http://www.bookofhook.com/Article/GameDevelopment/TheSecretLifeofGameScript.html

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