Caustics Generator: Fast Techniques for Photoreal Water & Glass Effects

Procedural Caustics Generator — Settings, Tips, and Workflow ExamplesCaustics — the shimmering patterns of light created when rays refract through or reflect off a curved, transmissive surface — are a hallmark of realism in rendered water, glass, and other translucent materials. A procedural caustics generator produces these patterns algorithmically, enabling flexible, resolution-independent, and often real-time solutions suitable for VFX, games, simulations, and architectural visualization.

This article covers core concepts, practical settings, optimization tips, and workflow examples for integrating a procedural caustics generator into your pipeline. Whether you’re a shader artist, technical artist, or indie developer, you’ll gain insight into making believable caustics without relying on pre-baked textures.

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1. Fundamentals of Caustics

Caustics occur when light is concentrated by refraction (through transparent media like water or glass) or reflection (off shiny surfaces). Two common visual types:

• Transmission caustics: Patterns on surfaces beneath water or behind glass due to refraction and internal focusing.
• Reflection caustics: Bright, often sharper highlights created by specular reflection off curved surfaces.

Key physical factors:

• Light direction and intensity
• Surface curvature and normals
• Index of refraction (IOR)
• Surface roughness and scattering
• Depth and absorption in participating media

A procedural generator simulates or approximates these factors using mathematical models inside shaders or compute routines.

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2. Approaches to Procedural Caustics

• Ray-traced caustics (physically accurate)
  • Pros: Highly accurate, physically plausible results.
  • Cons: Expensive; requires ray tracing or photon mapping.
• Shadow-map / projective methods
  • Pros: Fast and compatible with raster pipelines.
  • Cons: Artifacts where projection doesn’t match geometry or when surfaces move.
• Screen-space caustics
  • Pros: Real-time friendly; uses depth and normal buffers.
  • Cons: Limited to visible pixels; can miss off-screen light paths.
• Texture-based procedural patterns
  • Pros: Simple, artist-controllable, cheap.
  • Cons: Not physically driven; may tile or look repetitive.
• Hybrid systems
  • Combine offline/expensive solution for high accuracy with cheaper approximations at runtime.

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3. Core Settings and Their Effects

Below are the typical parameters exposed in a caustics generator and guidance on how to use them.

• Light Direction (vector)
  • Controls orientation of pattern; low-angle light stretches caustics.
• Light Intensity
  • Scales brightness; use tone-mapped values for HDR workflows.
• Caustic Strength (gain)
  • Multiplies generated pattern; useful to blend into scene lighting.
• Caustic Scale / Frequency
  • Controls size of caustic features. Smaller scale = finer ripples.
• Anisotropy / Stretch
  • Stretches pattern along one axis to simulate elongated specular paths.
• Smoothness / Blur Radius
  • Softens high-frequency detail; simulates scattering and roughness.
• IOR (Index of Refraction)
  • Influences focal behavior for refractive caustics; higher IOR intensifies bending.
• Absorption / Attenuation
  • Darkens caustics with depth; critical for colored water.
• Depth Falloff / Distance Influence
  • Reduces caustic contribution with distance to avoid far-field noise.
• Projection Mode (world, local, screen)
  • Determines how patterns are projected onto surfaces.
• Temporal Jitter / Animation Speed
  • Adds movement; match to water surface animation frequency.
• Sample Count / Quality
  • Controls accuracy in stochastic or ray-based techniques.

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4. Shader Implementation Patterns

1. Compute surface normals and curvature
   • Use analytic normals for parametric meshes or normal maps for complex surfaces.
2. Generate light interaction
   • For refraction, compute refracted ray direction using Snell’s law and trace toward receiving surfaces (approx via screen-space or ray-trace).
3. Accumulate energy
   • Map refracted/reflected rays to a light accumulation buffer (caustic map) using additive blending or splatting.
4. Blur and filter
   • Apply multi-scale blur / bilateral filtering to emulate scattering without losing pattern fidelity.
5. Composite
   • Blend caustic map into scene lighting using multiply/additive modes; modulate by albedo and shadowing.

Example pseudocode snippet (screen-space, conceptual):

// inputs: depthTex, normalTex, lightDir, causticParams vec3 worldPos = reconstructWorldPos(depthTex, uv); vec3 n = sampleNormal(normalTex, uv); vec3 refracted = refract(-lightDir, n, 1.0 / causticParams.ior); vec2 projUV = projectToSurface(refracted, worldPos); float energy = computeIntensity(n, lightDir) * occlusion(worldPos, refracted); causticMap[projUV] += energy; 

(Place actual implementation details according to engine/shader language requirements.)

