A crash-course in model nodes [tutorial].

[ThePsuedoMonkey]

When 0.20 was released we were given a

few tools for addon development, one of which was called "MODEL Nodes." This allows multiple parts models to be loaded as a single part, gives options to reposition, rescale, and rotate the models, and even the ability to overload their textures. Since this thing is seen as a single part there will be no physics calculations between the models in the node, which has the obvious application of improving performance of large vessels. This tool is called from a part config file, in place of the standard model definitions.

Premade model nodes

Kadrons

thrust plates.

UbioZur

has a bunch of assemblies and has been developing a plugin to do the work for you.

A few of

my

old ones.

Building from scratch:

For a relatively simple demonstration, lets combine a structural quad adapter with some aerospikes.

The first thing we need to know is that the position values of a model node are relative to its center of mass, and each model is centered on its own center of mass. To determine where to position the models in the node, we can open the config of the Toroidal Aerospike and check its node definitions

node_stack_top = 0.0, 0.0, 0.0, 0.0, 1.0, 0.0

This tells us that the aerospikes only attachment node is at its center of mass (first 3 numbers), and faces straight up (4th-6th numbers). The position values are in meters in the X,Y,Z coordinates, and the direction is a vector with the X,Y,Z components. Now we need to know how to place the aerospikes relative to the structural piece, so we look at its node definitions

node_stack_top = 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 2

node_stack_bottom01 = 0.625, -0.75, 0.625, 0.0, 1.0, 0.0, 1

node_stack_bottom02 = 0.625, -0.75, -0.625, 0.0, 1.0, 0.0, 1

node_stack_bottom03 = -0.625, -0.75, 0.625, 0.0, 1.0, 0.0, 1

node_stack_bottom04 = -0.625, -0.75, -0.625, 0.0, 1.0, 0.0, 1

If you looked closely, you saw a seventh number, which just refers to the visual size of the attachment node in the VAB/SPH. Once again, the top node is at the models center of mass, but the bottom nodes are 0.75m below it. The Y-values of the aerospikes and the adapter must be separated by the total distance between their centers of mass (i.e. 0.0 + [0.0 - -0.75] = 0.75), while the X and Y values should correspond to the offsets in the adapters attachment nodes. The last piece of information we need is where to define the center of mass; the aerospikes are 6T and the adaptor is 0.2T, so the CoM is [0.75 * 0.2 / 6.2 = 0.02419] above the Aerospikes. Lets round this to 0.025 for simplicity and we've determined that the positions of the models are

adaptor = 0.0, 0.725, 0.0

aerospike1 = 0.625, -0.025, 0.625

aerospike2 = 0.625, -0.025, -0.625

aerospike3 = -0.625, -0.025, -0.625

aerospike4 = -0.625, -0.025, 0.625

Now we just convert this data into the model node format, and include the rotation of the aerospikes that symmetry would create. It will be important later on that rotation is applied in the order Z,X,Y.

MODEL
{
        model = Squad/Parts/Structural/adapterLargeSmallQuad/model
        position = 0,0.725,0
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = 0.625, -0.025, 0.625
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = 0.625, -0.025, -0.625
        rotation = 0, 90, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = -0.625, -0.025, 0.625
        rotation = 0, 180, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = -0.625, -0.025, -0.625
        rotation = 0, 270, 0
}

If you were to load this into a part config, you'd find that the aerospikes are a little smaller than they should be: this is because their model was made before the switch to 1.25m parts so they must be rescaled. You will also find that this engine doesn't overheat as much as its counterpart. The position of the top attachment node should reflect its original position. The FX definitions will be mimicked for each engine, so we do not need to change them from the default settings in the Aerospike config. The cost, mass, and engine properties will need to be updated as well. When we fix these issues, the part config should look similar to this

-------------------------------------------------------------

PART
{
name = ZWeldedEngineA
module = Part
author = ThePsuedoMonkey

MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = 0.625, -0.025, 0.625
        scale = 1.25, 1.25, 1.25
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = -0.625, -0.025, 0.625
        scale = 1.25, 1.25, 1.25
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = 0.625, -0.025, -0.625
        scale = 1.25, 1.25, 1.25
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Engine/toroidalAerospike/model
        position = -0.625, -0.025, -0.625
        scale = 1.25, 1.25, 1.25
        rotation = 0, 0, 0
}
MODEL
{
        model = Squad/Parts/Structural/adapterLargeSmallQuad/model
        position = 0.0, 0.725, 0.0
        scale = 1, 1, 1
        rotation = 0, 0, 0
}

