OpenNest/docs/plans/2026-03-07-gpu-bitmap-bestfit-design.md

GPU Bitmap Best Fit Evaluation

Overview

Add GPU-accelerated bitmap-based overlap testing to the best fit pair evaluation pipeline using ILGPU. Parts are rasterized to integer grids; overlap detection becomes cell comparison on the GPU. Runs alongside the existing geometry-based evaluator, selectable via flag.

Architecture

New project OpenNest.Gpu (class library, net8.0-windows). References OpenNest.Core and OpenNest.Engine. NuGet: ILGPU, ILGPU.Algorithms.

Components

1. Polygon.ContainsPoint(Vector pt) (Core)

Ray-cast from point rightward past bounding box. Count edge intersections with polygon segments. Odd = inside, even = outside.

2. PartBitmap (OpenNest.Gpu)

• Rasterizes a Drawing to int[] grid
• Pipeline: ConvertProgram.ToGeometry() -> Helper.GetShapes() -> Shape.ToPolygonWithTolerance() -> Polygon.ContainsPoint() per cell center
• Dilates filled cells by spacing / 2 / cellSize pixels to bake in part spacing
• Default cell size: 0.05"
• Cached per drawing (rasterize once, reuse across all candidates)

3. IPairEvaluator (Engine)

interface IPairEvaluator
{
    List<BestFitResult> EvaluateAll(List<PairCandidate> candidates);
}

• PairEvaluator — existing geometry path (CPU parallel)
• GpuPairEvaluator — bitmap path (GPU batch)

4. GpuPairEvaluator (OpenNest.Gpu)

• Constructor takes Drawing, cellSize, spacing. Rasterizes PartBitmap once.
• EvaluateAll() uploads bitmap + candidate params to GPU, one kernel per candidate
• Kernel: for each cell, transform to part2 space (rotation + offset), check overlap, track bounding extent
• Results: overlap count (0 = valid), bounding width/height from min/max occupied cells
• IDisposable — owns ILGPU Context + Accelerator

5. BestFitFinder modification (Engine)

• Constructor accepts optional IPairEvaluator
• Falls back to PairEvaluator if none provided
• Candidate generation (strategies, rotation angles, slide) unchanged
• Calls IPairEvaluator.EvaluateAll(candidates) instead of inline Parallel.ForEach

6. Integration in NestEngine

• FillWithPairs() creates finder with either evaluator based on UseGpu flag
• UI layer toggles the flag

Data Flow

Drawing -> PartBitmap (rasterize once, dilate for spacing)
               |
Strategies -> PairCandidates[] (rotation angles x slide offsets)
               |
GpuPairEvaluator.EvaluateAll():
  - Upload bitmap + candidate float4[] to GPU
  - Kernel per candidate: overlap check + bounding box
  - Download results
               |
BestFitFilter -> sort -> BestFitResults

Unchanged

• RotationSlideStrategy and candidate generation
• BestFitFilter, BestFitResult, TileEvaluator
• NestEngine.FillWithPairs() flow (just swaps evaluator)