Grass

Grass is rendered as a set of 2D textures (blades), each of which represent a small grass tuft. Grass diversity is achieved by the probabilistic factor that influences the grass blades height, offset, color, etc., which grants convincing illusion of natural variation with fine details, while not monopolizing the GPU. The grass can be animated, which makes it realistically sway in the wind.

See Also

To add grass to the world, perform the following:

1. On the Menu bar, click Create -> Object -> Grass.

   

2. Place the object somewhere in the world.

   

The default grass field would appear with the grass_base material assigned.

Rendering of grass mostly depends on two textures:

• A diffuse texture defines the grass shape and color.
• A mask texture determines grass mapping and density distribution on the terrain.

For convenience, diffuse texture is subdivided to the chunks. Each chunk will be rendered depending on the used mask values. To utilize maximum variations of the grass species, the chunks are placed on the texture in the following way:

• The horizontal row can have up to 4 chunks. Each chunk would correspond to the red, green, blue and alpha channels of the mask.

  

• The vertical column can have any amount of chunks. The chunks in the column represent the variations of the grass per the mask channel. Usually, each channel corresponds to the particular species of grass (thiner, thicker, tangled, etc.). If there are several grass clusters in a vertical column, they will be randomly spread across the masked area.

  For the texture to be split on the correct number of chunks, the number of chunks that you've used should be specified in the Num textures field. For example, for the diffuse texture below, Num textures = 4 should be specified.

  

For example, we have the following diffuse and mask textures:

Diffuse texture

RBG and A channels of the mask

We see, that in the first column, chunks of the green shades correspond to the red channel of the mask, which is 9 red rhombuses; in the second column, chunks of the real grass variation correspond to the green channel of the mask, which is the square green mask frame; in the third column, chunks of the red shades correspond to the blue channel of the mask, which is the blue rhombuses frames; and in the fourth column, chunks of the yellow shades correspond to the alpha channel of the mask, which is the rest black areas (note that these parts won't re rendered, as there no alpha color data).

The result will be as follows:

To randomize appearance of the grass blades, two types of values are used:

• Mean value (set in the main field of the parameter:Aspect, Minimum/Maximum Height, Offset) defines the average value of the distribution. Four fields correspond to the red, green, blue and alpha channels of the diffuse texture correspondingly. With its help the designer can set setting milestones to control the parameter.
• Spread value defines the range for possible variation of the parameter. Four fields correspond to the red, green, blue and alpha channels of the diffuse texture correspondingly. The higher the value, the more diverse the final result will be.
  Notice

  Spread value is optional: if set to 0, it will not influence the simulation process and only the mean value will be uniformly used.

Probability

The probability of appearance of the grass chunk belonging to the definite mask channel (horizontal row) depends on the Probability parameter. The variations of the specie (vertical column) will be rendered randomly.

Probability of appearance of the grass slots belonging to the red, green, blue and alpha channels of the mask correspondingly

Lets check several variations of the set Probability parameter for the following diffuse texture:

As a mask the default RGBA8 white texture will be used (as there are 4 chucks in the diffuse texture, we would set Num textures to 4). As its value is white = 1, the whole grass area will be filled.

In the pictures below different options of the set Probability parameter are shown:

Probability = 1 1 1 1

Probability = 0,1 0 0 1

Probability = 0 1 0 1

Probability = 0,1 0,3 0,7 1

Minimum and Maximum Height

Aspect

To render the intended grass quad, besides the height, it should also have the width set. But in order to preserve the proportions more easily, the latter is not specified directly, but rather using the aspect parameter. So the final width is calculated by multiplying the Max height by the Aspect.

Aspect is set for each of the diffuse texture columns separately. Like the former, this parameter can be randomly varied if spread value set in addition to mean one. For example, setting spread to 0, yields the following result:

• If mean value is set to 1, the grass will be rendered in square proportions, with height equal width.
• If mean value is set to 2, the width of the grass quad will be twice longer than its height.

Aspect = 1; Spread = 0

Aspect = 2; Spread = 0

Offset

Texture Slots

On one single field, it may be necessary to simulate grass that had grown in good and bad conditions, shooting young grass blades and withered straw. The grass diffuse texture can have 4 x  n slots, each containing a separate grass cluster, plant or a flower. There can be any number ofhorizontal rows in the texture. However, the number of vertical columns may vary depending on the way the grass distribution is specified:

• If an image mask is used to specify the grass density distribution, the number of diffuse texture columnscan be less than 4.
• If a mesh mask is used to specify the grass distribution, the number of diffuse texture columnsmust be equal to 4.
• If both masks are used (the image mask sets the grass density distribution and the mesh mask is used to exclude some areas such as roads and so, the number of diffuse texture columns also must be equal to 4.

