@physion/app / Exports / FreeBodyDiagramMixin
Class: FreeBodyDiagramMixin
Draws the forces acting on a BodyNode as labelled arrows, on top of the scene.
Which forces are drawn, and how, is controlled by a single object-valued property
(freeBodyDiagram), edited in the inspector by FreeBodyDiagramEditor and settable from a
script. Each key is a types.ForceKey and each value is an options object switched on by
its enabled flag:
{
"gravity": { "enabled": true },
"normal": { "enabled": true, "color": "#ffffff", "scale": 0.05, "components": true }
}
The default value spells every force out with enabled: false, so that a scene records what it
was asked for rather than inheriting whatever the defaults become. Unrecognized keys are kept in
the stored value, so a typo survives a save/load round trip, but are ignored when drawing.
Gravity is computed from the body and the scene on every frame. The other four are sampled once per physics step, in the body's local frame, and are not discarded when the simulation stops: pausing to read a value works, and a frozen arrow follows its body if the body is then dragged.
By default each force is drawn from where it acts. Setting atCenter on every force instead
produces the textbook diagram, in which the body is a point and every arrow radiates from its
centre of mass; setting it on only some of them gives neither picture.
The overlay is drawn on the scene's freeBodyDiagrams layer rather than inside the body, so that
it is never hidden behind another body. Like TextMixin, this is a composition helper
rather than a TypeScript mixin: the host node owns one, forwards the property accessor to it,
and calls its lifecycle methods. The Mixin suffix keeps it out of
NodeFactory.getClassNames.
Constructors
constructor
• new FreeBodyDiagramMixin(node, freeBodyDiagram?): FreeBodyDiagramMixin
Parameters
| Name | Type | Default value |
|---|---|---|
node | BodyNode | undefined |
freeBodyDiagram | IFreeBodyDiagram | defaultFreeBodyDiagram |
Returns
Properties
DEFAULT_FREE_BODY_DIAGRAM
▪ Static Readonly DEFAULT_FREE_BODY_DIAGRAM: IFreeBodyDiagram = defaultFreeBodyDiagram
Accessors
freeBodyDiagram
• get freeBodyDiagram(): IFreeBodyDiagram
The raw, user-authored value. Replace it to change what is drawn; mutating the returned object in place does not trigger a redraw of the resolved configuration.
Returns
IFreeBodyDiagram
• set freeBodyDiagram(v): void
Parameters
| Name | Type |
|---|---|
v | IFreeBodyDiagram |
Returns
void
frictionForces
• get frictionForces(): readonly IContactForce[]
The contact friction forces of the last physics step, in the body's local frame.
Returns
readonly IContactForce[]
graphics
• get graphics(): Graphics
Returns
Graphics
isEmpty
• get isEmpty(): boolean
True when nothing at all is drawn.
Returns
boolean
jointForces
• get jointForces(): readonly IJointForce[]
The joint reaction forces of the last physics step, in the body's local frame.
Returns
readonly IJointForce[]
needsContactData
• get needsContactData(): boolean
True when the contact forces on the host body are being read.
Returns
boolean
needsJointData
• get needsJointData(): boolean
True when the reaction forces of the joints attached to the host body are being read.
Returns
boolean
needsSampling
• get needsSampling(): boolean
True when anything at all has to be sampled from the simulation. A diagram that is empty, or draws gravity alone, needs nothing, which keeps the feature free for the bodies not using it.
Returns
boolean
needsSpringData
• get needsSpringData(): boolean
True when the springs attached to the host body must report the force they apply.
Returns
boolean
node
• get node(): BodyNode
Returns
normalForces
• get normalForces(): readonly IContactForce[]
The contact normal forces of the last physics step, in the body's local frame.
Returns
readonly IContactForce[]
springForces
• get springForces(): readonly IBodyForce[]
The spring forces of the last physics step, in the body's local frame.
Returns
readonly IBodyForce[]
Methods
addSpringForce
▸ addSpringForce(point, force): void
Records the force a spring applies to the host body, from SpringNode.update(). A spring is the
one force in Physion that Box2D never sees, so the body cannot discover it on its own.
Parameters
| Name | Type | Description |
|---|---|---|
point | IPoint | Where the spring is anchored on the host body, in scene coordinates. |
force | IPoint | The force it applies there, in Newtons and scene coordinates. |
Returns
void
attach
▸ attach(scene): void
Parameters
| Name | Type |
|---|---|
scene | Scene |
Returns
void
destroyFreeBodyDiagram
▸ destroyFreeBodyDiagram(): void
Returns
void
detach
▸ detach(): void
Returns
void
draw
▸ draw(): void
Redraws the overlay, once per rendered frame. Everything is in scene coordinates: the samples are converted out of the body's frame here, against its current transform, so they follow it.
Returns
void
reset
▸ reset(): void
Discards everything sampled from the simulation. Called when a run starts, not when it stops: a stopped scene is a paused one, so its last sample is still the right thing to have on screen.
Returns
void
sample
▸ sample(): void
Samples the forces that cannot be computed from the body alone. Must be called from the host's
update(), which the Scene runs after the step, so the impulses describe the frame drawn next.
Returns
void
propertyDescriptors
▸ propertyDescriptors(): IPropertyDescriptor[]
Returns
IPropertyDescriptor[]