## Introduction

Its easy to draw a BezierSpline with GDI+ ( call `Graphics.DrawCurve(Points)`

), but how get any arbitrary Point on that drawn curve?

## What I'm talking about

Please take a look at what the screenshot shows:

A **Bezier-Splines-Curve** (brown), constructed by 7 **support-points**, which subdivide the BezierSplines into 6 **Bezier-Segments**.

The 19 **construction-points**, which model the curve (orange).

The
"Curve-Pointer" (red). It can be moved along the BezierSplines and
its location is displayed, computed by interpolating
the BezierSplines.

## How BezierSplines is constructed

**E****ach segment between two supportpoints is constructed as BezierCurve with 4 construction-points:**

The two support-points themselfes and two additional, which take care, that the grade of the bezier arriving the support-point is the same as the grade of the bezier, which leaves the support-point.

## Now interpolate it

To get the Y-value of an X-position I interpolate the BezierSplines in two steps:

First I search the Bezier-Segment, which contains the X-position. This is quickly done by a binary search:

Dim Indx = _Points.BinarySearch(New PointF(X, 0), Function(P1, P2) P1.X.CompareTo(P2.X))

(The Comparison of this search only compares the Point.X-Values.)

Indx will be the bit-complement of the index of the first point with `point.X > X`

.

This data I pass to the second step, to `InterpolateSegment() `

Protected Overrides Function InterpolateSegment( _
ByVal X As Single, ByVal Indx As Integer) As PointF
Indx = Indx * 3 Dim Pts = Me.DrawPath.PathPoints
Dim BezierSegment = New PointF() { _
Pts(Indx - 3), Pts(Indx - 2), Pts(Indx - 1), Pts(Indx)}
Dim Range = New Single() {0, 0, 1} Dim Dlt = -2.0F
Dim Pt As PointF
Do
Range(If(Dlt < 0, 0, 2)) = Range(1) Range(1) = (Range(0) + Range(2)) / 2 Pt = PointOfFraction(Range(1), BezierSegment) Dlt = Pt.X - X Loop Until Math.Abs(Dlt) < 0.5
Return Pt
End Function

Ups! Didn't I mention DrawPath? Its a GraphicsPath, which stores all the construction-points of the BezierSplines, I'd like to get drawn. Very useful! The construction-points are stored in the ".PathPoints" - Property.

Now we come to the Casteljau-algorithm, which computes a point on a Bezier.

I'm lucky: The commentation is sufficient, so I can do without to make still more words.

Private Shared Function PointOfFraction( _
ByVal Fraction As Single, ByVal ptBeziers() As PointF) As PointF
Dim Pts(ptBeziers.Length - 2) As PointF
For I = 0 To ptBeziers.Length - 2 Pts(I) = PointBetween(ptBeziers(I), ptBeziers(I + 1), Fraction)
Next
For UBord = Pts.Length - 2 To 0 Step -1 For I = 0 To UBord
Pts(I) = PointBetween(Pts(I), Pts(I + 1), Fraction)
Next
Next
Return Pts(0)
End Function
Private Shared Function PointBetween( _
ByVal Pt1 As PointF, ByVal Pt2 As PointF, ByVal Fraction As Single) As PointF
Return Pt1.Add(Pt2.Subtract(Pt1).Mult(Fraction)) End Function

## Reduction

To interpolate Bezier-Spline-Segments as an Y = f(X) - function is mathematically incorrect.

Although I keep the support-points ordered "from left to right" a
Segment can take forms, where it shows more than one Y-Value on certain
X-positions.

My "interpolation" ignores such, and simply returns the first found Y-Value.

The failure lets itself be seen by some parts of the curve, which cannot be reached by
interpolation.

Mathematically correct is to interpolate CubicSplines.

They are mathematically undefined only if two support-points set on the same X-position (as vertical line).

## Polygons, Cubic Splines

while interpolating Polygons is trivial, the same on CubicSplines is - ah - untrivial. There's no GDI+ - Function which draws it for you, so there is to deal with linear algebra to solve linear equation-systems - brrr! - I've done that without real understanding. For that:

## Credits

to Marco Roello.

My exercises on CubicSplines base on his **article.**

## Implementation-details

The project deals with several ownerdraw-requirements:

move- and highlight-able points, polygons, bezier-splines, cubic-splines, and a caption

I collected them into a little hierarchy of classes:

ControlCaption
/
DrawObjectBase --DrawPoint
\
\ BezierSpline
\ /
Polygon
\
CubicSpline

You see: programmically I consider a spline as a polygon with different way to draw and interpolate the line between two support-points. And if there are only two support-points - they actually display the same straight line.

## History

30/12/2007: submit article, sample-solution in VB2008 prof, beta

1/5/08: VB2005-version to zip-file added

##