Position of a point along a line

A utility function for drawing vector diagrams. Find position of an interpolated point along a line from x1 to x2.

pointOnLine(x1, x2, d, absolute = TRUE)

Arguments

x1

A vector of length 2 or 3, representing the starting point of a line in 2D or 3D space

x2

A vector of length 2 or 3, representing the ending point of a line in 2D or 3D space

d

The distance along the line from x1 to x2 of the point to be found.

absolute

logical; if TRUE, d is taken as an absolute distance along the line; otherwise it is calculated as a relative distance, i.e., a fraction of the length of the line.

Value

The interpolated point, a vector of the same length as x1

Details

The function takes a step of length d along the line defined by the difference between the two points, x2 - x1. When absolute=FALSE, this step is proportional to the difference, while when absolute=TRUE, the difference is first scaled to unit length so that the step is always of length d. Note that the physical length of a line in different directions in a graph depends on the aspect ratio of the plot axes, and lines of the same length will only appear equal if the aspect ratio is one (asp=1 in 2D, or aspect3d("iso") in 3D).

See also

Other vector diagrams: Proj(), arc(), arrows3d(), circle3d(), corner(), plot.regvec3d(), regvec3d(), vectors3d(), vectors()

Examples

x1 <- c(0, 0)
x2 <- c(1, 4)
pointOnLine(x1, x2, 0.5)
#> [1] 0.1212678 0.4850713
pointOnLine(x1, x2, 0.5, absolute=FALSE)
#> [1] 0.5 2.0
pointOnLine(x1, x2, 1.1)
#> [1] 0.2667892 1.0671568

y1 <- c(1, 2, 3)
y2 <- c(3, 2, 1)
pointOnLine(y1, y2, 0.5)
#> [1] 1.353553 2.000000 2.646447
pointOnLine(y1, y2, 0.5, absolute=FALSE)
#> [1] 2 2 2