Setting up Lightwave
Since Kray is relying on real world physics to compute global illumination it is important to set up a scene properly.
The first step is to turn on all raytracing flags in LightWave render options.
Ray recursion limit tells Kray how many times the light should bounce. In contrast to other render engines, a high number of light bounces doesn't increase the render time significantly.
The number of bounces can be overridden by adding the following command to the tailer line in Kray GUI: recurse 100; This will tell Kray to use 100 bounces of light.
The lighting of a scene should be set up realistically.
This means turning off ambient light and setting it to 0% because it makes renders look washed out and unnatural, and also, all lights should use Inverse Distance Square falloff which is closer to what happens in reality.
If LightWave is not set-up this way Kray will give a warning saying Physically incorrect light model in a scene with global illumination. This is just a warning and will not stop Kray from rendering.
Background can also act as a light source in global illumination scene when using Light mapping. Photons tracing does not support background lighting since there is no point from where photons can be emitted. You can overcome this by using a luminous dome instead of background light.
Since CG is not an exact science - it is all good as long as it looks good. For example sometimes you will want your lights to have linear falloff, to lighten up certain parts more and avoid over burned areas.
There is a switch in Kray GUI which converts Spotlights into area lights. This is especially handy when you want soft sun shadows. Using LightWave's “soft edge angle” you control how soft the shadows are.
Tip: For a nice realistic Sun light use Spotlight with soft edge angle to 0.55. Put sun 100km away (that's easiest done with Sun spot motion modifier) and set Light to have Inverse distance^2 falloff of 100km.
Since Kray does many different computations to avoid long render times it is necessary to obey some rules when building 3d models.
Here are some guidelines to follow:
- Try to model clean and if possible "watertight". This means that you should check that your points are merged together and polygons share edges. For example: if polygons don't share edges in room corner in a room, you may get light leaks, because Kray will sample points from both sides of polygon instead only from inner side.
- UV's can sometimes cause problems. So be take care when unwrapping your models.
- Try to avoid very long and thin polygons. This may cause extremely large memory consumption and hence long render times. If you run into this problem then just slice the object up in smaller more regular pieces.
For more help take a look into troubleshooting section of this manual.
Since Kray uses physically accurate computation to render the scene there are some guidelines that you should try to follow when setting up your surfaces.
Light and material models used in 3d packages (such as LightWave) are not always physically correct. For example in LightWave you can create materials that have diffuse 100% and reflection 100%. Diffuse + reflection = 200% and that means the material reflects (it reflects all light diffusely and reflectively) more light than it receives. Physical incorrectness in a non-GI scene helps to hide lack of global illumination, but in a GI scene such surfaces can look very unnatural.
Kray does not force user to use physically correct models. They are sometimes useful to make rendered images look more dramatic, it is important to know when the scene matches the laws of physics and what will happened if we break them.
The general balance rule that should be followed to avoid unrealistic materials is :
or a more complex version including transparency would be:
Color*Diffuse(100%-Transparency)+Transparency+Reflection+Translucency < 100%
Notice that it is "less than" not "less or equal than". In reality no material reflects all the light that hits its surface. The same rule is valid if we use gradients or textures for every gradient angle and texture pixel.
Balance rule does not affect Luminosity. Feel free to use 300% Luminosity and 95% Diffuse for example.
Objects with surface material Luminosity > 0%, emit light. On a GI enabled scene they can be used to illuminate the scene. Howerver you can also use luminous object to light a scene in Diffuse model Raytrace only mode but you need to switch to Compute as direct Luminosity model in the quality tab of the Kray settings which will make them emit light even without GI.
Generally it's not suggested to use luminosity to tweak the look of a material that normally doesn't emit light. Same as light intensity, you are not limited to 100% luminosity and you can use values over 100%. Unlike regular lights, you can also have them textured to emit multicolored light and with variable intensity.
Specularity is used to fake light reflections. It simulates how the hotspot of a light would be reflected on a surface. This is used because rendering blurry reflections was often very slow. It has important limitation though: it doesn't render light reflected from other surfaces.
In Kray however, you don't often need to use specularity because blurry reflections work very fast and look much more realistic. So instead of using specularity and glossiness you just have to add reflection and some blurring to get nice glossy surface. However there are still uses for specularity - in cases where you want to see light reflected in a surface and you don't have any physical model of the light (sun for example).