GUI - FG tab

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FG tab


Here is where you will spend most of the time in Kray GUI. This tab controls the most important stage of rendering – final gathering (FG).

GI resolution

See GI Resolution for more information.

FG preset

This selection list lets you choose predefined FG preset. When you choose Custom from the preset list advanced settings are unlocked.

FG threshold, min rays, max rays

Whenever you see threshold in Kray interface it always means the same.

Kray starts sampling point by shooting min number of rays into the scene. Threshold defines the how much difference between two pixels is "allowed". If difference is greater than that defined by threshold, Kray will increase rays, until the difference gets smaller than the threshold. It will also stop adding rays if it reaches max rays before reaching the threshold value.

The FG threshold should be usually left at default 0.0001. Increasing this number may cause splotches in the scene but may reduce render time.

Min rays define the least amount of rays each sampled point will fire. This value is usually kept low – around 100-300, but may be increased if there are artifacts in the scene that do not disappear when increasing max rays value.

Max is the maximum rays Kray will use for any sampled point. If this value is too low you splotches will appear. Increasing this value means longer render time. By increasing this number render time increases approximately linear. For example if you increase max rays from 500 to 1000 render time will double.


Prerender is the first step of FG which helps in defining which parts of the scene should be evaluated more thoroughly. When prerender value is 50% it means that it will render use 50% of the final resolution. You should also note that increasing prerender will cause longer prerender pass but shorter final render pass.

This value is best kept between 50%-100%.

It is important to understand how Kray chooses location of samples. To optimize rendering as much as possible samples are placed at “strategical” places. This are usually places where surfaces are close to each other or places of sudden light change. On large flat areas there is no need to compute samples for every pixel so Kray will only place only very few samples there. This way speed of rendering can be greatly improved. More samples means longer render time. Therefore it's important to find the right balance between sample density and render quality.'

There are several settings which control placement of samples in a scene:


Passes defines the number of passes Kray will render to find new sample points. In each pass kray will find more sample points and add them to solution. The number of new sample points found is displayed in render window. More passes used will make much cleaner global illumination and produce less splotches. Usual values are 1-3, more passes mean longer render time.

Note: when rendering animations don't use passes > 1 because this may cause very slow rendering times.

Splotch detection

Splotch detection controls how sensitive is Kray to detecting artifacts in global illumination solution. Increasing this number will make Kray less sensitive while making render times shorter. Values around 0.05 work well with 2-3 steps.


Sensitivity is another control that controls how sensitive Kray is to detect artifacts in global illumination solution. It compares neighbouring irradiance gradients and if the difference between them is higher than specified then it will add more samples. Values around 0.05 usually work well with 2-3 steps.

Spatial tolerance

In a simplified terms, spatial tolerance defines the smallest distance between two sampled points in space. When this number is too large the shadows will look blurred and undefined. If this number is too small then the render times can get much longer than needed. Good values range around 0.05-0.1 for typical scenes. I also depends on the scale of your scene. Smaller scene will require smaller Spatial tolerance.

Angle tolerance

Angle tolerance defines the placement of sample points on curved surfaces. The value defines how much can surface angle change before new sample is added.

Distance min / distance max

In simplified terms this parameters control the minimum and maximum distance allowed between any two sample points.

The true math behind it is a little more complex. Minimum distance is acctualy the shortest lenght of the ray fired from the sample point. Rays shorter than this will be treated as if they were 'min distance' long. The opposite is true for the maximum distance. Rays that travel longer than 'max distance' will be treated as if they are 'max distance' long.


Brightness/Density will add more samples in places of high light contrast. It's not very useful any more and is left here mostly for the legacy purposes. To get better density of samples in high contrast areas use more prerender steps as described above.


Will blur neighbouring samples and thus reduce artefacts such as splotches. The drawback is that it may also blur some very fine indirect shadows. Useful values are 0-5.

Show samples

Using Show All FG samples

This option lets you visualize placement of FG samples. This is primarily used for testing and troubleshooting.


Will not show any samples.


Will show only samples in corners.


Will show all samples in scene.

FG reflections

Will turn on or off tracing of FG rays on reflective surfaces. Turning this option off may speed up render times but it may also cause inaccurate reflections.

  • Note: When using material nodes changing FG reflections/refractions will not have any effect. The only way is to exclude shader from FG is by using Kray Indirect Rays shared and multiplier 0.

FG transparency/refractions

Will turn on or off tracing of FG rays on refractive or transparent surfaces. Turning this option off may speed up render times but it may also cause inaccurate calculation of GI through transparent/refractive surfaces.

  • Warning: Your windows may block FG rays if you turn this flag off. To work around put them on a separate layer and turn ON "unseen by radiostiy" in the object properties.
  • Note: When using material nodes changing FG reflections/refractions will not have any effect. The only way is to exclude shader from FG is by using Kray Indirect Rays shared and multiplier 0.

Corner distance/Paths

When Paths is set to > 0 Kray will turn to path tracing when rendering corners. This may reduce splotches or light leaks. Distance specify how far from a corner paths rendering will be used.