Create a grassy surface by using XGen Spline Primitives with V-Ray

In the realm of 3D computer graphics, achieving realism in environmental elements such as grass can be a challenging yet rewarding task. By utilizing tools like XGen Spline Primitives in conjunction with the powerful rendering capabilities of V-Ray, artists can breathe life into their scenes with lush, natural-looking grassy surfaces. 

Today, in this article, we will explore the step-by-step process of creating a realistic grassy surface using XGen Spline Primitives and V-Ray, delving into techniques for optimization and achieving photorealism. Let’s get started!

Xgen is a digital beauty tool developed by Foundry, a leading software company, that can be integrated into Autodesk Maya to create realistic hair and fur simulations. Xgen enables artists to design complicated hair and fur behaviors like hairdos, wind, and cloth interactions. It has several capabilities, including node-based reconstruction, Alembic support, and Arnold/Houdini compatibility. By integrating Xgen and Maya, visual effects artists may build breathtaking characters and creatures for film, television, and video game production. The connection allows for smooth collaboration between the technologies, expanding artistic possibilities in the digital field.

Create a polygon plane with 20 subdivisions in height and width. This will allow us to shape it into a plane, creating a varied base for our grass. Scale the plane to about 10 in X and Z. You create a plane first and then use XGen to assign a description with splines distributed randomly around the surface. Choose Splines (for long hair, vines, etc.). Leave the other default options alone and click Create.

Note: Groomable splines are fine, but we’ll use normal splines to show how to change their characteristics procedurally with expressions/maps.

Lowering the Percentage setting in the Preview/Output tab is a good idea before continuing. If the percentage is too high, your computer may stall because of a huge amount of primitives generated.

Increase the Density to approximately 5000 to cover the plane with grass. Reduce the Width to approximately 0.05 to make the grass appear thinner. Increase the spline length to 2. Adjust the Taper and Taper Start to 0.5 to make the splines like grass blades.

Assign a VRayMtl material to the description by selecting it from the Outliner, right-clicking in the viewport, and selecting Assign New Material. Change the Diffuse Color to Green.

Make a V-Ray Dome Light in the scene and render. Your rendering should look similar to the one below.

Splines can be colorized in a unique way using expressions and texture maps, as well as custom shader parameters.

Each strand’s color is controlled by two parameters: root_color and tip_color. If different colors are applied to each, the color of each hair strand will be interpolated from the root to the tip. If only one color is assigned to both, the strand will be uniformly colored with that color.

Here are the steps for colorizing splines:

Assign a V-Ray Hair Sampler to the diffuse color of the material.

Go to the Preview/Output tab of the XGen window and scroll down to the Custom Shader Parameter space. Add root_color as the type color, as shown.

Your parameter should then appear above the textbox, and an expression can now be applied to it. Then you start render the scene.

We can also use both the tip_color and the root_color.

A texture map could alternatively be utilized.

If you don’t want to be limited to just two colors along the strand, try a ramp texture. Instead of directly linking the V-Ray Hair Sampler, assign a ramp texture to the material’s diffuse color. Connect the distanceAlongStrand of the V-Ray Hair Sampler to the vCoordinate of the ramp texture. Your shading network should appear like this:

XGen’s spline primitives offer great versatility for creating a variety of effects, but they do come with some limitations and potential problems. Large numbers of XGen spline primitives can impact performance, especially in complex scenes or when rendering with many instances of the primitives. Using a large number of XGen spline primitives can consume significant amounts of memory, potentially leading to performance issues.

While these limitations and problems exist, it’s worth noting that XGen’s spline primitives still offer powerful tools for creating a wide range of effects, and many artists have successfully used them in production environments. As with any tool, it’s important to be aware of their constraints and to work within their capabilities to achieve the best results.

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Reference Sources: Chaos and Autodesk Documentation