rescaleFactor = 1

// --- node definitions ---
node_stack_top = 0.0, 0.725, 0.0, 0.0, 1.0, 0.0, 2
// --- FX definitions ---
fx_exhaustFlame_blue = 0.0, -0.0, 0.0, 0.0, 1.0, 0.0, running
fx_exhaustLight_blue = 0.0, -0.0, 0.0, 0.0, 0.0, 1.0, running
fx_smokeTrail_light = 0.0, -0.0, 0.0, 0.0, 1.0, 0.0, running
fx_exhaustSparks_flameout = 0.0, -0.0, 0.0, 0.0, 1.0, 0.0, flameout
// --- Sound FX definition ---
sound_vent_medium = engage
sound_rocket_hard = running
sound_vent_soft = disengage
sound_explosion_low = flameout

cost = 4200
category = Propulsion
subcategory = 0
title = Quadraspike
manufacturer = KittyHawk United
description = Just a bunch of Aerospikes welded together, move along people.
// attachment rules: stack, srfAttach, allowStack, allowSrfAttach, allowCollision
attachRules = 1,0,1,0,0

mass = 6.2
dragModelType = default
crashTolerance = 12
maxTemp = 3400
breakingForce = 50
breakingTorque = 50

MODULE
{
        name = ModuleEngines
        thrustVectorTransformName = thrustTransform
        exhaustDamage = True
        ignitionThreshold = 0.1
        minThrust = 0
        maxThrust = 700
        heatProduction = 600
        fxOffset = 0, 0, 0.25
        PROPELLANT
        {
                name = LiquidFuel
                ratio = 0.9
                DrawGauge = True
        }
        PROPELLANT
        {
         name = Oxidizer
         ratio = 1.1
        }
        atmosphereCurve
        {
         key = 0 390
         key = 1 388
        }
}
MODULE
{
      name = ModuleAnimateHeat
}
}

In essence, you can think of it as modifying a part config file and telling it where to place the models.

Building from a *.Craft file

Rocket engines are cool and all, but you know what's awesome? Motorbikes.

If you open the craft file, you get a lot of data that you don't really need and some that needs interpretation. For instance, here is the data on the probe core.

PART
{
	part = probeCoreCube_4294680264
	partName = Part
	pos = -0.1005701,5.050443,0.2725831
	rot = 0.7071068,1.256074E-15,1.256074E-15,0.7071068
	attRot = -3.247939E-15,-4.385655E-23,1.776357E-15,0.9999999
	mir = 1,1,1
	istg = 0
	dstg = 0
	sidx = -1
	sqor = -1
	attm = 0
	link = SmallGearBay_4294680234
	EVENTS
	{
	}
	ACTIONS
	{
	}
	MODULE
	{
		name = ModuleCommand
		isEnabled = True
		controlSrcStatusText = 
		EVENTS
		{
			MakeReference
			{
				active = True
				guiActive = True
				guiIcon = Control From Here
				guiName = Control From Here
				category = Control From Here
				guiActiveUnfocused = False
				unfocusedRange = 2
				externalToEVAOnly = True
			}
			RenameVessel
			{
				active = True
				guiActive = True
				guiIcon = Rename Vessel
				guiName = Rename Vessel
				category = Rename Vessel
				guiActiveUnfocused = False
				unfocusedRange = 2
				externalToEVAOnly = True
			}
		}
		ACTIONS
		{
		}
	}
	MODULE
	{
		name = ModuleReactionWheel
		isEnabled = True
		stateString = Active
		WheelState = Active
		EVENTS
		{
			OnToggle
			{
				active = True
				guiActive = True
				guiIcon = Toggle Torque
				guiName = Toggle Torque
				category = Toggle Torque
				guiActiveUnfocused = False
				unfocusedRange = 2
				externalToEVAOnly = True
			}
		}
		ACTIONS
		{
			Activate
			{
				actionGroup = None
			}
			Deactivate
			{
				actionGroup = None
			}
			Toggle
			{
				actionGroup = None
			}
		}
	}
	MODULE
	{
		name = ModuleSAS
		isEnabled = True
		EVENTS
		{
		}
		ACTIONS
		{
		}
	}
	MODULE
	{
		name = ModuleTripLogger
		isEnabled = False
		EVENTS
		{
		}
		ACTIONS
		{
		}
		Surfaced
		{
		}
		Flew
		{
		}
		FlewBy
		{
		}
		Orbited
		{
		}
		SubOrbited
		{
		}
	}
}

The pertinent data approximates to

part = probeCoreCube_4294680264

pos = -0.1005701,5.050443,0.2725831

rot = sqrt(2)/2, ~0, ~0, sqrt(2)/2

attRot = ~0, ~0, ~0, ~1

It's listed position is relative to the center of the floor in the SPH, and the rotation values are split into two fields and appear to be given as either the sine or cosine of an angle. I'm not exactly sure how to interpret it, so I'll just ignore it for now and just use the position information and knowledge of the directory structure to get a starting point.

