Rhea - Geometry Shader Grass

Rhea is a geometry-shader based grass for Unity's Universal Render Pipeline (URP).

Features

Procedural and atlas-based texturing
- Atlas biasing for non-uniform texture distribution
- Vertex color tinting

Customizable blade shape and warping
View-based tessellation for increasing grass density efficiently
Texture-based trimming and height mapping
Vector wind displacement
Object-based displacement
Normal-based spawning and jittering
- Works well on non-planar meshes such as spheres, sharp terrain, etc

Several example materials are included, with examples of texture atlasing and procedural setups
For object displacement, attach the SetDisplacementLocation.cs script to your player, or the object that grass should be displaced from
The grass height map and wind vector map can be set to RenderTextures instead, allowing for a trail of grass following an object, dynamic wind from real-time flowmaps, or pre-baked height maps
This repository is intended as a technical demonstration of geometry shaders for terrain grass. However, it should be noted that platform support and performance of this implementation, and geometry shaders in general, may not be ideal for general use in games!
Use of this asset is under the CC0 Creative Commons License

Blade Texture - A transparent pixel texture or atlas used for texturing each blade of grass. This can be left unassigned if procedural texturing is desired.
- If the texture is to be used as an atlas, the individual textures should be aligned in a horizontal row
  - The textures should be arranged in order of occurrence if biasing is used (most commonly occurring texture on the left, least common on the right)
- Textures can be provided at any aspect ration, but should be scaled as necessary to fill the full UDIM
Blade Coloring - Two colors used for gradient tinting, linearly interpolated between the base and tip of the blade
Luma Jitter - Randomly darkens some blades
Height Brightness - Darkens blades based on blade height: lower blades are darker, taller blades are brighter
Hue Jitter - Randomly varies the hue of each blade (only affects R and G channels to preserve more natural coloration)

Atlas Length - The number of textures arranged horizontally in the blade texture
Bias - Creates a non-uniform distribution of textures from the atlas: textures on the left side will occur more frequently than textures on the right side
Height Bias - Applies texture biasing based on height instead of random selection: lower blades will have textures selected from the left hand side, higher blades from the right.

Width - The upper and lower bounds of the width of the blades
Height - The upper and lower bounds of the height of the blades
Tapered Blades - Changes geometry from a mesh ribbon to a segmented triangle, useful for procedurally textured blades

Bending - The overall amount of flex in each blade (can be positive or negative)
Curvature - How curved the blade is across it's flex. Higher values will give straighter bases and more arched ends
Variation - Controls how much the blades flow together, higher values will give a denser, bushier appearance
Position Jitter - Randomly varies the position of each blade, useful to break up tessellation patterns

Density - The overall maximum amount of tessellation applied
View Distance - Min and maximum distances for LOD fading: minimum determines the distance at which fading starts, upper value determines the distance at which it ends

Trim Map - A grayscale texture used to determine the height and culling of grass blades (darker values are shorter, brighter values are taller)
Threshold - The darkness threshold at which grass blades will be culled entirely
Falloff - Controls the contrast of the texture when used as a heightmap for blades

To be used with SetDisplacementLocation.cs.

Radius - The radius from the objects location at which grass will be displaced
Strength - The amount of displacement on the grass from the object
Falloff - How sharply the displacement around the object ends