p5.Stroke(color.Black)
		p5.Fill(color.White)

		p5.Curve(
			p1.x, p1.y,
			p2.x, p2.y,
			p3.x, p3.y,
			p4.x, p4.y,
		)

var (
	// gproc is the global Proc instance used by the p5js-like API.
	gproc = newProc(defaultWidth, defaultHeight)
)

/ Curve draws a curved line starting at (x2,y2) and ending at (x3,y3).
// (x1,y1) and (x4,y4) are the control points.
//
// Curve is an implementation of Catmull-Rom splines.
func (p *Proc) Curve(x1, y1, x2, y2, x3, y3, x4, y4 float64) {
	// Convert the Catmull-Rom curve into a cubic Bézier curve according to
	//
	//  "Conversion Between Bézier and Catmull-Rom Splines"
	//  S. Arasteh and A. Kalisz
	//
	// An electronic version is available at:
	//  https://arxiv.org/pdf/2011.08232.pdf

	tau := p.stk.cur().tau
	if tau == 1 {
		p.Line(x2, y2, x3, y3)
		return
	}

	var (
		cr1 = p.pt(x1, y1)
		cr2 = p.pt(x2, y2)
		cr3 = p.pt(x3, y3)
		cr4 = p.pt(x4, y4)

		itau = 1 / (6 * (1 - tau))

		p1 = cr2
		p2 = cr2.Add(cr3.Sub(cr1).Mul(itau))
		p3 = cr3.Sub(cr4.Sub(cr2).Mul(itau))
		p4 = cr3

		path clip.Path

		sp  = p1
		cp0 = p2.Sub(sp)
		cp1 = p3.Sub(sp)
		ep  = p4.Sub(sp)
	)

	defer op.Save(p.ctx.Ops).Load()

	path.Begin(p.ctx.Ops)
	path.Move(sp)
	path.Cube(cp0, cp1, ep)

	paint.FillShape(
		p.ctx.Ops,
		rgba(p.stk.cur().stroke.color),
		clip.Stroke{
			Path:  path.End(),
			Style: p.stk.cur().stroke.style,
		}.Op(),
	)
}

// CurveTightness determines how the curve fits to the Curve vertex points.
// CurveTightness controls the Catmull-Rom tau tension.
//
// The default value is 0.
func (p *Proc) CurveTightness(v float64) {
	p.stk.cur().tau = float32(v)
}