The highest level of a VSL material hierarchy consists of Shaders. A Shader is a VSL object that has only one property: 'Type'. 'Type' has values like 'Surface properties', 'Surface illumination' and 'Volume properties'. These names, quite obviously, reflect the actions that the rendering engine performs.

The order in which the shaders appear in the hierarchy is not significant. For example, the rendering engine needs to know surface properties before it starts computing surface illumination, and therefore the corresponding shaders are executed in this order, although the user may have inserted them in reversed order.

The set of channels, which is available for use in each shader type, varies. This helps to understand when channels become initialized and which channels are sensible to change. For example, in the 'Volume properties' shader, the surface normal channel is not available at all. This is because it does not make sense to deal with a surface attribute inside an object. In this way, the shader structure guides the user and helps to understand the rendering process.

The shader system is expandable. Plugin modules may insert new shader types.

The default shader types are listed below. These shaders describe the steps of the rendering process quite accurately, so rendering can be customized quite thoroughly using the shaders. For example, the default surface lighting model (standard diffuse lighting) can be redefined completely using suitable objects in the 'Surface illumination' shader.

Included is also a list of 'system' channels that are initialized by the rendering system particularly for that shader, or by an object, which is typically evaluated before the shader. In addition to these, other channels may also contain initialized information written by user defined special operations, materials, plugin classes or by other means.

Material Initialization

This is a special shader in that it is executed only once; at the beginning of rendering. Therefore, it is suitable for initializing constant data, which is stored in root level variables.

Initialized channels:

Common Properties

The VSL code included in this shader is always executed with all other shaders of the material. You can place general-purpose operations and preparations that affect all other shaders into this shader. A typical use of this shader is defining the scope of a material - how much the material affects the target object.

Note, that the position of the common properties shader in the VSL hierarchy defines its evaluation time: if it is placed above other shaders, it is evaluated first. If it is at the end of the VSL tree, the code is evaluated as the last action of every other shader.

If the material contains no other shaders as the common properties shader, the evaluation defaults to the surface properties shader.

Initialized channels:

Output channels:

Surface Geometry

This shader can be used to modify surface geometry related attributes.

Initialized channels:

Channels used by the rendering system after the shader:

Surface Properties

This is probably the most commonly needed shader, because the appearance of most objects is defined by their surface. Normally, properties such as color or transparency are defined in this shader.

Initialized channels:

Channels used by the rendering system after the shader:

Volume Properties

This shader is used when examining interiors of objects. The shader is needed when creating fog effects, including volumetric light effects (fog glowing in light, shadows casted in fog). Volume sampling slows down rendering, and therefore there is a switch on render settings, 'Volume sampling', which must be enabled in order to let volume properties shaders to operate.

Initialized channels:

Channels used by the rendering system after the shader:

Light Properties

This shader defines the properties of light emitted from a light source. For example, it is possible to define light falloff rate, color, additional light property channels (a light source casting gamma rays), etc.

Initialized channels (all in Light sample):

Channels used by the rendering system after the shader:

Surface Filtering

This shader controls the shadow creation. A ray tracer executes the shader when a light ray coming from a light source hits an object. The purpose is to find out if the material applied to the object lets the light go through and how the light changes. The decision is made by examining the transparency channel. If the color is white (1.0, 1.0, 1.0), the light continues in an unmodified state, whereas black (0,0,0) stops the light and causes a full shadow. Materials can define custom filter effects by computing suitable filter color values in the filtering shader.

A Ray tracer always executes the 'Surface properties' shader before 'Surface filtering' shader. Therefore, transparency defined in 'Surface properties' is conveniently used as a default value for 'Surface filtering'.

Initialized channels:

Channels used by the rendering system after the shader:

Volume Filtering

This shader is executed to examine how a volumetric material shadows the light. The sampling density channel defines the accuracy of computation, just like in other volume effects. Furthermore, the 'volume shadows' option in render settings needs to be switched on. The transparency channel defines the amount of shadow. Transparency has the default value defined in the 'Surface properties' shader, which was evaluated when the light ray entered the volume.

