Grass (PhotoRendering Option)

These settings are available in the Detailed view of PhotoRendering Settings for the CineRender engine. 

Use these controls to define the rendering parameters for the Grass surface in the scene, if the Grass surface channel is being used. See Grass (CineRender Surface Channel).


When grass is rendered, light shadow and color are calculated at different locations on the grass and subsequently interpolated:

Pixel: No interpolation will take place - each pixel will be calculated individually. This method delivers the better results but also take the longest to render.

Otherwise the Pixel option should be selected in the following instances:

-If a texture in the Color channel is projected onto each individual hair.

-If the Specular Lights are too blurred, regardless of changes made to the Reflectance channel.

-In general, render quality will increase when very few grass segments are used.

Vertex: The start and endpoints of segments are sampled and interpolated. This setting offers a good compromise between quality and render time and will lead to good render results in most cases. 

Left Vertex, right Pixel. Note how individual grass shadows are taken into consideration
at the right, which are ignored on the left.


These options are displayed when the Physical Renderer is used. They are designed to deliver faster rendering but will result in corresponding loss of quality. Generally speaking, you should simply activate the options, see if the results meet your standards and enjoy the faster rendering.

The Cache options temporarily save the grass illumination surface properties - diffusion, specular and indirect lighting (GI) - to the RAM (primarily at the grass vertices). These one-time calculated values will then be used multiple times during rendering, which saves time. However, the calculation is simplified, which can have negative effects on render quality. These effects are very minimal and barely noticeable. 

Left: Cache Primary Rays disabled; right: enabled.

Note how the fur on the mouse at the right appears more coarse. The grass consists of only 3 segments and the respective Grass surface has the color range to the right assigned to it. You can see how the colors of the grass on the right blend. Here, the color is calculated on each of the 4 grass vertices and interpolated. Now imagine this “blurriness” applied not only to the color but also to other grass properties such as specular, etc., and you have an impression of how the Cache options work.

The following image shows various Cache option combinations and their effect: 

Note how the color range for the grass at the top left looks best with no caching but requires the most render time, and how the other examples have poorer quality but shorter render times. The reduction in quality has been exaggerated in these examples by using only 3 grass segments. Normally you will have a far greater number of segments, which means that the loss in quality will be much less.

Cache Primary Rays

Effects of the rays as seen from the angle of view of the camera will be cached. This includes shading and shadow effects as well as the grass’s specularity (see image above).

Cache Secondary Rays

Secondary rays are used to calculate reflections, refractions, blurriness effects and GI. If this option is enabled, these elements will be cached. Secondary rays are a superset of matte and GI rays, which means that the following two options can be disabled separately if this option is enabled. Secondary rays cannot read/use the primary rays’ cached specularity. This will then come after transparency and reflection.

Cache Blurry Rays

Blurry rays are needed in order to calculate Blurriness. These will then be cached if this option is enabled.

Cache GI Rays

This option caches the GI sampler’s effect and is only effective if used in conjunction with Global Illumination. The visible effect of the cached GI rays is very minimal, which means that this option can actually remain enabled. This in turn results in much faster rendering in conjunction with QMC.

GI Sampling 

The top row is illuminated using a light source, the bottom row using a polygon light

You can define where GI should be rendered or cached on grass:

Pixel: The entire grass will be rendered with GI. This is the slowest and most precise method.

Vertex: Only the grass vertices will be rendered with GI. The result is very similar to that of the Pixel mode but is generally rendered brighter because the grass normals are, internally, always oriented towards the camera.

GI Quality

This value represents a multiplier for the Samples parameter. It is designed for use with QMC, which requires more samples in conjunction with grass. When the aforementioned Cache options are used, this setting will be taken into consideration. However, if the Cache GI Rays option is enabled, this setting will have no effect.