OpenNest/docs/plans/2026-03-08-remnant-fill-investigation.md

Remnant Fill Optimization Investigation

Problem

When filling remnant strips on a partially-nested plate, NestEngine.Fill(NestItem, Box) produces fewer parts than the UI's Ctrl+F fill. On a test case (N0308-008.zip, 36x36 plate, "Converto 3 YRD DUMPERSTER HINGE PLATE #2" 5.89x3.39):

• MCP fill_remnants: 7 parts in the right-side strip
• UI Ctrl+F (ActionClone.Fill): 9 parts in the same strip

Test Setup

Load C:/Users/AJ/Desktop/N0308-008.zip — 75 parts on plate 0, 5 columns of 15. The remnant strip is at (31.0, 0.8) 4.7x35.0.

Use the OpenNest MCP tools to reproduce:

load_nest("C:/Users/AJ/Desktop/N0308-008.zip")
get_plate_info(0)  → should show 75 parts, remnant at (31.0, 0.8) 4.7x35.0
fill_remnants(0, "Converto 3 YRD DUMPERSTER HINGE PLATE #2")  → gets 7 parts

Root Cause Analysis

The MCP's fill_remnants calls NestEngine.Fill(NestItem, Box) which:

1. Tries FillLinear with best rotation + 90° (and angle sweep if strip is narrow)
2. Tries FillRectangleBestFit (mixes horizontal/vertical in bin packing)
3. Tries FillWithPairs (paired part combinations via BestFitCache)
4. Picks the best result

The UI's ActionClone.Fill() (OpenNest/Actions/ActionClone.cs:171-201) does something different:

1. Gets Helper.GetLargestBoxVertically(pt, bounds, boxes) and GetLargestBoxHorizontally(pt, bounds, boxes) from the cursor position
2. Picks the largest area
3. Calls NestEngine.Fill(groupParts, bestArea) — note: passes List<Part> not NestItem

The Fill(List<Part>, Box) overload uses RotationAnalysis.FindHullEdgeAngles(groupParts) which may produce different/better rotation candidates than Fill(NestItem, Box) which uses RotationAnalysis.FindBestRotation(item).

Key Differences to Investigate

1. Rotation candidate generation

• Fill(NestItem, Box) uses FindBestRotation → best angle + 90° + optional sweep
• Fill(List<Part>, Box) uses FindHullEdgeAngles → edge angles from convex hull
• The hull edge angles may include rotation angles that pack better in the narrow strip

2. Region selection

• UI uses cursor position to find the largest obstacle-free rectangle via GetLargestBoxVertically/GetLargestBoxHorizontally
• MCP uses Plate.GetRemnants() which returns edge strips from global part boundaries
• The UI region may differ in exact bounds from the remnant strip

3. Part grouping

• UI's ActionClone can pass multi-part groups to Fill(List<Part>, Box), enabling pattern-based tiling
• MCP passes single NestItem to Fill(NestItem, Box)

Files to Read

• OpenNest.Engine/NestEngine.cs — both Fill overloads, FillWithPairs, FillPattern
• OpenNest/Actions/ActionClone.cs:171-201 — the UI fill path
• OpenNest.Engine/BestFit/RotationAnalysis.cs — FindBestRotation vs FindHullEdgeAngles
• OpenNest.Engine/FillLinear.cs — the linear tiling engine
• OpenNest.Core/Helper.cs:1098+ — GetLargestBoxVertically/GetLargestBoxHorizontally

Possible Fixes

1. Use hull edge angles in the NestItem overload — merge rotation candidates from both FindBestRotation and FindHullEdgeAngles
2. Improve GetRemnants — instead of global edge strips, scan per-column to find the actual free space shape
3. Add a smarter fill_remnants — have the MCP tool use GetLargestBox* helpers to find free rectangles from multiple scan points, similar to how the UI does it