MODEL
{
        model = Squad/Parts/Command/probeCoreCube/model
        position = -0.1005701, 5.050443, 0.2725831
}
MODEL
{
        model = Squad/Parts/Wheel/SmallGearBay/model
        position = -0.10057,4.642117,0.4109806
}
MODEL
{
        model = Squad/Parts/Utility/spotLight1/model
        position = -0.1005701,5.039511,0.7625586
}
MODEL
{
        model = Squad/Parts/Structural/structuralPanel1/model
        position = -0.10057,4.713505,-0.807771
}
MODEL
{
        model = Squad/Parts/Command/sasModule/model
        position = -0.10057,4.923228,-0.807772
}
MODEL
{
        model = Squad/Parts/Command/seatExternalCmd/model
        position = -0.10057,5.274733,-0.6029134
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.6008557,4.713508,-1.33992
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.7094498,4.713508,-1.578224
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.8164898,4.713508,-1.813576
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.923505,4.713508,-2.050247
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.923505,4.713508,-2.307719
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.9235049,4.713508,-2.565187
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.9235051,4.476118,-2.586868
}
MODEL
{
        model = Squad/Parts/Wheel/wheelMed/model
        position = -0.8157229,4.476118,-2.54761
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.8229024,4.923228,-0.8520064
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.8846188,4.692904,-0.8666103
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -0.9512572,4.444208,-0.8666108
}
MODEL
{
        model = Squad/Parts/Structural/strutCube/model
        position = -1.017895,4.195512,-0.8666106
}
MODEL
{
        model = Squad/Parts/Electrical/RTG/model
        position = -0.5352382,4.713508,-0.8125257
}
MODEL
{
        model = Squad/Parts/Electrical/RTG/model
        position = 0.3340983,4.713508,-0.8125257
}

The first task is to calculate the CoM and then convert these values using that information. The CoM for those values would be at (0.10057,4.863508, -0.76329), so we subtract that from each coordinate and check the result in the SPH. It turns out that the Y and Z coordinates were flipped, and when we fix that we see that the Z has been mirrored, so multiply those by -1 and all of the parts should be in their proper position. Now we need to deal with rotation. The axes in the SPH are {X = parallel to floor and entrance, Y = parallel to floor and walls, Z = parallel to walls and entrance} and rotations are in degrees and seem to resolve in the order Z, X, Y. Put it together and what have you got?

PART
{
name = ZWeldedCommandB
module = Part
author = ThePsuedoMonkey

MODEL
{
    model = Squad/Parts/Command/probeCoreCube/model
    position = 0.0, 1.03590, -0.1870
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Wheel/SmallGearBay/model
    position = 0.0, 1.17225, 0.2214
	rotation = 90, 0, 180
}
MODEL
{
    model = Squad/Parts/Utility/spotLight1/model
    position = 0.0, 1.52585, -0.1760
	rotation = 15, 0, 180
}
MODEL
{
    model = Squad/Parts/Structural/structuralPanel1/model
    position = 0.0, -0.0445, 0.15
	rotation = 90, 0, 0
}
MODEL
{
    model = Squad/Parts/Command/sasModule/model
    position = 0.0, -0.0445, -0.0597
	rotation = 90, 0, 0
}
MODEL
{
    model = Squad/Parts/Command/seatExternalCmd/model
    position = 0.0, 0.1604, -0.4112
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.50025, -0.5767, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.60885, -0.815, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.71590, -1.0503, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.82290, -1.287, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.82290, -1.5445, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.82290, -1.8025, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.82290, -1.8236, 0.3874
	rotation = 90, 0, 0
}
MODEL
{
    model = Squad/Parts/Wheel/wheelMed/model
    position = -0.7151, -1.7843, 0.3874
	rotation = 90, 0, 180
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.72230, -0.0888, -0.0597
	rotation = 0, 0, 90
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.78700, -0.1035, 0.1706
	rotation = 0, 80, 90
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.85065, -0.1035, 0.4193
	rotation = 0, 80, 90
}
MODEL
{
    model = Squad/Parts/Structural/strutCube/model
    position = -0.91730, -0.1035, 0.668
	rotation = 0, 80, 90
}
MODEL
{
    model = Squad/Parts/Electrical/RTG/model
    position = -0.435, -0.05, 0.15
	rotation = 0, 0, 0
}
MODEL
{
    model = Squad/Parts/Electrical/RTG/model
    position = 0.435, -0.05, 0.15
	rotation = 0, 0, 0
}
rescaleFactor = 1

