RTS Game Design Difficulties

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RTS Game Design Difficulties

• October 20, 2004
• Stephen Lee-Urban

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Overview

• 7.4 Random Map Generation for Strategy Games
• 7.5 Transport Unit AI for Strategy Games
• Stainless Steel Studios Empire Earth
• Shawn Shoemaker

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Overview

• 7.4 Random Map Generation for Strategy Games
• 7.5 Transport Unit AI for Strategy Games
• Stainless Steel Studios Empire Earth
• Shawn Shoemaker

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Shawn Shoemaker

• 6th year in game industry
• B.S. and M.S. in computer engineering from
  Virginia Tech.
• Worked for Intel, now works for Stainless Steel
  Studios
• shansolox_at_yahoo.com

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Stainless Steel Studios

• Dedicated exclusively to designing RTS games
• RT 3D game engine TITAN 2.0
• Windows, complete source (c), 40 person years
• 3 million units sold (got cash?)
• Empire Earth
• Released Nov 2001
• a new high-water mark for realtime strategy
  fans. GameSpys Dave Kosak
• Same lead game designer (Rick Goodman) as AOE
• Empires Dawn of the Modern World

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Overview

• 7.4 Random Map Generation for Strategy Games
• 7.5 Transport Unit AI for Strategy Games
• Stainless Steel Studios Empire Earth
• Shawn Shoemaker

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7.4 Maps?

• Subset of map generation in general
• Successfully implemented in E.E.
• Examples

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Birds-Eye
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Fancy 3D View
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If You Were Asked

• Make a system to generate maps
• Tile-based
• RTS
• Multi-player
• What qualities should these maps have?

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These Maps Must be ______?

• Fair
• Land per player
• Resources per player
• No strategic advantage
• Predictable
• Similar by type
• Allow for diff. outcomes

• Reproducible
• Seed type size
• Number of players
• Varied
• Size
• Type
• Relatively Quick
• Many in-game systems not running
• Impatient to start

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Shoemakers Mapmaker

• 1 .dll 50 script files
• Different .dll can be specified
• 7 scripts for each map type
• Inputs
• players, teams
• Size
• Climate
• Type
• Random seed

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Intermediate Results

• Visualization tool created for 2D arrays
• Land/Water
• Height
• Height/Water
• Combined
• Map engine integrated late into game

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(No Transcript)
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Solution Overview

• Step 1 Place players (blank map of H2O)

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Solution Overview

• Step 2 Grow player land

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Solution Overview

• Step 3 Add flatlands to map

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Solution Overview

• Step 4 Generate elevations map

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Solution Overview

• Step 5 Combine the two (obeying rules)

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Solution Overview

• Step 6 Fix cliffs and other anomalies

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Solution Overview

• Step 7 Distribute resources to players
• Step 8 Place trees
• Step 9 Paint map with terrain textures
• Step 10 Place initial units for players
• Step 11 Place wildlife
• Steps 9 through 11 wont be covered.

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Step 1 Details

• Placement directly effects land growth
• How would you do this?
• Algorithm
• Inscribe one large disk onto map
• Assign one random loc. per player w/in disk
• Repeatedly push closest of player points apart
  until distribution sufficiently optimal
• optimal is a map attribute
• Specifies how evenly players must be placed

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Step 1 Results

• Predictable player locations. Desirable?
• Keeps maps balanced
• Tunable through map attributes
• Assign player to points after placed. Why?
• Teams adjacent to each other
• Enemies somewhat symmetrically opposed
• What about colonization? Dummy players
• Placed after real players, on in/outside of disk.
  Why?
• Dummies land grown after real players. Why?
• Give the dummies resources too.

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Step 2 Details

• Land is a resource in RTS games!
• Allocate fairly
• Must appear natural
• Clumps, modifiable script attributes
• clumpSize
• clumpNumber
• Chaos level
• World starts as H2O
• Whats a clump?

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Step 2 Clumps

• Grow until
• size clumpsize
• Create new clump some dist away at random,
  chaotic angle
• Cant grow any larger
• Make new clump of limited size
• Continue until number clumpNumber
• Two growth methods, step completion

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Step Clump Growth

• 1st tile is players starting location
• Choose one valid random edge per iteration

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Valid Clumps

• Different types have different rules
• Dont take owned land
• Dont make peninsulas
• Island Clumps

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Step Clump Discussion

• Why is stepwise growth desirable?
• Fair
• each player has equal chance to grow
• Preventative
• Hinders ones land surrounding anothers
• Avoids starvation
• Opportune
• Makes good on the importance of placing players!
• Remember the dummies!

