University of Pennsylvania, CIS 565: GPU Programming and Architecture, Project 4
- Xiaoyue Ma
- Tested on: Windows 10, i7-12700H @ 2.30 GHz 16GB, GTX3060 8GB
In this project, I developed a grass simulator and renderer using the Vulkan API, drawing inspiration from the paper Responsive Real-Time Grass Rendering for General 3D Scenes. The simulation represents each grass blade with three control points: v0, v1, and v2, facilitating efficient tessellation. The core components encompass a vertex shader for Bezier control point transformations, tessellation shaders for grass geometry generation, and a fragment shader for grass blade shading.
In our project, grass blades are represented using Bezier curves with three control points: v0 indicating the blade's position on the geometry, v1 serving as an upward vector guide above v0, and v2 acting as a directional guide. We simulate forces on these blades while they remain as Bezier curves, primarily applying transformations to v2 and subsequently adjusting v1 to ensure the blade's consistent length.
Using a specified gravity direction, D.xyz, and acceleration magnitude, D.w, the environmental gravity gE, is calculated as gE = normalize(D.xyz) * D.w. The front-facing gravity influence on the blade, termed "front gravity", is gF = (1/4) * ||gE|| * f. The overall gravity acting on the grass blade is then g = gE + gF.
The recovery force, bringing the grass blade back to equilibrium, is based on Hooke's law. It compares the current position of v2 with its initial position, iv2, set at the simulation's start. Initially, v1 and v2 are aligned at the blade height along the up vector. The recovery forces are then calculated as r = (iv2 - v2) * stiffness.
The wind's direction in the simulation is dynamic, defined by windFunc (vec3(2.0, 4.0, 2.0) * sin(totalTime * 0.1)). Grass blades aligned with the wind direction experience a greater impact, represented by the windAlignment term. The cumulative wind force, w, is given by windDirection * windAlignment.
When the grass blade's front face direction is perpendicular to the view vector, it can cause aliasing artifacts due to the blades having no width and appearing smaller than a pixel. To address this, blades are culled using a dot product test checking for perpendicularity between the view vector and the blade's direction. The threshold for this culling is set at 0.9.
Blades outside the view-frustum are culled since they won't be visible in the frame. To ascertain a grass blade's position in relation to the view-frustum, three points: v0, v2, and m = (1/4)v0 * (1/2)v1 * (1/4)v2) are assessed. If all these points lie outside the view-frustum, the blade is culled.
For grass blades at significant distances that appear smaller than a pixel, rendering can introduce artifacts. To counter this, blades are culled based on their distance from the camera, similar to orientation culling.
To optimize performance, the tessellation level for a grass blade can be adjusted based on its distance from the camera. Blades farther away require less detail, reducing the need for high tessellation levels.
#if DYNAMIC_LOD
if (dist > 20.0)
LOD = 4;
else if (dist > 15.0)
LOD = 8;
else if (dist > 10.0)
LOD = 12;
else if (dist > 5.0)
LOD = 16;
#endif
View-frustum culling offers a minor performance enhancement, as it primarily culls blades at the image's periphery. Distance culling and Orientation culling is more effective based on the camera position. Dynamic LOD significantly improves performance by reducing polygons for distant grass blades. Most rear blades are obscured, and reduced tessellation often goes unnoticed, yielding a substantial speed increase with minimal visual impact.
Utilizing all culling methods amplifies the frame rate to almost 10 times the original. These methods substantially enhance performance while minimally affecting visual quality.
