MFS Engine - Ray Tracing Documentation

Overview

The MFS Engine now includes comprehensive ray tracing support across multiple graphics backends, with particular emphasis on Vulkan 1.3 KHR ray tracing and DirectX Ray Tracing (DXR). This implementation follows modern ray tracing best practices and supports hardware-accelerated ray tracing on compatible GPUs.

Supported Backends

Vulkan 1.3 Ray Tracing (VK_KHR_ray_tracing)

Primary backend for cross-platform ray tracing
Supports the latest Vulkan 1.3 ray tracing extensions
Hardware-accelerated on RTX, RDNA2+, and Intel Arc GPUs
HLSL to SPIR-V shader compilation via DXC

DirectX Ray Tracing (DXR)

Windows-native ray tracing implementation
Supports DXR 1.0 and DXR 1.1 features
Direct HLSL shader support
Optimized for Windows gaming and enterprise applications

Metal Ray Tracing

macOS-native ray tracing implementation
Supports Metal Performance Shaders ray tracing
Hardware-accelerated on Apple Silicon and modern AMD GPUs

Software Fallback

CPU-based ray tracing for compatibility
Embree-based high-performance software ray tracing
Automatic fallback when hardware acceleration is unavailable

Architecture

Core Components

// Main ray tracing context
const RayTracingContext = struct {
    allocator: std.mem.Allocator,
    backend_type: BackendType,
    capabilities: RayTracingCapabilities,
    device_handle: *anyopaque,
};

// Acceleration structures
const AccelerationStructure = struct {
    handle: *anyopaque,
    backend_type: BackendType,
    as_type: AccelerationStructureType, // BLAS or TLAS
    size: u64,
    device_address: u64,
};

// Ray tracing pipeline
const RayTracingPipelineState = struct {
    raygen_shader: *anyopaque,
    miss_shaders: []*anyopaque,
    hit_groups: []HitGroup,
    callable_shaders: []*anyopaque,
    max_recursion_depth: u32,
};

Shader Compilation

The engine supports HLSL ray tracing shaders compiled to SPIR-V for Vulkan using Microsoft's DXC compiler:

# Compile HLSL ray tracing shader to SPIR-V
dxc.exe -T lib_6_4 raytrace.rchit.hlsl -spirv -Fo raytrace.rchit.spv -fvk-use-scalar-layout -fspv-extension="SPV_KHR_ray_tracing"

Usage Examples

Basic Ray Tracing Setup

const std = @import("std");
const mfs = @import("mfs");

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();
    
    // Initialize engine with ray tracing
    const engine_config = mfs.EngineConfig{
        .graphics = .{
            .preferred_backend = .vulkan,
            .enable_ray_tracing = true,
        },
    };
    
    const engine = try mfs.init(allocator, engine_config);
    defer mfs.deinit(engine);
    
    // Initialize ray tracing
    const rt_config = mfs.graphics.RayTracingConfig{
        .enable_hardware_acceleration = true,
        .max_recursion_depth = 8,
    };
    
    const rt_context = try mfs.graphics.ray_tracing.init(
        allocator,
        rt_config,
        engine.graphics.getDeviceHandle(),
    );
    defer mfs.graphics.ray_tracing.deinit(rt_context);
    
    // Create geometry
    const vertices = [_]Vertex{
        .{ .position = .{ -1, -1, 0 }, .normal = .{ 0, 0, 1 } },
        .{ .position = .{  1, -1, 0 }, .normal = .{ 0, 0, 1 } },
        .{ .position = .{  0,  1, 0 }, .normal = .{ 0, 0, 1 } },
    };
    
    const indices = [_]u32{ 0, 1, 2 };
    
    // Create geometry description
    const geometry_desc = mfs.graphics.GeometryDesc{
        .geometry_type = .triangles,
        .flags = .{ .opaque = true },
        .vertex_buffer = &vertices,
        .vertex_stride = @sizeOf(Vertex),
        .vertex_format = .float3,
        .vertex_count = vertices.len,
        .index_buffer = &indices,
        .index_format = .uint32,
        .index_count = indices.len,
    };
    
    // Build bottom-level acceleration structure
    const build_flags = mfs.graphics.BuildFlags{
        .prefer_fast_trace = true,
    };
    
    const blas = try rt_context.createBottomLevelAS(
        &[_]mfs.graphics.GeometryDesc{geometry_desc},
        build_flags,
    );
    defer blas.deinit(allocator);
    
    // Create instance for top-level acceleration structure
    const instance_desc = mfs.graphics.InstanceDesc{
        .transform = mfs.math.Mat4.identity(),
        .instance_id = 0,
        .acceleration_structure_reference = blas.device_address,
    };
    
    // Build top-level acceleration structure
    const tlas = try rt_context.createTopLevelAS(
        &[_]mfs.graphics.InstanceDesc{instance_desc},
        build_flags,
    );
    defer tlas.deinit(allocator);
    
    std.log.info("Ray tracing setup complete!", .{});
}

Ray Tracing Shaders

#### Ray Generation Shader (HLSL)

#include "raycommon.hlsl"

[[vk::binding(0, 0)]] RaytracingAccelerationStructure topLevelAS;
[[vk::binding(1, 0)]] RWTexture2D image;

struct RayPayload {
    float3 color;
    int depth;
};