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5. Performance & Optimization Tips

• Use screen-space or projective methods for most real-time needs; reserve ray tracing for high-end targets.
• Render caustics at a lower resolution and upsample with edge-aware upscaling or bilateral filters.
• Temporal accumulation: accumulate noisy caustics over frames and denoise with spatial-temporal filtering.
• Limit the area affected by caustics (mask by water volume or bounding box).
• Cache static or slow-moving caustics as lightmaps or atlases when possible.
• Use analytic approximations (e.g., normal derivatives or curvature maps) instead of tracing per pixel.
• Reduce sample counts with importance sampling; bias direction toward light lobes.
• Combine multiple LODs: high-detail caustics near camera, low-detail far away.

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6. Artistic Controls and Practical Tips

• Match caustic scale to real-world scale: smaller ripples on a puddle produce smaller caustic spots than ocean waves.
• Use color absorption to convey depth and material (e.g., green/blue for water).
• Blend caustics with diffuse lighting rather than simply adding brightness; multiply by surface albedo to avoid blown-out highlights on dark surfaces.
• For glass objects, increase sharpness and reduce blur; for murky water, soften and tint the pattern.
• Animate parameters subtly: slight phase shift in pattern or light direction mimics sun movement and water flow.
• Balance realism and readability: strong caustics can distract from other scene details — dial strength accordingly.

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7. Workflow Examples

Example A — Real-time Game Engine (Screen-Space Caustics)

1. Render scene depth and normals.
2. From the water surface mesh, compute surface normals (optionally via normal map).
3. For each screen pixel under water, compute refracted ray direction using Snell’s law in view space.
4. Project refracted ray to the receiving plane/depth buffer and splat energy into a low-res caustic render target.
5. Temporal accumulate and bilateral blur to denoise.
6. Composite into lighting pass using multiply with surface albedo and add specular contribution.

Practical tips:

• Use a ⁄₂ or ⁄₄ resolution buffer.
• Use a simple binary mask of water surface to skip non-water pixels.
• Use temporal reprojection to reduce noise.

Example B — Offline Render / VFX (High-Quality)

1. Use photon mapping or path tracing with dedicated caustic photons.
2. Emit photons from light sources; store hits after refracting through glass/water.
3. Build a photon map or use density estimation to render caustic irradiance.
4. Filter photons with kernel density estimation; composite into final render with physically-based shading.

Practical tips:

• Increase photon count for small-scale, high-frequency caustics.
• Use importance sampling near sharp curvature or high IOR materials.

Example C — Hybrid (Precomputed + Dynamic)

1. Precompute high-frequency caustic detail for static parts (like a pool floor) into an atlas.
2. For dynamic elements (moving water, glass), use a screen-space overlay blended with the atlas.
3. Switch between precomputed and dynamic based on camera proximity.

Practical tips:

• Bake multiple tiles for different lighting angles or use spherical harmonics to approximate directional dependence.
• Use blending weights to smoothly transition between precomputed and dynamic maps.

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8. Troubleshooting Common Issues

• Tiling/repetition: increase noise or use multiple layered procedural frequencies.
• Flat, unrealistic caustics: check normals and curvature; low normal variance produces weak patterns.
• Flicker when using temporal accumulation: ensure correct motion vectors and temporal reprojection.
• Caustics too bright: reduce gain or use HDR tone mapping; modulate by surface albedo.
• Missing caustics on thin geometry: project onto nearest large receiving surface or render a thin proxy plane for accumulation.

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9. Example Parameter Presets (Starting Points)

• Calm pond: scale = 0.5, strength = 0.6, blur = 0.8, IOR = 1.33, absorption = low
• Choppy sea: scale = 2.5, strength = 0.9, blur = 0.3, IOR = 1.33, temporal jitter = medium
• Thick glass: scale = 0.8, strength = 1.2, blur = 0.1, IOR = 1.5, absorption = very low
• Murky pool: scale = 1.0, strength = 0.5, blur = 1.5, IOR = 1.33, absorption = high

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10. Further Reading & Tools (Topics to Explore)

• Photon mapping and bidirectional path tracing (for offline accuracy)
• Screen-space algorithms and depth reprojection
• Wave and surface synthesis (Gerstner waves, FFT)
• Denoising (spatial-temporal bilateral filters, wavelet denoising)
• Engine-specific implementations: Unity shader graphs/HDRP caustics, Unreal Engine water/real-time ray tracing caustics, custom Vulkan/DirectX compute pipelines

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Practical procedural caustics are a balance between physical principles and artistic control. Start with cheap, screen-space approaches for interactivity, then layer quality where needed with hybrid or ray-traced solutions. With careful masking, temporal filtering, and artist-friendly parameters, you can achieve convincing, performant caustics for a wide range of projects.