Instructions - Vulkan Grass Rendering

This is due Sunday 11/5, evening at midnight.

Summary: In this project, you will use Vulkan to implement a grass simulator and renderer. You will use compute shaders to perform physics calculations on Bezier curves that represent individual grass blades in your application. Since rendering every grass blade on every frame will is fairly inefficient, you will also use compute shaders to cull grass blades that don't contribute to a given frame. The remaining blades will be passed to a graphics pipeline, in which you will write several shaders. You will write a vertex shader to transform Bezier control points, tessellation shaders to dynamically create the grass geometry from the Bezier curves, and a fragment shader to shade the grass blades.

The base code provided includes all of the basic Vulkan setup, including a compute pipeline that will run your compute shaders and two graphics pipelines, one for rendering the geometry that grass will be placed on and the other for rendering the grass itself. Your job will be to write the shaders for the grass graphics pipeline and the compute pipeline, as well as binding any resources (descriptors) you may need to accomplish the tasks described in this assignment.

You are not required to use this base code if you don't want to. You may also change any part of the base code as you please. This is YOUR project. The above .gif is just a simple example that you can use as a reference to compare to.

Important:

• If you are not in CGGT/DMD, you may replace this project with a GPU compute project. You MUST get this pre-approved by Austin Eng before continuing!

Contents

• src/ C++/Vulkan source files.
  • shaders/ glsl shader source files
  • images/ images used as textures within graphics pipelines
• external/ Includes and static libraries for 3rd party libraries.
• img/ Screenshots and images to use in your READMEs

Installing Vulkan

In order to run a Vulkan project, you first need to download and install the Vulkan SDK. Make sure to run the downloaded installed as administrator so that the installer can set the appropriate environment variables for you.

Once you have done this, you need to make sure your GPU driver supports Vulkan. Download and install a Vulkan driver from NVIDIA's website.

Finally, to check that Vulkan is ready for use, go to your Vulkan SDK directory (C:/VulkanSDK/ unless otherwise specified) and run the cube.exe example within the Bin directory. IF you see a rotating gray cube with the LunarG logo, then you are all set!

Running the code

While developing your grass renderer, you will want to keep validation layers enabled so that error checking is turned on. The project is set up such that when you are in debug mode, validation layers are enabled, and when you are in release mode, validation layers are disabled. After building the code, you should be able to run the project without any errors. You will see a plane with a grass texture on it to begin with.

Requirements

Ask on the mailing list for any clarifications.

In this project, you are given the following code:

• The basic setup for a Vulkan project, including the swapchain, physical device, logical device, and the pipelines described above.
• Structs for some of the uniform buffers you will be using.
• Some buffer creation utility functions.
• A simple interactive camera using the mouse.

You need to implement the following features/pipeline stages:

• Compute shader (shaders/compute.comp)
• Grass pipeline stages
  • Vertex shader (`shaders/grass.vert')
  • Tessellation control shader (shaders/grass.tesc)
  • Tessellation evaluation shader (shaders/grass.tese)
  • Fragment shader (shaders/grass.frag)
• Binding of any extra descriptors you may need

See below for more guidance.

Base Code Tour

Areas that you need to complete are marked with a TODO comment. Functions that are useful for reference are marked with the comment CHECKITOUT.

• src/main.cpp is the entry point of our application.
• src/Instance.cpp sets up the application state, initializes the Vulkan library, and contains functions that will create our physical and logical device handles.
• src/Device.cpp manages the logical device and sets up the queues that our command buffers will be submitted to.
• src/Renderer.cpp contains most of the rendering implementation, including Vulkan setup and resource creation. You will likely have to make changes to this file in order to support changes to your pipelines.
• src/Camera.cpp manages the camera state.
• src/Model.cpp manages the state of the model that grass will be created on. Currently a plane is hardcoded, but feel free to update this with arbitrary model loading!
• src/Blades.cpp creates the control points corresponding to the grass blades. There are many parameters that you can play with here that will change the behavior of your rendered grass blades.
• src/Scene.cpp manages the scene state, including the model, blades, and simualtion time.
• src/BufferUtils.cpp provides helper functions for creating buffers to be used as descriptors.

