Pick a point adjacent to a selected endpoint, on a given side of a segment

Given an endpoint (typically the start or end of a previously selected path), this function computes a target point located one perpendicular step away from the endpoint, on the "left" or "right" side relative to the directed segment \(A \rightarrow B\). It then returns the closest spot to that target among the remaining candidates `df_rest`.

Usage

adjacent_endpoint(df_rest, endpoint, A, B, spacing, side = c("left", "right"))

Arguments

df_rest

A data frame of candidate spots containing at least columns `x` and `y`.

endpoint

A one-row data frame with columns `x` and `y` representing the endpoint from which to step sideways. If it has multiple rows, only the first row is used.

A

A data frame with columns `x` and `y` representing point A defining the direction of the segment. Only the first row is used.

B

A data frame with columns `x` and `y` representing point B defining the direction of the segment. Only the first row is used.

spacing

Numeric scalar; step size (in the same coordinate units as `x`/`y`) used to move perpendicularly from `endpoint`.

side

Character; which side to step to relative to the vector \(A \rightarrow B\). Must be `"left"` or `"right"`.

Value

A one-row data frame (same columns as `df_rest`) corresponding to the spot closest to the computed perpendicular target point.

Details

The unit direction vector is computed from \(A \rightarrow B\) and a unit left normal \((-vy, vx)/||v||\) is derived. The target point is: $$T = endpoint + sign \cdot spacing \cdot n$$ where `sign = +1` for `"left"` and `-1` for `"right"`.

The closest spot is selected with closest_spot() using squared Euclidean distance.

Examples

df <- data.frame(x = c(0, 1, 1, 2, 2), y = c(0, 0, 1, 0, 1), id = 1:5)
A <- data.frame(x = 0, y = 0)
B <- data.frame(x = 2, y = 0)
endpoint <- data.frame(x = 1, y = 0)

# Step "left" of A->B (here, toward positive y)
adjacent_endpoint(df, endpoint, A, B, spacing = 1, side = "left")
#>   x y id
#> 3 1 1  3

# Step "right" of A->B (here, toward negative y)
adjacent_endpoint(df, endpoint, A, B, spacing = 1, side = "right")
#>   x y id
#> 2 1 0  2