如何在 .NET 中绘制线性渐变圆？

Question

我可以在 VB.NET 或 C# 中画出这样的圆圈吗？

Answer 1

您可以用许多梯形来近似环，并为每个矩形使用不同的填充颜色。

Imports System.Drawing.Drawing2D

Public Class Form1

    Private Sub Form1_Paint(sender As Object, e As PaintEventArgs) Handles MyBase.Paint

        Dim centre = New PointF(160, 140)
        Dim r1 = 100
        Dim r2 = 105

        Dim colourStart = 90 * Math.PI / 180

        Dim col1 As Color
        col1 = Color.FromArgb(0, 0, 0)

        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias

        Using br As New SolidBrush(col1)

            Dim nSteps = 180
            Dim aInc = 2 * Math.PI / nSteps

            For i = 0 To nSteps - 1
                Dim a = i * aInc

                Dim t1 = a
                Dim t2 = a + aInc

                Dim p1 = New PointF(CSng(r2 * Math.Cos(t1)) + centre.X, CSng(r2 * Math.Sin(t1)) + centre.Y)
                Dim p2 = New PointF(CSng(r1 * Math.Cos(t1)) + centre.X, CSng(r1 * Math.Sin(t1)) + centre.Y)
                Dim p3 = New PointF(CSng(r1 * Math.Cos(t2)) + centre.X, CSng(r1 * Math.Sin(t2)) + centre.Y)
                Dim p4 = New PointF(CSng(r2 * Math.Cos(t2)) + centre.X, CSng(r2 * Math.Sin(t2)) + centre.Y)

                Dim pts = {p1, p2, p3, p4}

                col1 = HSL2RGB(a + colourStart, 1, 0.5)
                br.Color = col1

                e.Graphics.FillPolygon(br, pts)

            Next

        End Using

    End Sub

    ' HSL2RGB from https://dotnetfiddle.net/aORkec (with translation and bugfix).
    ''' <summary>
    ''' Convert HSL to RGB.
    ''' </summary>
    ''' <param name="h">Hue 0..2pi</param>
    ''' <param name="s">Saturation 0..1</param>
    ''' <param name="l">Lightness 0..1</param>
    ''' <returns>RGB color.</returns>
    <DebuggerHidden>
    Function HSL2RGB(h As Double, s As Double, l As Double) As Color
        If h > 2 * Math.PI Then h -= 2 * Math.PI
        If h < 0 Then h += 2 * Math.PI

        h = h / (2 * Math.PI)
        Dim v As Double

        Dim r, g, b As Double
        r = l
        g = l
        b = l
        v = If(l <= 0.5, l * (1.0 + s), l + s - l * s)

        If (v > 0) Then
            Dim m As Double
            Dim sv As Double
            Dim sextant As Integer
            Dim fract, vsf, mid1, mid2 As Double
            m = l + l - v
            sv = (v - m) / v
            h *= 6.0
            sextant = CInt(Math.Floor(h))
            fract = h - sextant
            vsf = v * sv * fract
            mid1 = m + vsf
            mid2 = v - vsf
            Select Case sextant
                Case 0, 6
                    r = v
                    g = mid1
                    b = m
                Case 1
                    r = mid2
                    g = v
                    b = m
                Case 2
                    r = m
                    g = v
                    b = mid1
                Case 3
                    r = m
                    g = mid2
                    b = v
                Case 4
                    r = mid1
                    g = m
                    b = v
                Case 5
                    r = v
                    g = m
                    b = mid2
            End Select
        End If

        Return Color.FromArgb(Convert.ToByte(r * 255.0F), Convert.ToByte(g * 255.0F), Convert.ToByte(b * 255.0F))

    End Function

End Class

结果：

使用允许每个矩形在正确方向上的颜色渐变的画笔可能会获得更好的结果。

---

也可以用环形扇形制作圆，它给出了准确的边界，而不是梯形的直边：

    Function Rad2Deg(x As Double) As Single
        Return Convert.ToSingle(x * 180 / Math.PI)
    End Function

    Private Sub Form1_Paint(sender As Object, e As PaintEventArgs) Handles MyBase.Paint

        Dim centreF = New PointF(160, 140)
        Dim centre = New Point(CInt(centreF.X), CInt(centreF.Y))
        Dim r1 = 100
        Dim r2 = 105

        Dim boundingRectInner = New Rectangle(-r1, -r1, r1 * 2, r1 * 2)
        boundingRectInner.Offset(centre)

        Dim boundingrectOuter = New Rectangle(-r2, -r2, r2 * 2, r2 * 2)
        boundingrectOuter.Offset(centre)

        Dim colourStart = 90 * Math.PI / 180

        Dim col1 As Color
        col1 = Color.FromArgb(0, 0, 0)

        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias

        Using br As New SolidBrush(col1)

            Dim nSteps = 12
            Dim aInc = 2 * Math.PI / nSteps
            Dim sweepAngle = Rad2Deg(aInc)

            For i = 0 To nSteps - 1
                Dim a = i * aInc

                Dim t1 = a
                Dim t2 = a + aInc

                Dim p3 = New PointF(CSng(r1 * Math.Cos(t2)) + centreF.X, CSng(r1 * Math.Sin(t2)) + centreF.Y)
                Dim p4 = New PointF(CSng(r2 * Math.Cos(t2)) + centreF.X, CSng(r2 * Math.Sin(t2)) + centreF.Y)

                Using gp As New GraphicsPath()
                    gp.AddArc(boundingRectInner, Rad2Deg(a), sweepAngle)
                    gp.AddLine(p3, p4)
                    gp.AddArc(boundingrectOuter, Rad2Deg(a + aInc), -sweepAngle)
                    gp.CloseFigure()

                    col1 = HSL2RGB(a + colourStart, 1, 0.5)
                    br.Color = col1

                    e.Graphics.FillPath(br, gp)
                End Using

            Next

        End Using

    End Sub

注意步数越少，颜色越少，但边缘仍然是圆形：