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Completion Clumps

• Why bother, steps are awesome?!
• How about step 3?

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Step 4 Details

• 2D height map array
• Fractals (esp. diamond sq.)
• Quick easy
• Realistic looking elevation
• Once again, map attributes
• Min, max, initial elevation
• What about noise?
• Filter used to smooth out jagged artifacts

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Step 5 Details

• Remember obeying rules?
• Algorithm
• if( landNH2Oij FLAT ) ignore heightMap
• if( landNH2Oij LAND heightij
  WATER )
• Raise value in heightij to preserve natural
  look
• if( landNH20ij WATER heightij
  LAND )
• Lower heightij to landNH2Oij level
• Why not negate in the last two cases?

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Step 6 Details

• Weve got anomalies again!
• Wouldnt negating have worked?
• Re-apply filter

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Step 7 Resource Allocation

• Each player gets same amt. of each w.r.t. other
  players
• Randomly placed within rings centered on players
  starting locs.
• Validation
• Some resources cant be placed on terrain
  features
• Resources restricted in min. distance to other
  resources
• Resources restricted in min. dist to start loc
• Once again, map attributes
• Amount of each resource
• Size of ring

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Step 8 Place trees

• Plentiful Restrict pathfinding
• Can the clump class be re-used?
• Map attributes
• Number forests/player
• Size of forests
• Number of clumps/forest
• Some areas of map marked as forest free
• Player-owned land does not restrict growth

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Scripts

• Language to manipulate map attribs
• Basic text parser and data structures
• Things like
• resources per player
• Max altitude in height map
• Player land allocation
• Climate (terrain/forest/animal texture set)
• Small changes yield map variations

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Scripts Pros

• Allow for incremental map type devel.
• New attribs for new maps
• Add attribs to parser
• Matching code added to map generator
• Offload some tasks from programmers to designers
• Modability/extensibility/variety
• Next steps
• Island map w/teams allow land for team-members
  to merge
• Tweak elevations island maps become canyon maps
• Tweak clump size island maps become river maps

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Scripts Cons

• Unfortunately
• One master script per type of random map
• One script per size of type of map
• In each script, subsections for each qty. of
  players ( berries? gold?)
• You can imagine
• Scripts can become hard to manage
• Manuals needed to explain types attribs
• Additional tools to edit attribs across files
• Designers are not programmers
• Programmers must create map templates
• Designers simply tweak

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Buyer Beware!

• Cant explicitly place particular terrain
  features
• super-tile design approach
• Fairer/more visually appealing
• Designers are not programmers
• Chokepoints are possible unfair
• Resource placement can still yield advantages

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Overview

• 7.4 Random Map Generation for Strategy Games
• 7.5 Transport Unit AI for Strategy Games
• Stainless Steel Studios Empire Earth
• Shawn Shoemaker

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7.5 Transport Unit AI

• Move passengers around map
• Passengers are land based units
• Unit AI ? Micro level for specific piece
• What are some of the challenges?
• Many units must work together to achieve common
  goal.
• Attempt to minimize player frustration.
• Adhere to expected RTS transport standards.

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Transport Hierarchy

• Transport types?
• Most difficult?

Transport
Air Transport
Building Transport
Land Transport
Naval Transport
helicopter
bunker
horse
ships
balloon
city hall
armored personnel carrier
subs
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If You Were Asked

• Make a transportation strategy
• Unit level AI
• Passengers are land based units
• Minimize input needed from player
• What would you do?
• Use FSM based system?
• Use HTN based system?