We left out descriptions for a couple files that you likely won't have to modify. Feel free to investigate them to understand their importance within the scope of the project.

Grass Rendering

This project is an implementation of the paper, Responsive Real-Time Grass Rendering for General 3D Scenes. Please make sure to use this paper as a primary resource while implementing your grass renderers. It does a great job of explaining the key algorithms and math you will be using. Below is a brief description of the different components in chronological order of how your renderer will execute, but feel free to develop the components in whatever order you prefer.

Representing Grass as Bezier Curves

In this project, grass blades will be represented as Bezier curves while performing physics calculations and culling operations. Each Bezier curve has three control points.

• v0: the position of the grass blade on the geomtry
• v1: a Bezier curve guide that is always "above" v0 with respect to the grass blade's up vector (explained soon)
• v2: a physical guide for which we simulate forces on

We also need to store per-blade characteristics that will help us simulate and tessellate our grass blades correctly.

• up: the blade's up vector, which corresponds to the normal of the geometry that the grass blade resides on at v0
• Orientation: the orientation of the grass blade's face
• Height: the height of the grass blade
• Width: the width of the grass blade's face
• Stiffness coefficient: the stiffness of our grass blade, which will affect the force computations on our blade

We can pack all this data into four vec4s, such that v0.w holds orientation, v1.w holds height, v2.w holds width, and up.w holds the stiffness coefficient.

Simulating Forces

In this project, you will be simulating forces on grass blades while they are still Bezier curves. This will be done in a compute shader using the compute pipeline that has been created for you. Remember that v2 is our physical guide, so we will be applying transformations to v2 initially, then correcting for potential errors. We will finally update v1 to maintain the appropriate length of our grass blade.

Binding Resources

In order to update the state of your grass blades on every frame, you will need to create a storage buffer to maintain the grass data. You will also need to pass information about how much time has passed in the simulation and the time since the last frame. To do this, you can extend or create descriptor sets that will be bound to the compute pipeline.

Gravity

Given a gravity direction, D.xyz, and the magnitude of acceleration, D.w, we can compute the environmental gravity in our scene as gE = normalize(D.xyz) * D.w.

We then determine the contribution of the gravity with respect to the front facing direction of the blade, f, as a term called the "front gravity". Front gravity is computed as gF = (1/4) * ||gE|| * f.

We can then determine the total gravity on the grass blade as g = gE + gF.

Recovery

Recovery corresponds to the counter-force that brings our grass blade back into equilibrium. This is derived in the paper using Hooke's law. In order to determine the recovery force, we need to compare the current position of v2 to its original position before simulation started, iv2. At the beginning of our simulation, v1 and v2 are initialized to be a distance of the blade height along the up vector.

Once we have iv2, we can compute the recovery forces as r = (iv2 - v2) * stiffness.

Wind

In order to simulate wind, you are at liberty to create any wind function you want! In order to have something interesting, you can make the function depend on the position of v0 and a function that changes with time. Consider using some combination of sine or cosine functions.

Your wind function will determine a wind direction that is affecting the blade, but it is also worth noting that wind has a larger impact on grass blades whose forward directions are parallel to the wind direction. The paper describes this as a "wind alignment" term. We won't go over the exact math here, but use the paper as a reference when implementing this. It does a great job of explaining this!

Once you have a wind direction and a wind alignment term, your total wind force (w) will be windDirection * windAlignment.

Total force

We can then determine a translation for v2 based on the forces as tv2 = (gravity + recovery + wind) * deltaTime. However, we can't simply apply this translation and expect the simulation to be robust. Our forces might push v2 under the ground! Similarly, moving v2 but leaving v1 in the same position will cause our grass blade to change length, which doesn't make sense.

Read section 5.2 of the paper in order to learn how to determine the corrected final positions for v1 and v2.

Culling tests

Although we need to simulate forces on every grass blade at every frame, there are many blades that we won't need to render due to a variety of reasons. Here are some heuristics we can use to cull blades that won't contribute positively to a given frame.

Orientation culling

Consider the scenario in which the front face direction of the grass blade is perpendicular to the view vector. Since our grass blades won't have width, we will end up trying to render parts of the grass that are actually smaller than the size of a pixel. This could lead to aliasing artifacts.

