Calculating the prism of a plant

Just click here:

And wait a little bit (1 minute max for large prisms).. A prism is a multi-faceted impostor that replace a real plant mesh in the distance. Note that due to the high quality of these impostors, they may be used even very close to the camera position. For small plants such as grasses, it's very common to only use prisms and not source meshes, relaxing all polygon count constraints that way.

A prism's facet contain most of the parameters needed to do a proper shading of the plant. Even if geometrically it's a plane, it stores all lighting informations and it reacts to the lighting and it's rendered in 3D with depth. In the image below we have focused on a prism's facet and we see how it reacts to the variation of the sun direction:

If we move around the prism, we can see its facets:

Several parameters are to be considered when calculating a prism:

Reducing a prism pixel size

A prism has a quite high memory cost due to the number of impostor facets being used to define it and also due to the pixel size used for it. Therefore, it's good to look forward to reduce the size of a prism. It's worth noting that if a mesh switches at a given distance - for instance 50 meters - then a prism will never be visible closer than 50 meters. Therefore, it may be unnecessary to use a too large texture size for it since it'll never be that large on screen. This can be setup for each plant to balance the wished quality / performance ratio.

Using only prisms

Sometimes, there's no need to switch to use meshes. The prism - in which case must have a high resolution - can look good enough even when close. Below is an example of a grassy field looked from below 1 meter, only made with prisms:

Prisms are transparent, blended and have smooth edges, so they tend to work pretty well for this kind of display. Note that unlike most impostors, prisms are designed to be looked from a side angle and also from a top view.

Keeping a prism up to date

These markers in the information part of the prism panel indicate whether the prism is ready or not:

The first icon indicates whether the ambient occlusion field of the prism is correct or not. The second icon indicates whether the prism is correct or not. Modifying a value in the panel may invalidate the AO or the prism or both. For instance changing the resolution of the prism will invalidate it. An invalid prism will remove the display of the plant from Biomes that are using it. Regenerating the prism will make the plant appear again.

Note that changing a material of the source mesh of a plant does not invalidate its prism. Said otherwise it's possible to have a prism that does not exactly match to the mesh seen when close.