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Rendering Countless Blades of Waving Grass

Jeff Schmidt CS 680. Rendering Countless Blades of Waving Grass. Project Problem. Nature scenes are a prevalent topic in computer graphics (for example, computer games)

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Rendering Countless Blades of Waving Grass

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  1. Jeff Schmidt CS 680 Rendering Countless Blades of Waving Grass

  2. Project Problem Nature scenes are a prevalent topic in computer graphics (for example, computer games) In addition, to realistic trees and flowing water effects, we want to render a high-quality grass effect in real-time that looks realistic from all angles

  3. Project Problem In general, grass needs to cover a vast amount of the scene. This makes modeling each individual blade of grass with polygons unrealistic A simple, flat grass texture will only look realistic from certain angles

  4. Solution We create a “star” organization of quads and cover them with grass textures

  5. Solution The star formation gives us the same effect no matter which side of the grass object we are viewing

  6. Solution Finally, we want to simulate a wind effect to make the grass feel more realistic To simulate the blowing of grass, we shift the upper vertices of our grass object

  7. My implementation I have implemented both a CPU only and a GPU version of the program I use OpenGL/GLUT for rendering I used the GPU to speed up various tasks

  8. My implementation I have been running my project on double/float CPU: Intel(R) Xeon(R) CPU GPU: GeForce GTX 580 Timings were taken using cudaEvent’s No compiler flags

  9. Where I use the GPU – Texture Loading For the grass texture, I use .ppm files for their simple r, g, b, r, g, b,… file format However, ppm files do not support an alpha channel Solution: Pick a color that does not appear in any textures and fill the background with it. Then parse the texture file, and fill in alpha values where you used the “transparent” color

  10. Where I use the GPU – Texture Loading Each (r, g, b) triple is independent, therefore, splitting it amongst threads is simple We only read each r, g, b value once, and we only write each alpha value once Therefore, I used zero-copy host memory to eliminate copying the texture from the CPU to the GPU and back

  11. Where I use the GPU – Texture Loading Experiment ran with varying texture sizes. GPU version has 64 blocks, 64 threads

  12. Where I use the GPU – Texture Loading

  13. Where I use the GPU – Scene Initialization Again, each grass object is independent of one another, therefore, each thread can create it’s own set of grass objects in parallel Each grass object has an initial position that is perturbed by some random value, which gives slightly non-uniform distribution PROBLEM: Creating a random number on the GPU

  14. Where I use the GPU – Scene Initialization To generate random numbers, I create an array on the host filled with random numbers Then I place the array in texture memory Each block/thread indexes into the texture to retrieve the desired random numbers

  15. Where I use the GPU – Scene Initialization Experiment ran with varying numbers of grass objects. GPU version has 64 blocks, 64 threads

  16. Where I use the GPU – Scene Initialization

  17. Where I use the GPU – Wind Simulation Create random wind vectors crossing the viewing area Calculate vector to shift the grass objects by measuring their distance from wind vector After the wind blows, “spring” back towards resting position Include some slight randomness, so that all grass doesn’t move exactly the same speed/direction

  18. Where I use GPU – Wind Simulation Unfortunately, due to time, I was unable to simulate wind Instead, my grass objects wave randomly

  19. Where I use the GPU – Wind Simulation • My “wind” simulation actually runs slower on the GPU version, due to creating the texture of random numbers and copying them to the GPU • For this reason, I did not include any timings • For a more in-depth wind simulation, the GPU version would likely out-perform the CPU

  20. Lessons Learned • Strategies to get random numbers from the GPU: • Store them in a texture generated by the host • Implement a simple psuedo-random number generator that runs on the GPU (example: linear congruential generators)

  21. Lessons Learned • I got to implement some new (for me) CUDA features • Zero-copy host memory – used for updating my textures with transparency • Texture memory – used for storing random numbers and passing them to the GPU • Use __host__ __device__ for functions you want on both the CPU and GPU

  22. Future Improvements • I would have liked to use CUDA’s interoperability features with OpenGL to have the GPU render the scene without going through the CPU • I would have liked to explore CUDA streams for overlapping of GPU calculations and memory copying

  23. Future Improvements • Due to time constraints, I was unable to implement wind simulation. I would have liked to use a real simulation formula and better grass textures to make the scene look more realistic

  24. A screenshot

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