// --- node definitions ---
node_stack_top = 0.0, -0.072, 0.15, 0.0, 0.0, 1.0, 0

cost = 5110
category = Pods
subcategory = 0
title = KU Motorbike
manufacturer = KittyHawk United
description = Not sure if functional...
// attachment rules: stack, srfAttach, allowStack, allowSrfAttach, allowCollision
attachRules = 1,0,1,1,0

mass = 0.73
dragModelType = default
maximum_drag = 0.2
minimum_drag = 0.2
angularDrag = 2
crashTolerance = 20
breakingForce = 200
breakingTorque = 200
maxTemp = 3500
vesselType = Rover

MODULE
{
	name = ModuleLight
        lightName = spotlight
	useAnimationDim = true
	lightBrightenSpeed = 2.5
	lightDimSpeed = 2.5
	resourceAmount = 0.04
	animationName = LightAnimation
	useResources = true
}
MODULE
{
	name = ModuleLight
        lightName = LandingLight
	useAutoDim = true
}
MODULE
{
	name = ModuleLandingGear
}
MODULE
{
	name = ModuleGenerator
	isAlwaysActive = true
	OUTPUT_RESOURCE
	{
	   name = ElectricCharge
	   rate = 1.5
	}	
}
MODULE
{
	name = ModuleWheel
	hasMotor = true
	resourceName = ElectricCharge
        resourceConsumptionRate = 0.7
	canSteer = true
	controlAxisType = Forward
	steeringModeType = AutomaticSteer
	brakeTorque = 500
	brakeSpeed = 2.5
	impactTolerance = 300
	overSpeedDamage = 60

	WHEEL
	{
	    wheelName = wheel
	    wheelColliderName = wheelCollider
	    suspensionTransformName = suspensionTraverse
	    suspensionNeutralPointName = suspensionNeutralPoint
	    damagedObjectName = bustedwheel
	    rotateX = 1
	    rotateY = 0
	    rotateZ = 0
	}

	steeringCurve
 	{
   	 key = 0 10
	 key = 10 6
  	 key = 30 2
 	}

	torqueCurve
	{
	  key = 0 100 0 0 
	  key = 2.5 70 0 0
	  key = 30 0 0 0
	}

}

MODULE
{
	name = FXModuleConstrainPosition
	matchRotation = true
	matchPosition = false
	CONSTRAINFX
	{
		targetName = steering
		moversName = trackSteering
	}

}
MODULE
{
	name = KerbalSeat
	seatPivotName = seatPivot		
	ejectDirection = 0, 1, 0.2
}
MODULE
{
	name = ModuleCommand
	minimumCrew = 0
	RESOURCE
	{
		name = ElectricCharge
		rate = 0.02777778
	}
}
MODULE
{
	name = ModuleReactionWheel
	PitchTorque = 20.5
	YawTorque = 20.5
	RollTorque = 20.5
	RESOURCE
	{
		name = ElectricCharge
		rate = 0.325
	}
}
MODULE
{
	name = ModuleSAS
}
RESOURCE
{
 name = ElectricCharge
 amount = 410
 maxAmount = 410
}
}

Something like this:

Caveats

If you played with the motorbike, you will notice a few things are very different from the craft we based this on. You are only able to toggle the light on the landing gear, and the driving power is coming from the landing gear instead of the electric wheel. These are both a result of being unable to resolve multiple instances of MODULE functions: since the landing gear was the first applicable model node for each of those in the definitions list, they are only applied to the landing gear. This is also why we didn't define four engine modules in the quadraspike. There may be ways around this, but I've not learned them yet. Another problem is that trying to turn with the wheels doesn't work, since it is seen as neither behind nor ahead of the CoM, which also prevents some uses of RCS thrusters. Remember that this system isn't meant to replace the way KSP works, but it may help us take on more grandiose missions with better performance.