Graphics FillPolygon exterior? - vb.net

Question

I've created a simple test app that will draw a polygon onto an image given the points I provide. I've created a brush that will fill in the polygon how I want to. Now I want to fill in everything BUT the polygon. So, using my brush, I want to draw around the polygon so all that is visible is what's inside the polygon. Does anyone know how I may be able to accomplish this?
Thanks in advance!

Answer 1

I was surprised not to find this answer anywhere but in looking at the documentation for System.Drawing.Region
the answer seemed very simple.
We can Exclude the polygon (which I assume would need to be a GraphicsPath), from an infinite region. Region.XOR should work the same as Exclude in this case:
Region region = new Region();
region.MakeInfinite();
GraphicsPath polygonPath = GetYourPolygon();
region.Exclude(polygonPath);
e.Graphics.FillRegion(Brushes.Black, region);
In my case I just needed to exclude a plain RectangleF but this did the trick, it filled the surrounding area and left the excluded area alone.

Answer 2

I think that System.Drawing.Graphics.Clip is what you want.
Here is a code sample from that link:
Private Sub SetAndFillClip(ByVal e As PaintEventArgs)
' Set the Clip property to a new region.
e.Graphics.Clip = New Region(New Rectangle(10, 10, 100, 200))
' Fill the region.
e.Graphics.FillRegion(Brushes.LightSalmon, e.Graphics.Clip)
' Demonstrate the clip region by drawing a string
' at the outer edge of the region.
e.Graphics.DrawString("Outside of Clip", _
New Font("Arial", 12.0F, FontStyle.Regular), _
Brushes.Black, 0.0F, 0.0F)
End Sub
To fill everything outside of the region, then you would have to determine the extents of the DC that you are drawing to and then fill that rect, after having set Graphics.Clip to a Region created from your points.
So, you code might look something like this:
Private Sub SetAndFillClip(ByVal e As PaintEventArgs)
' Set the Clip property to a new region.
e.Graphics.Clip = GetRegionFromYourPoints()
' Fill the entire client area, clipping to the Clip region
e.Graphics.FillRectangle(Brushes.LightSalmon, GetWindowExtentsFromYourWindow())
End Sub
This link shows how to create a Region from an array of points:
http://www.vb-helper.com/howto_net_control_region.html

Answer 3

Those who didn't find the solution yet, have a look at this. Worked for me, out of the box. Do as follows,
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
var points = new []
{
new PointF(150, 250),
new PointF( 50, 500),
new PointF(250, 400),
new PointF(300, 100),
new PointF(500, 500),
new PointF(500, 50),
};
using (var path = new GraphicsPath())
{
path.AddPolygon(points);
// Uncomment this to invert:
// p.AddRectangle(this.ClientRectangle);
using (var brush = new SolidBrush(Color.Black))
{
e.Graphics.FillPath(brush, path);
}
}
}