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@@ -1,7 +1,80 @@ --- -pagetitle: yummers +title: yummers lang: en --- +# fft water: the plan {data-date="7 Aug 2026"} + +About a year ago I took a stab at reimplementing Tessendorf's ocean +water[^tessendorf]. I got some decent results, but the implementation was +sloppy and not particularly organized. I also never implemented Bruneton's +"geometry to BRDF" method [^bruneton], partially because the implementation was +sloppy. + +I've gotten the itch to take another stab at implementing this water +system. This time, I will try harder to proceed along principled, logical +steps, and check my work more thoroughly along the way. + +I've also decided to document this process. I expect it to take a few months to +complete this project. Hopefully in the future, these notes might help someone +trying to implement some nice deep-ocean water in their engine/game/whatever. + +I'll implement this renderer as follows (this list is likely to change): + +* Derive a high-performance GPU-based FFT implementation on CPU. + * Check against a simple reference implementation of Cooley-Tukey. +* Measure the impact of radix on the numerical precision of the FFT. + * Ideally, generate some graphs. + * Also look at the impact of float precision - 8-bit, 16-bit, etc. +* Implement this FFT algorithm in slang + webGPU. Render unlit. Measure + performance. + * Implement image export from webGPU harness. Measure error - verify that it + matches expectations. +* Generate a wave energy spectrum using Horvath's viscous shallow water wave + dispersion relation. +* Generate one frame of wave displacement using slang + webGPU. + * Validate feature scale, energy, etc. You probably want some histograms. +* Generate one frame of analytic normals using slang + webGPU. + * Validate using finite differences of the heightmap as an approximation of + ground truth. The two images should match within some small epsilon. +* Generate chop and chop normals. + * Validate feature size and normals using finite differences (again). +* Implement stdev (per geometry-to-brdf paper). + * (Note to self: this is a static image based on the energy spectrum. We + calculate the ddx/ddy of the offset [meters/px], then divide 2 \* pi by + that number to get a wave number. That is then used as the index to the + LUT. The LUT contains, for each wave number, the sum of the variances of + all waves with higher or equal wave numbers.) +* Render a simple scene in webGPU and in Mitsuba 3. + * Implement a simple brdf. + * Implement frame export. + * Implement image diffing / measurement. + * Implement hard shadows. + * Implement soft shadows. + * Validate point lighting. + * Validate directional lighting. + * Implement and validate IBL. + * Implement and validate DFG LUT (energy-preserving roughness). + * Implement vertex deformation and normals using baked heightmap & tangents. + Validate against Mitsuba. + * Make a new scene with a highly subdivided quad. +* Port to Unity. + * Implement tooling to blit a texture through a RenderTexture using a shader. + * Automation should generate quads, materials, and rendertextures on behalf + of the user. + * Port compute shader to shaderlab pixel shader. Validate. + * Port lit shader to shaderlab. + * Sample scene, frame export, exhaustive validation... the works. + * Validate point, directional, and IBL. + * Add light volumes. + * Add LTCGI. + +So... yeah. A lot of work. I'll get started tomorrow! + +--- + +[^tessendorf]: Tessendorf, Jerry. *Simulating Ocean Water*. 2004. [PDF](https://people.computing.clemson.edu/~jtessen/reports/papers_files/coursenotes2004.pdf). +[^bruneton]: Bruneton, Eric et. al. *Real-time Realistic Ocean Lighting using Seamless Transitions from Geometry to BRDF*. 2010. [PDF](https://inria.hal.science/inria-00443630/PDF/article-1.pdf). + # how do you evenly tile a column? {data-date="12 Jul 2026"} While walking through town the other day, I saw a pillar that looks a bit like this: |