---
jupyter:
  description: 'Merging collinear lines in planex2d: MergeCollinearEdges joins nearly collinear pieces of a straight run
    within a tolerance, shown on a rectangle whose bottom comes in four pieces.'
---

# Collinear lines

Straight runs often come in many pieces. Sketches and Boolean operations already merge pieces that lie exactly on one
line, and `MergeCollinearEdges` also merges nearly collinear ones, as they occur in imported data. Two lines meeting at
a vertex become one if the vertex lies within the tolerance of the line through their far ends.

```python
import planex as px
```

```python nbsphinx="hidden"
from docs_figures import Row, Vertices
```

## A bottom in four pieces

The example is a rectangle 4 × 1 whose bottom is drawn as four lines, with the vertex at x = 3 lifted by 10⁻⁶.
`FromPolyline` already joins the two exactly collinear pieces, so the face has six edges, and the default tolerance of
10⁻⁷ changes nothing (Figure 1a). A tolerance of 10⁻⁵ makes the whole bottom one line (Figure 1b). The vertices are
marked.

```python
face = px.Face([px.FromPolyline([(0, 0), (1, 0), (2, 0), (3, 1e-6), (4, 0), (4, 1), (0, 1)], closed=True)])
merged = face.Copy().MergeCollinearEdges()
straight = face.Copy().MergeCollinearEdges(1e-5)
print(len(face.edges), len(merged.edges), len(straight.edges))
```

```python nbsphinx="hide-input"
Row((merged, "Default tolerance", {"marks": Vertices(merged)}),
    (straight, "Tolerance 10⁻⁵", {"marks": Vertices(straight)}), caption="Nearly collinear lines merged")
```