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openpencil/openspec/specs/pen-tool/spec.md

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pen-tool Specification

Purpose

Pen tool for vector path creation using a vector network data model. Supports bezier curves with tangent handles, open/closed paths, and Figma-compatible vectorNetworkBlob binary encoding.

Requirements

Requirement: Pen tool with vector network model

Pressing P SHALL activate the pen tool. Clicking places corner points, click+drag places curve points with bezier handles. The underlying data model SHALL use vector networks (not simple paths).

Scenario: Place corner points

  • WHEN user clicks three positions with the pen tool
  • THEN a path with three straight-line segments is created

Scenario: Place curve point

  • WHEN user clicks and drags with the pen tool
  • THEN a bezier curve point is created with tangent handles matching the drag direction and length

Requirement: Open and closed paths

Clicking the first point SHALL close the path. Pressing Escape SHALL commit an open path.

Scenario: Close path

  • WHEN user clicks on the first point of an in-progress path
  • THEN the path closes into a loop

Scenario: Open path

  • WHEN user presses Escape during pen tool drawing
  • THEN the current path is committed as an open path

Requirement: Vector network blob encoding

Vector network data SHALL be encoded/decoded using the Figma vectorNetworkBlob binary format for .fig compatibility.

Scenario: Encode vector data

  • WHEN a vector node is created with the pen tool
  • THEN the vector data can be encoded to the binary vectorNetworkBlob format

Scenario: Decode imported vector data

  • WHEN a .fig file containing vector nodes is imported
  • THEN the vectorNetworkBlob is decoded into editable vector network data

Requirement: Open path rendering

Open paths SHALL be rendered as open strokes, not closed and filled.

Scenario: Open path stroke

  • WHEN an open vector path has a stroke applied
  • THEN the path is rendered as an open stroke line, not as a closed filled shape

Requirement: Preview line

During pen tool drawing, a preview line SHALL connect the last placed point to the current cursor position. The preview SHALL NOT start from (0,0).

Scenario: Preview line from last point

  • WHEN user has placed one point and moves the cursor
  • THEN a preview line extends from the last placed point to the cursor position