Center of circle/ellipsis using VB.NET

Question

I am attempting to complete a project where, on mouse click, 200 circles are drawn from the clicked location as the center point, growing larger from 1 radius to 200.

I don't need help with the mouse part of this program.

My current roadblock is that it would seem that although there were circle methods in previous releases of VB, VB.NET only uses the System.CreateGraphics.DrawEllipse method to create circles, and this method uses an x and y coordinate as a starting location for the upper lefthand corner of an invisible rectangle and uses two more integers/singles to decide the distance to the right and down (x + and y +) to determine the size and location of the circle.

I'm hoping I am just epically missing a built in way to create a circle/ellipsis using a center point and declaring the radius from that center point in order to complete my program.

Is there a way to draw a circle onto a FORM using a center point as the point of reference then declare the radius?

Answer 1

Essentially, the only thing you need to do is to is to decrease the X and Y values of the point with the radius, and use radius * 2 for the width and height:

graphics.DrawEllipse(
    pen, location.X - radius, location.Y - radius, radius * 2, radius * 2)

Given that, it's pretty easy to make an extension method giving you the interface that you are looking for:

Imports System.Runtime.CompilerServices

Module GraphicsExtensions

    <Extension()>
    Public Sub DrawCircle(ByVal graphics As Graphics, pen As Pen, location As Point, radius As Integer)
        graphics.DrawEllipse(
            pen, location.X - radius, location.Y - radius, radius * 2, radius * 2)
    End Sub

End Module

...and use it in your form:

Dim pos As Point = Me.PointToClient(MousePosition)

Using g As Graphics = Me.CreateGraphics()
    g.DrawCircle(Pens.Black, pos, 15)
End Using

Answer 2

I'm hoping I am just epically missing a built in way to create a circle/ellipsis using a center point and declaring the radius from that center point in order to complete my program.

Is there a way to draw a circle onto a FORM using a center point as the point of reference then declare the radius?

Sure, just create a Rectangle() at that point with a size of (1,1) and repeatedly call the Inflate() method:

Public Class Form1

    Private Center As Point
    Private MouseClicked As Boolean = False

    Private Sub Form1_MouseDown(sender As Object, e As MouseEventArgs) Handles MyBase.MouseDown
        If e.Button = Windows.Forms.MouseButtons.Left Then
            MouseClicked = True
            Center = New Point(e.X, e.Y)
            Me.Refresh()
        End If
    End Sub

    Private Sub Form1_Paint(sender As Object, e As PaintEventArgs) Handles Me.Paint
        If MouseClicked Then
            Dim rc As New Rectangle(Center, New Size(1, 1))
            For i As Integer = 1 To 200
                e.Graphics.DrawEllipse(Pens.Black, rc)
                rc.Inflate(1, 1)
            Next
        End If
    End Sub

End Class

Answer 3

If you have the center point and radius, then you can calculate corner point easily. The following might help you.

Private Sub Form1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
    Dim centerX, centerY As Integer
    Dim cornerX, cornerY As Integer
    Dim radius As Integer

    centerX = 100
    centerY = 100
    Dim i As Integer
    For i = 20 To 200 Step 20
        radius = i
        cornerX = centerX - radius / 2
        cornerY = centerY - radius / 2

        e.Graphics.DrawEllipse(Pens.Black, cornerX, cornerY, radius, radius)
    Next

End Sub