In order to remedy this, we can cull these blades! Simply do a dot product test to see if the view vector and front face direction of the blade are perpendicular. The paper uses a threshold value of 0.9 to cull, but feel free to use what you think looks best.

View-frustum culling

We also want to cull blades that are outside of the view-frustum, considering they won't show up in the frame anyway. To determine if a grass blade is in the view-frustum, we want to compare the visibility of three points: v0, v2, and m, where m = (1/4)v0 * (1/2)v1 * (1/4)v2. Notice that we aren't using v1 for the visibility test. This is because the v1 is a Bezier guide that doesn't represent a position on the grass blade. We instead use m to approximate the midpoint of our Bezier curve.

If all three points are outside of the view-frustum, we will cull the grass blade. The paper uses a tolerance value for this test so that we are culling blades a little more conservatively. This can help with cases in which the Bezier curve is technically not visible, but we might be able to see the blade if we consider its width.

Distance culling

Similarly to orientation culling, we can end up with grass blades that at large distances are smaller than the size of a pixel. This could lead to additional artifacts in our renders. In this case, we can cull grass blades as a function of their distance from the camera.

You are free to define two parameters here.

• A max distance afterwhich all grass blades will be culled.
• A number of buckets to place grass blades between the camera and max distance into.

Define a function such that the grass blades in the bucket closest to the camera are kept while an increasing number of grass blades are culled with each farther bucket.

Occlusion culling (extra credit)

This type of culling only makes sense if our scene has additional objects aside from the plane and the grass blades. We want to cull grass blades that are occluded by other geometry. Think about how you can use a depth map to accomplish this!

Tessellating Bezier curves into grass blades

In this project, you should pass in each Bezier curve as a single patch to be processed by your grass graphics pipeline. You will tessellate this patch into a quad with a shape of your choosing (as long as it looks sufficiently like grass of course). The paper has some examples of grass shapes you can use as inspiration.

In the tessellation control shader, specify the amount of tessellation you want to occur. Remember that you need to provide enough detail to create the curvature of a grass blade.

The generated vertices will be passed to the tessellation evaluation shader, where you will place the vertices in world space, respecting the width, height, and orientation information of each blade. Once you have determined the world space position of each vector, make sure to set the output gl_Position in clip space!

** Extra Credit**: Tessellate to varying levels of detail as a function of how far the grass blade is from the camera. For example, if the blade is very far, only generate four vertices in the tessellation control shader.

To build more intuition on how tessellation works, I highly recommend playing with the helloTessellation sample and reading this tutorial on tessellation.

Resources

Links

The following resources may be useful for this project.

• Responsive Real-Time Grass Grass Rendering for General 3D Scenes
• CIS565 Vulkan samples
• Official Vulkan documentation
• Vulkan tutorial
• RenderDoc blog on Vulkan
• Tessellation tutorial

Third-Party Code Policy

• Use of any third-party code must be approved by asking on our Google Group.
• If it is approved, all students are welcome to use it. Generally, we approve use of third-party code that is not a core part of the project. For example, for the path tracer, we would approve using a third-party library for loading models, but would not approve copying and pasting a CUDA function for doing refraction.
• Third-party code MUST be credited in README.md.
• Using third-party code without its approval, including using another student's code, is an academic integrity violation, and will, at minimum, result in you receiving an F for the semester.

README

• A brief description of the project and the specific features you implemented.
• At least one screenshot of your project running.
• A performance analysis (described below).

Performance Analysis

The performance analysis is where you will investigate how...

• Your renderer handles varying numbers of grass blades
• The improvement you get by culling using each of the three culling tests

Submit

If you have modified any of the CMakeLists.txt files at all (aside from the list of SOURCE_FILES), mentions it explicity. Beware of any build issues discussed on the Google Group.

Open a GitHub pull request so that we can see that you have finished. The title should be "Project 6: YOUR NAME". The template of the comment section of your pull request is attached below, you can do some copy and paste:

• Repo Link
• (Briefly) Mentions features that you've completed. Especially those bells and whistles you want to highlight
  • Feature 0
  • Feature 1
  • ...
• Feedback on the project itself, if any.