gopatterns/pkg/pattern/path/splines.go

package path

import (
	"fmt"

	"git.wtrh.nl/patterns/gopatterns/pkg/pattern/point"
	"github.com/tdewolff/canvas"
	splines "gitlab.com/Achilleshiel/gosplines"
)

const resolution = 40

// Spline defines a smooth curved path through points.
type Spline struct {
	*Path
	start, end point.Point
}

// NewSpline returns a new spline through points. Start and end points can be provided as
// the start and stop direction of the spline. When start or end point arguments are left nil there
// are no constraints on the direction.
func NewSpline(start, end point.Point, points ...point.Point) Spline {
	s := Spline{
		Path:  NewPath(points...),
		start: start,
		end:   end,
	}

	if start == nil && len(points) > 1 {
		s.start = points[0]
	}

	if end == nil && len(points) > 1 {
		s.end = points[len(points)-1]
	}

	return s
}

// Draw the spline to the provided [canvas.Canvas].
func (p Spline) Draw(c *canvas.Canvas) error {
	if len(p.points) < 2 {
		return nil
	}

	x := make([]float64, len(p.points))
	y := make([]float64, len(p.points))

	for i, point := range p.points {
		x[i], y[i] = point.Vector().Values()
	}

	diffStart := p.start.Vector().Subtract(p.points[0].Vector())
	diffEnd := p.points[len(p.points)-1].Vector().Subtract(p.end.Vector())

	xCoefficient, err := splines.SolveSplineWithConstraint(x, diffStart.X, diffEnd.X)
	if err != nil {
		return fmt.Errorf("unable to calculate coefficients for x: %w", err)
	}

	yCoefficient, err := splines.SolveSplineWithConstraint(y, diffStart.Y, diffEnd.Y)
	if err != nil {
		return fmt.Errorf("unable to calculate coefficients for y: %w", err)
	}

	points := make([]point.Point, 0, len(x)*resolution)
	stepSize := 1.0 / float64(resolution-1)

	for i := range len(p.points) - 1 {
		points = append(points, p.points[i])

		for t := stepSize; t < 1.0; t += stepSize {
			xCalculated := xCoefficient[i].Calculate(t)
			yCalculated := yCoefficient[i].Calculate(t)
			points = append(points, point.NewAbsolutePoint(xCalculated, yCalculated, 0, "0"))
		}
	}

	points = append(points, p.points[len(p.points)-1])

	err = NewPath(points...).Draw(c)
	if err != nil {
		return fmt.Errorf("draw spline points to canvas: %w", err)
	}

	return nil
}