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==== First steps on light modelling ====


This tutorial we show you the basics on how to do light modelling in GroIMP. For some more theoretical background pleas refer to the [[:Tutorials:light-modeling-introduction| Introduction - A little bit of Theory ]] page. For an advanced tutorial on spectral light modelling check out the [[:Tutorials:basic-spectral-light-modeling| Spectral light modelling ]] tutorial.

GroIMP integrates two two main light model implementations, namely:

  * Twilight, a CPU-based implementation
  * GPUFlux, a GPU-based implementation

While they are both integrating various renderer and light model implementations, they differ in a few details we are not gonna be discuss here. Please refer to the [[:Tutorials:light-modeling-differences| Differences between the CPU and GPU light model ]] page for details. The two main difference however are first that the GPU implementation is several times faster than the CPU implementation and second, that the GPU version is, beside the three channel RGB implementation, able to simulate the full visible light spectrum (spectral light modelling).

In the following, we focus on the three channel RGB versions of both the CPU and GPU implementations. 

To set up a light model basically three steps are needed. 

  * Definition/Initialization of the light model
  * Running the light model
  * Checking the scene objects for their light absorption

In GroIMP/XL, this can be done as following:

For the Twilight (CPU-based) implementation:

<code java>
import de.grogra.ray.physics.Spectrum;

//constants for the light model: number of rays and maximal recursion depth
const int RAYS = 1000000;
const int DEPTH = 10;

//initialize the scene
protected void init() {

  //create the actual 3D scene
  [
    Axiom ==> Box(0.1,1,1).(setShader(BLACK)) M(2) RL(180) 		
    LightNode.(setLight(new SpotLight().(setPower(100),setInnerAngle(0.02),setOuterAngle(0.055))));
  ]

  //make sure the changes on the graph are applied...
  {derive();}
  //so that we directly can continue and work on the graph

	
  // initialize the light model
  LightModel CPU_LM = new LightModel(RAYS, DEPTH);

  CPU_LM.compute(); // run the light model
	
  //check the scene objects for their light absorption
  Spectrum ms;
  [
    x:Box::> { ms = CPU_LM.getAbsorbedPower(x); }
  ]
  print("absorbed = "+ms);println(" = "+ms.integrate(), 0xff0000);
}
</code>

Direct after saving the code, the light model will be executed and the following will be printed in the XL Console Window:

{{ :tutorials:console_output.png?direct&500 |}}


While in the above example the object is black, what means that all incoming radiation is absorbed (and nothing reflected), the line below will shange the object colour to some orange

<code java>
  Axiom ==> Box(0.1,1,1).(setShader(new RGBAShader(1,0.5,0))) M(2) RL(180) 		
</code>

what means that all of red and 50% of green are reflected while all blue radiation is absorbed as the resulting output confirms.


{{ :tutorials:console_output2.png?direct&500 |}}


For the GPUFlux (GPU-based) implementation:

<code java>
import de.grogra.gpuflux.tracer.FluxLightModelTracer.MeasureMode;
import de.grogra.gpuflux.scene.experiment.Measurement;

//constants for the light model: number of rays and maximal recursion depth
const int RAYS = 1000000;
const int DEPTH = 10;

//initialize the scene
protected void init() {

  //create the actual 3D scene
  [
    Axiom ==> Box(0.1,1,1).(setShader(BLACK)) M(2) RL(180)
        LightNode.(setLight(new SpotLight().(setPower(100),setInnerAngle(0.02),setOuterAngle(0.055))));
  ]

  //make sure the changes on the graph are applied...
  {derive();}
  //so that we directly can continue and work on the graph

	
  // initialize the light model
  FluxLightModel GPU_LM = new FluxLightModel(RAYS, DEPTH);
  GPU_LM.setMeasureMode(MeasureMode.RGB); // user the Flux model in three channel 'RGB mode'

  GPU_LM.compute(); // run the light model
	
  //check the scene objects for their light absorption
  Measurement ms;
  [
    x:Box::> { ms = GPU_LM.getAbsorbedPowerMeasurement(x); }
  ]
  print("absorbed = "+ms);println(" = "+ms.integrate(), 0xff0000);
}
</code>

To be continued...