ppt.md

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Real-Time Global Illumination

(using Voxel Cone Tracing)

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Introduction

• Raytracing is slow
• Optimizations:
  • Brute Force: more computing power!
  • Approximation: cone tracing

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VXGI Pipeline

• Voxelization using the GPU Hardware Rasterizer
• Hybrid Rendering Pipeline
  • Rasterized primary rays for direct lighting
  • Cone Traced secondary rays for indirect GI

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THE VXGI ENGINE

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VOXELIZATION

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One Pass Voxelization Pipeline

1. Rasterization
2. Dominant Axis Projection
3. Direct Light Injection and Mipmapping

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Rasterization

• Use GPU to generate voxels
• Thin surface voxelization of triangle B with voxel V:
  • B’s plane intersects V
  • 2D projection of B along it's dominant axis intersects the 2D projection of V.
• Disable depth testing (z-buffer)

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Hardware Conservative Rasterization

• Generate fragments for every pixel touched by a primitive
• GL_NV_conservative_raster Extension
• NVIDIA’s new Maxwell architecture (2014 onwards)

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Dominant Axis Selection

$$l_{{x,y,z}} = |n · v_{{x,y,z}} |$$

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Inside the Geometry Shader

...

// select the dominant axis
float axis = max(geometryNormal.x, max(geometryNormal.y, geometryNormal.z));

if (axis == geometryNormal.x) {
    gl_Position = vec4(outputPosition.zy, 1.0, 1.0);
} else if (axis == geometryNormal.y) {
    gl_Position = vec4(outputPosition.xz, 1.0, 1.0);
} else if (axis == geometryNormal.z) {
    gl_Position = vec4(outputPosition.xy, 1.0, 1.0);
}
EmitVertex();

...

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Direct Lighting and 3D Mipmapping

• Each projected triangle passed through the standard rasterization pipeline to perform 2D scan conversion
• Compute 3D integer coordinate within the destination voxel image
• imageStore() (OpenGL 4.2+) to write direct lighting to 3D image
• glGenerateMipmap() for automatic mipmap generation

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CONE TRACING

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The Rendering Equation

$$ L_o(\mathbf{p}, \omega_o) = L_e(\mathbf{p}, \omega_o) + \int_{\Omega} f(\mathbf{p}, \omega_i, \omega_o) \cdot L_i(\mathbf{p}, \omega_i) \cdot (\mathbf{n} \cdot \omega_i) , d\omega_i $$

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The hemispherical integration can be represented as a summation of 𝑁 cones

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$$ L_o(\mathbf{x}, \omega) \approx L_e(\mathbf{x}, \omega) + \frac{1}{N} \sum_{i=1}^{N} V_c(\mathbf{x}, \omega_i) $$

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Raymarching the 3D Mipmap

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We define the diameter d as:

$$d = 2t \times \tan(\frac{\theta}{2})$$

Mipmap LOD for sampling: $$V_{\text{level}} = \log_2(\frac{d}{V_{\text{size}}})$$

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Volumetric front-to-back accumulation:

$$C = \alpha C + (1 - \alpha)\alpha_2C_2$$ $$\alpha = \alpha + (1 - \alpha)\alpha_2$$

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Indirect Diffuse Lighting

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Indirect Specular Lighting

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Complete Global Illumination

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RESULTS

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