Blitz3D+ Command Reference

PhysicsBodyMass# ( body )

Parameters

body - physics-body handle returned by CreatePhysicsBody

Description

Returns a body's mass in kilograms.

Mass normally comes from the volume and density of every shape attached to the body, including sensors. A 1x1x1 box at density 1 weighs 1 kilogram. PhysicsShapeDensity can apply density changes immediately or stage them for RecalculatePhysicsBodyMass. PhysicsBodyMassData overrides the complete mass, centre and inertia of a dynamic body. Adding/freeing a collider, changing body mode, explicit mass recalculation or entering/leaving full angular locking replaces that override.

Static and kinematic bodies always report 0, as do bodies whose only shapes are triangle meshes or terrain (those shapes have no volume).

The classic use is scaling pushes: PhysicsBodyImpulse changes velocity by impulse divided by mass, so multiply your desired velocity change by this value and light and heavy props respond identically.

Requires Extended mode.

See also: PhysicsBodyBox, PhysicsBodySphere, PhysicsBodyImpulse, PhysicsBodyForce, PhysicsBodyGravityScale.

Example

; PhysicsBodyMass Example
; -----------------------
; Requires Extended mode.

Graphics3D 640,480,0,2
SetBuffer BackBuffer()

Const PHYSICS_STATIC=0
Const PHYSICS_DYNAMIC=2

camera=CreateCamera()
PositionEntity camera,0,3,-9

light=CreateLight()
RotateEntity light,45,-30,0
AmbientLight 50,50,50

; Independent Box3D world with Earth-like gravity
world=CreatePhysicsWorld()
PhysicsGravity world,0,-9.81,0

; Blue static ground slab
ground=CreateCube()
ScaleEntity ground,4,0.5,4
PositionEntity ground,0,-0.5,0,True
EntityColor ground,80,160,255
ground_body=CreatePhysicsBody(world,ground,PHYSICS_STATIC)
ground_shape=PhysicsBodyBox(ground_body,8,1,8,0)

; Orange crate whose density - and so mass - you can change
density#=1
crate=CreateCube()
ScaleEntity crate,0.5,0.5,0.5
PositionEntity crate,0,0.5,0,True
EntityColor crate,255,120,70
crate_body=CreatePhysicsBody(world,crate,PHYSICS_DYNAMIC)
crate_shape=PhysicsBodyBox(crate_body,1,1,1,density)

While Not KeyDown(1)

    ; Keys 1/2/3 rebuild the shape with a new density
    new_density#=0
    If KeyHit(2) Then new_density=0.5
    If KeyHit(3) Then new_density=1.0
    If KeyHit(4) Then new_density=4.0
    If new_density>0
        density=new_density
        FreePhysicsShape crate_shape
        crate_shape=PhysicsBodyBox(crate_body,1,1,1,density)
    EndIf

    ; Space kicks with a FIXED impulse - heavier crates jump less
    If KeyHit(57) Then PhysicsBodyImpulse crate_body,0,4,0

    ; Advance the Box3D simulation by one 60 Hz step
    StepPhysics world,1.0/60.0,4

    ; Read the mass Box3D calculated from shape volume times density
    mass#=PhysicsBodyMass(crate_body)

    ; Arrow keys move the camera
    If KeyDown(200) Then MoveEntity camera,0,0,0.1
    If KeyDown(208) Then MoveEntity camera,0,0,-0.1
    If KeyDown(203) Then TurnEntity camera,0,1,0
    If KeyDown(205) Then TurnEntity camera,0,-1,0

    RenderWorld

    Text 0,0,"1/2/3: density   Space: fixed kick   Arrow keys: camera   Esc: exit"
    Text 0,20,"Density: "+density+"   PhysicsBodyMass = "+mass+" kg"

    Flip

Wend

FreePhysicsWorld world
FreeEntity crate
FreeEntity ground
End

Index