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Solution Overview

• Implemented via FSMs
• Jarret Raim and Chad Hogg covered these
• Other options available
• One FSM for passengers, one for transports
• All units have high-level goals
• Based on player input
• Decomposed into small steps

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Solution Overview

• High level goals, part 1
• Loading
• Units instructed to board become passengers
• Complications
• Mult. units meeting as one
• Coordination / Interaction of unit AI
• Prevent exploits

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Solution Overview

• High level goals, part 2
• Unloading
• Transport instructed to disembark passengers
• Complications
• Transport must move within range of unload loc.
• Guess intended unload loc. modify as necc.
• Prevent exploits
• Unreachable locations
• Teleportation through walls

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Passenger FSM
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Passenger FSM

• Note no unload action! Why?
• Other considerations
• Dont wait forever
• Keep trying to reach rendezvous point
• Freeloaders pretty easy

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(No Transcript)
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Transport FSM

• Note both FSMs have a load goal
• Other considerations
• Pick up passengers along way to rendezvous
• Inform excess passengers when full
• Continue to move toward unload point even if in
  range

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Loading

• High level goal to enter transport
• Both passengers transport given goal
• Passengers in range of transport load
• Key agree on rendezvous point

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Rendezvous Point

• Where transport/passengers will meet
• Particular transport class determines initial
  point
• Passengers then validate that point
• If unreachable permanent blocking units
• Adjust point by considering tiles surrounding
  initial rendezvous point (same validation)
• First valid tile chosen, or original if none found

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Transport Type R.P.

• Air Transport
• Do most of moving
• Not slowed by terrain
• Dont consider collisions
• Passenger nearest transport used as R.P.
• Building Transport
• Building location

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Transport Type R.P.

• Land Transport Algorithm
• Nearest passenger determined (assumes)
• Line cast from nearest pass. to transport
• Tiles underneath line determined
• One tile on line is selected as R.P.
• Based on transport nearest passengers
  relative max. velocities
• Goal make travel distance for nearest
  transport the same since their speeds are nearly
  equal
• Does this make sense?

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Transport Type R.P.

• Naval Transport Algorithm
• Land/H2O interface locations needed (2 RPs)
• Dist. btw. RPs lt transport load range
• Choose shore tile closest to passengers as Land
  RP
• Water tile adjacent to Land RP picked
• Adjust both RPs until reachable by everyone
• Potential problems?

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Loading Range

• Distance from transport to passenger
• Land Air
• Performance-friendly bounded pathfinder from
  passenger to transport
• Prevents teleportation
• Building
• Same as Land Air
• Considers border tiles of transport upon failure

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Loading Range

• Naval Algorithm

• Cast Ray
• 1st shore tile test tile

T
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Loading Range

• Naval Algorithm

• a ? false
• if( D1 lt LR )
• if( D2D1 lt LR )
• if( !pathfind( p, T ) )
• a ?lineOfFireOK( p,T )
• else
• a ? true
• return a
• Problems?

D2
T
D1
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Unloading

• High level goal to disembark passengers
• Only transport given goal
• As passengers exit, use subtile placement
• Prevents collisions (passengers, terrain, etc.)
• Prevents teleporting
• Key determine unload point

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Unload Range

• Distance check btw. ULR and UP
• When in range
• Verify first passenger can path to UP from T
• Verification uses pathfinder
• Illustration of Unload Points

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Unload Points

• Each transport class determines TDP, FPP, and UP
• Points validated after calculation
• Validated for each passenger/transport
• Points must be (nearly) reachable
• Final UP may be changed from CP

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Transport Type U.P.s

• Air Transport
• Transport destination not constraining
• FPP UP
• Points subject to pathfinding of land units
• Building Transport
• Placement code same as unit production
• T TDP FPP
• Remember sub-tile placement!

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Transport Type U.P.s

• Land Transport
• Same pathfinding restrictions as cargo
• TDP FPP UP CP
• Different pathfinder for Siege tower unload
• Allow placement over wall unit
• Special power

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Transport Type U.P.s

• Naval Transport
• TDP closest water tile from CP (ray)
• FPP (adjacent land tile to TDP)
• Adjust UP from CP
• Ray btw. FPP and CP
• Tiles under ray are potential UPs
• Actual up ?
• Potential UP closest to CP and lt ULR
• Use pathfinding to verify CP from UP

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Transport System Pros

• Decent job at naval transport
• Supports special design (siege tower)
• Reusable for garrisoning

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Transport System Cons

• Imperfect
• No coordination btw. multiple transports
• Cut each other off
• Hog unload location
• Pathfinding tests get expensive

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Sources

• AI Game Programming Wisdom 2
• http//www.stainlesssteelstudios.com
• http//www.3dgamers.com/games/empireearth