Global Options Overview
The global switches allow you to control various aspects of the renderer globally.
Geometry Section
Displacement - enables (default) or disables V-Ray displacement mapping.Indirect Illumination
Don't render final image - when this option is on, V-Ray will only calculate the relevant global illumination maps (photon maps, light maps, irradiance maps). This is a useful option if you are calculating maps for a fly-through animation.
Displacement
Allow negative colors used in texture for displacement - when enabled V-Ray will recognize negative colors in textures that are used for displacement and displace the geometry in the opposite direction.
Alpha
Output alpha channel to separate file - when enabled a separate output image file will be generated for the alpha render element during the final rendering.
Raytracing Section
Secondary rays bias - a small positive offset that will be applied to all secondary rays; this can be used if you have overlapping faces in the scene to avoid the black splotches that may appear. See the example below for a demonstration of the effect of this parameter.
Example: Secondary Rays Bias
This example shows the effect of the Secondary rays bias parameter. The scene below has a box object with a height of 0.0, which makes the two sides of the box to occupy exactly the same region in space. Due to this, V-Ray cannot resolve unambiguously intersections of rays with these surfaces.
The first image shows what happens when you try to render the scene with the default settings. You can see the splotches in the GI solution, caused by the fact that rays randomly intersect one or the other surface:
In the second image below, the Secondary rays bias is set to 0.001, which offsets the start of each ray a little bit along its direction. In effect, this makes V-Ray skip the problematic surface overlaps and render the scene correctly:
Note that the Secondary rays bias affects only things like GI, reflections, etc. In order to render the scene properly, the material assigned to the box has its 2-sided option checked. This is so that the object looks in the same way regardless of whether the camera rays hit the top or the bottom of the box. If the material did not have this option checked, it would appear "noisy" even though the Secondary rays bias is greater than 0.0:
Clamp max ray intensity - this option allows to suppress the contribution of very bright rays, which may typically cause excessive noise (fireflies) in the rendered image. Its effect is similar to the Subpixel mapping + Clamp output options of the Color Mapping section, but the Max ray intensity is applied to all secondary (GI/reflection/refraction) rays, as opposed to the final image samples. This allows fireflies to be effectively suppressed but without loosing too much HDR information in the final image. Similar to the Subpixel mapping option, the Max ray intensity introduces bias in the rendered image, as it may turn out to be darker than the actual correct result.
Lighting Section
Lights - enables or disables lights globally. Note that if you uncheck this, V-Ray will use the default lights. If you do not want any direct lighting in your scene, you must uncheck both this and the Default lights parameters.
Default lights - enables or disables the usage of the default lights when there are no light objects in the scene or when you have disabled lighting globally (see the Lights parameter).
Hidden lights - enables or disables the usage of hidden lights. When this is checked, lights are rendered regardless of whether they are hidden or not. When this option is disabled, any lights that are hidden for any reason (either explicitly or by type) will not be included in the rendering.
Shadows - enables or disables shadows globally.
Show GI only - when this option is on, direct lighting will not be included in the final rendering. Note that lights will still be considered for GI calculations, however in the end only the indirect lighting will be shown.
Texture Filtering
Multiplier - this is a multiplier for the blur filter applied to all textures which are either loaded from a file (file-based) or loaded from the memory (when Softimage ClipFX is involved)
Materials Section
Reflection/refraction - enables or disables the calculation of reflections and refractions in V-Ray maps and materials.
Max depth - allows the user to globally limit the reflection/refraction depth. When this is unchecked, the depth is controlled locally by the materials. When this option is checked, all materials use the Max depth specified here.
Maps - enables or disables texture maps.
Filter maps - enables or disables texture map filtering. When enabled, the depth is controlled locally by the settings of the texture maps. When disabled, no filtering is performed.
Max. transp. levels - this controls to what depth transparent objects will be traced. Reducing the depth will cause faster rendering but may produce artifacts.
Transp. cutoff - this controls when tracing of transparent objects will be stopped. If the accumulated transparency of a ray is below this threshold, no further tracing will be performed.
Override mtl - turning this option on will override all scene materials with a single diffuse material when rendering. This feature can be useful during the lighting placement phase of the pipeline.
Glossy effects - enables and disables the rendering of glossy effects.