GraphicsPath.IsVisible broken?

[Mike]

The .NET class libary presents this as an example of GraphicsPath.IsVisible:

Public Sub IsVisibleExample(ByVal e As PaintEventArgs)
Dim myPath As New GraphicsPath
myPath.AddEllipse(0, 0, 100, 100)
Dim visible As Boolean = myPath.IsVisible(50, 50, e.Graphics)
MessageBox.Show(visible.ToString())
End Sub

This seems to work quite well, it pops up a box that is true, as it should
be. However, if you alter the code:

Public Sub IsVisibleExample(ByVal e As PaintEventArgs)
Dim myPath As New GraphicsPath
myPath.AddEllipse(0.0, 0.0, 1.0, 1.0)
Dim visible As Boolean = myPath.IsVisible(0.5, 0.5, e.Graphics)
MessageBox.Show(visible.ToString())
End Sub

To use those floating points on a smaller scale, the returned value is
false. Now, obviously, that point is within the bounds of the ellipse, and
method explicitly takes Singles, it's not doing a conversion here or anything.

Does anyone know what the reason for this discrepancy is?

[Larry Lard]

Mike wrote:

The .NET class libary presents this as an example of GraphicsPath.IsVisible:

Public Sub IsVisibleExample(ByVal e As PaintEventArgs)
Dim myPath As New GraphicsPath
myPath.AddEllipse(0, 0, 100, 100)
Dim visible As Boolean = myPath.IsVisible(50, 50, e.Graphics)
MessageBox.Show(visible.ToString())
End Sub

This seems to work quite well, it pops up a box that is true, as it should
be. However, if you alter the code:

Public Sub IsVisibleExample(ByVal e As PaintEventArgs)
Dim myPath As New GraphicsPath
myPath.AddEllipse(0.0, 0.0, 1.0, 1.0)
Dim visible As Boolean = myPath.IsVisible(0.5, 0.5, e.Graphics)
MessageBox.Show(visible.ToString())
End Sub

To use those floating points on a smaller scale, the returned value is
false. Now, obviously, that point is within the bounds of the ellipse, and
method explicitly takes Singles, it's not doing a conversion here or anything.

I wouldn't be so sure about that. Remember that in GDI+ the unit of
measure is the pixel. If you check what an ellipse produced with those
arguments actually looks like (eg, new WinForms project, drop a
picturebox on the form, add this code

Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As
System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
With e.Graphics
.FillEllipse(Brushes.Red, 0, 0, 1.0, 1.0)
End With
End Sub

and play with the parameters) you will see that it doesn't actually
include any pixels at all, so IsVisible will always return false (I
suspect). These aren't idealised mathematical coordinates - they are
very much tied to the actual pixels produced.

--
Larry Lard
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