Num textures parameter sets how many horizontal rows with grass clusters, plants or flowers are presented in the diffuse texture. After specifying the number, each grass cluster is rendered as a separate quad on the field.

4 Texture Slots; Num textures = 1	8 Texture Slots; Num textures = 2

Variation

For grass quads to repeat the relief, the following parameters are set up:

• Orientation. Grass quads can be oriented both along its own normal vector and along the normal vector of its parent node (usually - a terrain). However, on steep hills the grass oriented along the terrain normal can look unnatural (as the grass growth direction is up), so you will need to choose an appropriate value for the Angle parameter.

  On the right picture below, the Angle parameter is set incorrectly and the grass grows almost perpendicular to the slope:

  Grass Polygons Oriented Along Normal of Grass Object

  Grass Polygons Oriented Along Normal of Terrain

• Angle. This parameter affects to grass distribution that varies depending on the ground slope angle. This parameter simulates the natural tendency of the grass to strike root in more flat places.
• Intersection. By reproducing the hilly landscape, it is essential that the grass follows the relief it grows on. For the grass to be rendered upon the ground, the following is necessary:
  1. Make the grass a child of the terrain or any arbitrary mesh (static or dynamic one). If there are some nodes in the hierarchy between a terrain or mesh node and grass, intersections will still be checked.
  2. Check the Intersection box.

The grass can shoot as a few separate blades, or form a field of green verdure receding into horizon. The size of the field determines the extent of rectangular area, which will be covered with grass.

• Size X specifies the length of the grass area along the X axis.
• Size Y specifies the length of the grass area along the Y axis.

Visually attractive grass, however, may reduce performance if its is rendered as whole. To allow rendering at interactive frame rates, the field is split into cells that form a grid. Each cell is estimated, whether it contributes to the grass field or stays grassless. The grassless cells are disregarded, while for the cells with grass all the required computations are done. Depending on the density, the blades are planted in the cell with random coordinates, imitating the natural vegetation sprouting. The cells are rendered one by one, starting from the camera and further forward.

To get the resulting number of cells, Size of the field (both alongX andY axes) issplit by the Step.

Controlling the step for dividing field into cells allows you to change the field size, while not influencing the set density for planting the grass blades. On the other hand, that is why if we change the step and consequently the number of cells, while not altering the density value, the number of grass quads rendered will be different.

To choose the optimal size of the cell step (especially when a grass object is used as a distant LOD of World Clutter or Mesh Clutter), it is recommended to use one of the following formulas:

If the Step value is too small and the maximum visibility distance is large, cells will be rendered too long. Also the number of DIP calls will increase significantly. On the other hand, if the Step value is too large, there will be performance spikes due to the increased number of objects per cell. So the coefficient in range from 3 to 4 presented in the formula above will help to find the right balance.

Cell Subdivision

Image Mask for the Grass Field

Mesh Mask

Field Mask

To achieve the sufficient richness of a grass field, Density parameter can be tweaked. It determines how many grass quads are to be rendered per square unit.

• The higher the value, the more grass blades quads are presented and the more closely they are positioned. But it should be kept in mind, that the final result also depends on the Step of division and the number of cells it defines.

To randomly position grass blades quads across an area by an image mask, set up the Seed value, which is actually an integer value used to generate pseudo-random successions of numbers (in other words, arandom seed). To get a random seed value, press the Randomize button.

To achieve the predominance of one type of the grass over the other types, you can use the Probability parameter. This parameter sets the grass rendering probability per diffuse texture column: you can specify a probability factor for each column of the diffuse texture separately.

For example, if you set the probability factor to 5 for the 1st column of the diffuse texture, the grass blades quads stored in slots of this column will appear more often.

Probability = 1

Probability for the 1st column = 5

As it was mentioned above, rendering all grass cells at once (and therefore thousands of grass polygons) drastically drops performance. So, to reduce the number of polygons rendered each frame, it is necessary to set the Spawn rate that controls the number of cells rendered per frame. Thehigher the spawn rate, the more likely a performance spike will occur. However, at a low spawn rate and FPS values the grass will be generated right in front of your eyes (creating an effect of grass quads popping) on the approach of the camera to the grass field.

Except reducing the number of cells rendered per frame, the grass field can be thinned out with a distance: you can check the Thinning box in order not to render random grass polygons across the grass Fade distance.

Choosing the optimal Step for cells division is also important when optimizing the grass. Depending on the purpose of the grass usage, you should set different Step values:

• For the normal grass (that is used properly), the optimal step is in range [10..25].
• For the grass that is used as a distant LOD (for a forest, for example), the optimal step should be calculated according to the formula.

Using Grass as Low-Poly LOD