Question

I’m drawing directly onto a form with two FillPolygon statements to create two arrows – one black, one white. (The white arrow is slightly smaller and drawn over the black arrow.)

Here’s the code in the form’s OnPaint.

protected override void OnPaint(PaintEventArgs e)
{
    Graphics g = e.Graphics;

    g.FillPolygon(brushBlack, travelArrow);
    g.FillPolygon(brushWhite, featureArrow);
}

Works great. Now since the white arrow is going to be drawn several times in different rotations, I decided to use double-buffering to avoid as much flicker as possible.

I first created a DrawFeatureArrow method that I call in OnPaint.

protected override void OnPaint(PaintEventArgs e)
{
    Graphics g = e.Graphics;

    g.FillPolygon(brushBlack, travelArrow);
    DrawFeatureArrow(this, e);
}

Note: There’s no rotation of the white arrow coded yet, I’m just trying to get the double-buffering set up.

DrawFeatureArrow looks like this.

private void DrawFeatureArrow(object sender, PaintEventArgs e)
{
    Bitmap buffer = new Bitmap(60, 159);
    Graphics gOff = Graphics.FromImage(buffer);

    gOff.FillRectangle(brushGreen, 0, 0, buffer.Width, buffer.Height);
    gOff.FillPolygon(brushWhite, featureArrow);

    ImageAttributes attr = new ImageAttributes();
    attr.SetColorKey(Color.Green, Color.Green);

    Rectangle srcRect = new Rectangle(0, 0, bugger.Width, buffer.Height);
    Rectangle destRect = new Rectangle(90, 66, 60, 159);

    Graphics f = e.Graphics;

    // Should draw green rectangle and white arrow
    f.DrawImage(buffer, 90, 66);  // Draws just a green rectangle

    // If uncommented, should draw just white arrow (green rectangle hidden by SetColorKey)
    // f.DrawImage(buffer, destRect, 0, 0, buffer.Width, buffer.Height, GraphicsUnit.Pixel, attr); 

    f.Dispose();
    gOff.Dispose();
    buffer.Dispose();
}

When run, the green rectangle is drawn but not the white arrow.

Strangely enough, in DrawFeatureArrow, if you replace this

gOff.FillPolygon(brushWhite, featureArrow);

With this

gOff.FillRectangle(brushWhite, 10, 10, 20, 20);

You get a tiny white rectangle in the upper left of the green rectangle.

Very strange behavior. Hope someone can point out what I’m doing incorrectly.

Thanks in advance for the help.

Answer 1

You’re drawing your feature arrow over buffer that has dimensions of 60, 159. Maybe the location of the feature arrow is outside of those dimensions and therefore did not end in the bitmap.