[shader("raygeneration")]
void RayGenMain() {
    const float2 pixelCenter = float2(DispatchRaysIndex().xy) + float2(0.5, 0.5);
    const float2 inUV = pixelCenter / float2(DispatchRaysDimensions().xy);
    float2 d = inUV * 2.0 - 1.0;
    
    float4 origin = float4(0, 0, -2, 1);
    float4 target = float4(d.x, d.y, 0, 1);
    float4 direction = normalize(target - origin);
    
    RayDesc ray;
    ray.Origin = origin.xyz;
    ray.Direction = direction.xyz;
    ray.TMin = 0.001;
    ray.TMax = 10000.0;
    
    RayPayload payload;
    payload.color = float3(0, 0, 0);
    payload.depth = 0;
    
    TraceRay(topLevelAS, RAY_FLAG_CULL_BACK_FACING_TRIANGLES, 0xFF, 0, 0, 0, ray, payload);
    
    image[DispatchRaysIndex().xy] = float4(payload.color, 1.0);
}

#### Miss Shader (HLSL)

#include "raycommon.hlsl"

struct RayPayload {
    float3 color;
    int depth;
};

[shader("miss")]
void MissMain(inout RayPayload payload) {
    // Sky color gradient
    float3 unitDirection = normalize(WorldRayDirection());
    float t = 0.5 * (unitDirection.y + 1.0);
    payload.color = (1.0 - t) * float3(1.0, 1.0, 1.0) + t * float3(0.5, 0.7, 1.0);
}

#### Closest Hit Shader (HLSL)

#include "raycommon.hlsl"

struct RayPayload {
    float3 color;
    int depth;
};

struct Attributes {
    float2 bary;
};

[shader("closesthit")]
void ClosestHitMain(inout RayPayload payload, in Attributes attr) {
    const float3 barycentrics = float3(1.0 - attr.bary.x - attr.bary.y, attr.bary.x, attr.bary.y);
    
    // Simple diffuse material
    payload.color = float3(0.8, 0.8, 0.8) * barycentrics.x;
}

Performance Considerations

Hardware Requirements

Minimum Requirements:

NVIDIA RTX 20-series or newer
AMD RDNA2 (RX 6000 series) or newer
Intel Arc A-series
Apple Silicon M1/M2 (for Metal backend)

Recommended:

NVIDIA RTX 30-series or newer
AMD RDNA3 (RX 7000 series) or newer
8GB+ VRAM for complex scenes

Optimization Guidelines

1. Acceleration Structure Optimization

Use prefer_fast_trace for static geometry
Use prefer_fast_build for dynamic geometry
Enable compaction for memory-constrained scenarios

2. Shader Optimization

Minimize ray recursion depth (≤8 levels)
Use early ray termination when possible
Optimize shader binding table layout

3. Memory Management

Batch acceleration structure builds
Reuse scratch buffers when possible
Monitor VRAM usage for large scenes

Cross-Platform Compatibility

Vulkan 1.3 Extensions Required

VK_KHR_acceleration_structure
VK_KHR_ray_tracing_pipeline
VK_KHR_ray_query
VK_KHR_deferred_host_operations
VK_KHR_buffer_device_address
VK_KHR_spirv_1_4
VK_KHR_shader_float_controls

DirectX Requirements

Windows 10 version 1903 or later
DirectX 12 Ultimate compatible driver
DXR 1.1 support recommended

Metal Requirements

macOS 12.0 or later
Metal 3.0 compatible hardware
Apple Silicon or AMD RDNA2+ GPU

Future Enhancements

Planned Features

1. Ray Query Support

Inline ray tracing in compute shaders
Hybrid rasterization + ray tracing techniques

2. Mesh Shaders Integration

Modern geometry pipeline support
Optimized culling and LOD

3. Variable Rate Shading

Adaptive quality for VR/AR applications
Performance scaling for mobile platforms

4. Real-Time Global Illumination

Multi-bounce lighting
Temporal accumulation
Denoising integration

Advanced Techniques

ReSTIR (Reservoir-based Spatiotemporal Importance Resampling)
RTXGI integration for dynamic global illumination
Hardware-accelerated denoising (DLSS, FSR)
Mesh shaders for modern geometry processing

Troubleshooting

Common Issues

1. Driver Compatibility

Ensure latest GPU drivers are installed
Verify ray tracing support in device capabilities

2. Memory Issues

Monitor acceleration structure sizes
Use memory debugging tools for leaks

3. Performance Problems

Profile shader execution times
Optimize acceleration structure builds
Reduce ray recursion depth

Debug Tools

NVIDIA Nsight Graphics for RTX debugging
AMD Radeon GPU Profiler for RDNA debugging
Vulkan Validation Layers for API validation
PIX for DirectX ray tracing debugging

References

Examples

The examples/ray_tracing_demo/ directory contains a comprehensive ray tracing demonstration that showcases:

Multi-backend ray tracing support
Cornell box scene rendering
HLSL shader compilation
Real-time ray tracing pipeline
Performance monitoring and debugging

Run the demo with:

zig build run-ray-tracing-demo

This implementation represents a state-of-the-art ray tracing system that leverages the latest GPU hardware capabilities while maintaining broad compatibility across different platforms and graphics APIs.