Games Development 2 Entity Update

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Games Development 2Entity Update Rendering
Practicalities

• CO3301
• Week 10

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Entity Update

• We give every entity an update function, which is
  called periodically
• Maybe every frame
• Less frequently in the case of distant,
  non-visible or less important entities
• The update function defines the entity behaviour
• Perform movement rotation
• Set and play animations
• Receive and process messages
• Maintain state and consider AI

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Scene Update to Entity Update

• We have shifted to entities after using models
• Models are just graphical - contain no behaviour
• All model behaviour controlled in a single global
  Scene Update function
• No attempt to encapsulate behaviour
• Becomes a bloated, hard to maintain function
• Using entity-based Update is another shift of
  functionality in our generalised game engine
• Although we retain SceneUpdate to do certain
  global update work, e.g. manage user/system input

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Pros and Cons

• Entity Update
• Entity behaviour (code) and entity state (data)
  encapsulated together within each entity
• Easier to maintain, especially for a team
• Simpler to add scripted entity behaviour
• Game behaviour distributed, unexpected effects
• Scene Update
• Global function easy to find and work with
• But ultimately clumsy and bloated team
  unfriendly
• Model state tends to become a set of globals

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Entity Rendering

• We also give each entity a render function
• This is called every frame
• Provided the entity is visible
• Typically, the entity passes its current position
  / animation / textures etc. to its entity
  template
• The template class can render any of its own
  entities given these specifics
• Recall that the template stores the mesh
• This is fairly similar to model/mesh rendering

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Pre / Post Rendering

• Modern engines often perform one or more passes
  of pre or post-rendering
• E.g. entities might have a PreRender function
• Would called for all (visible) entities prior to
  the main render calls
• Different purposes for different entities
• A models blends its animations to get the final
  matrices for rendering
• Cameras calculate their view/projection matrix
• Lights perform a shadow rendering pass (dynamic
  shadow mapping)
• Some entities might do nothing

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Games Development 2 Entity Update Example
Targeting Turning and Movement

• CO3301
• Week 10

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Contents

• Forward Movement to a Target
• Turning
• Dot product method
• Turning Speed
• Acceleration / Deceleration
• Practicalities

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Forward Movement to a Target

• Have an object that needs to reach a target point
• However, the object cant move freely
• Has a forward velocity - can accelerate /
  decelerate
• Can also turn
• In a ground-based situation, can only turn left
  or right
• Like a car driving towards a target

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Forward Movement to a Target

• At any point in time we want to know
• Whether to turn left or right, and by how much
• Whether to accelerate or decelerate
• Want to reach the target swiftly
• But avoid excessively large turning circles
• Worst case circling around the target

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Turning Dot Product Method

• Flat surface - use a simple method for turning
• Assume we have the objects local Z axis (facing
  vector) and we can get the local X axis
• Extract X-axis it from the objects world matrix
• If local X unavailable, use this in 2D
• If Z (a,b), then X (b, -a) in a left handed
  system
• In 3D can use cross products (c.f. targeting)

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Turning Dot Product Method

• Ensure X Z axes are normalised
• Then get normalised vector, source to target (T)
• Calculate the angles a and ß using dot products
• If ß lt 90 then the target is to the right
• and if ß lt 90 then cos(ß) gt 0, so

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Turning Dot Product Method

• Calculate the angle a
• Indicates how much to turn left or right to face
  target
• Basic turning method
• Rotate object (around Y) to turn towards the
  target
• Rotate by ve angle if cos(ß) gt 0, -ve otherwise

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Turning Speed

• Want to turn gradually to face the target
• Dont want to snap instantly to face it
• Set a maximum turn speed for the object and cap
  turning by this value
• I.e. turn by min(turn speed, a)
• Can consider angular acceleration / deceleration
• Acceleration to max turn speed when starting to
  turn
• Deceleration to 0 when almost facing target
• Similar to the movement acceleration /
  deceleration that follows

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Acceleration / Deceleration

• When to accelerate and decelerate?
• Simple approach
• The stopping distance is the distance covered
  before stopping if we apply maximum deceleration
• If the stopping distance is less than the
  distance to the target, then apply maximum
  acceleration
• Otherwise maximum deceleration
• If the current speed (not velocity) is s and max
  deceleration d, then
• Stopping Distance s2 / 2d
• Since average speed while stopping s / 2
• And time to stop s / d

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Practicalities

• Use this method for static or dynamic tracking
• When tracking a moving object, consider tracking
  a point in front of it (depending on its speed)
• Need tweaking
• Wont target perfectly so stop when within a
  certain range use the length of the target
  vector
• The stopping distance as given doesnt quite
  match algorithm, should be s2 / 2d s / 2
• Needs careful insertion into a timed game loop
• Only use update time when applying movement /
  turning, dont alter angle / stopping distance
  formulae

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Further Development

• This is a very basic method, which can easily be
  refined / altered
• Angular acceleration / deceleration
• Decelerate to make a sharp turns (when a is
  large)
• Adapt the acceleration algorithm to consider a
• Or even smarter acceleration / deceleration /
  maximum speed based on distance and
  manoeuvrability of target
• Adapt to 3D space use cross products, see Space
  Game entity code or Van Verth p39