Initialized channels:

Channels used by the rendering system after the shader:

Begin Illumination

This shader starts processing surface illumination by light sources. It is executed once, and after that usually multiple Surface illumination evaluations follow. In this shader, you can initialize channels for collecting lighting information.

Initialized channels:

Surface Illumination

Initialized channels:

Channels used by the rendering system after the shader:

End Illumination

This shader finishes processing surface illumination by light sources. You can examine and store the total influence of light sources to the surface in this shader. For example:

    Begin illumination /* Store illumination level before light source contribution */
        surface:my_lighting_channel = copy(surface:illumination)
    Surface illumination
        .... /* Lighting actions here */
    End illumination /* Compute how much illumination increased */
        surface:my_lighting_channel = substract(surface:illumination, surface:my_lighting_channel)

Initialized channels:

Surface Finishing

This shader is the very last step of surface shading. In this shader, one can perform actions that control the total surface shading. For example, one can compute all illumination contributions to a buffer channel and then define the actual illumination using this shader.

Initialized channels:

Channels used by the rendering system after the shader:

Volume Illumination

This is similar to the surface illumination, except that this time, illumination of an interior point of a volume is examined. By default, ray tracer does the diffuse shading effect by the given color channel value: light makes the fog glow if the color channel is not black. The diffuse shading depends only on the intensity of the light in the examined point, it does not depend on the lighting direction.

Initialized channels:

Channels used by the rendering system after the shader:

Volume Shading

This shader is executed as the last step of volume processing. At this point, the Volume sample contains the fully illuminated fog in the 'Volume:Illumination' channel. The remaining step is the default shading, which blends the fog illumination with the result of ray tracing using the distance and turbidity. The thicker the turbidity and the longer the distance, the more fog illumination affects the result. VSL code in 'Volume shading' can make the final adjustments before the default shading. It can also compute completely customized fog shading and then set the 'Turbidity' to zero to eliminate default shading.

Initialized channels:

Channels used by the rendering system after the shader:

Initialize Ray

This shader is called when tracing of a new ray (reflection, GI ray etc.) starts. It is not called for rays originating from the camera. In this shader, you can move data from the originating surface into the traced ray.

Initialized channels:

Channels used by the rendering system after the shader:

Secondary Ray

This shader is executed when tracing of a ray is finished and the remaining step left is to add the new information to the overall result of shading. For example, fading of reflection by distance can be done in this shader.

Initialized channels:

Channels used by the rendering system after the shader:

The default shading operation, which combines the two generations of rays, is simply

Surface:Illumination += Traced ray:Illumination*Signalcolor

Here 'Signalcolor' is the channel that created the new ray, such as transparency or reflection.

Ray Hit

This shader is quite a special one. The ray tracer executes it when computing distances to a surface. By modifying the distance, one can distort the surface. If the distance is set to a negative value, the surface disappears and so-called clip mapping effect is achieved.

Initialized channels:

Channels used by the rendering system after the shader:

Ray Termination

This shader can be used for some fine detail effects for transparent objects. The ray tracer executes it when the tracing of a ray is stopped because of finite recursion depth. Usually, one could add the average of the color of the overall scene to the illumination channel.

Initialized channels:

Channels used by the rendering system after the shader:

Scanline

This shader allows rendertime displacement of scanline objects using materials. The density of points which are passed for this shader depend on the particle type. For example, a particle curve evaluates point displacement densely along the curve, and this shader can be applied to make it curly. Particles may also support other control channels (diameter channel etc.)

Initialized channels:

Channels used by the rendering system after the shader:

Post Particles

This shader is executed when rendering particles in the post-processing module. For example, it is possible to define the color of particles in this shader. The purpose of this shader is similar to that of the 'surface properties' shader, but as the particle rendering in post processing is a much simpler operation than surface rendering in the ray tracer, less channels are available.

Control channels:

Image Processing

This special shader is plugged in by one post processing effect, 'material effect'. The purpose is to let the user manipulate pixels of the rendered image by a shader system. As described before, the material editor of the program includes a powerful programming language, and this shader allows the user to define custom image processing effects with the language. All the channels are available in this shader.

Rendering Diagram

The picture below shows how the rendering system moves data between the samples. Thick orange arrows represent internal default shading operations between samples, filling the gaps in the data flow.