Find Bounds of GDI+ Shape - vb.net

Question

Hi I'm trying to get a rectangle that completely surrounds a shape drawn via GDI+ when the pen is thick.
This is my test code to demo which I just added to mybase.load to test out..
Dim bmp As New Bitmap(500, 500)
Dim g As Graphics = Graphics.FromImage(bmp)
g.Clear(Color.Transparent)
g.SmoothingMode = SmoothingMode.HighQuality
g.PixelOffsetMode = PixelOffsetMode.Default
' Setup Matrix
Dim mat As New Matrix
mat.Translate(100, 100)
g.Transform = mat
' Draw Lines and Fill
Dim gp As New GraphicsPath
gp.AddLines({New Point(50, 0), New Point(100, 100), New Point(0, 100)})
gp.CloseAllFigures() ' Closes Open Path (ie join back to origin)
g.FillPath(New SolidBrush(Color.Beige), gp)
' Add Border to Graphics Path
g.DrawPath(New Pen(New SolidBrush(Color.Black), 20), gp)
g.DrawRectangle(New Pen(New SolidBrush(Color.Red), 1), Rectangle.Truncate(gp.GetBounds))
' Tidy Up
g.ResetTransform()
mat.Dispose()
' Set Picturebox Value
PictureBox1.Image = bmp
I'd post the image but the forum doesn't let me.
I want the bounding rectangle to include the pen width too. I used the above triangle to demo that I cant just add penwidth/2.
Any ideas?
Julian

Answer 1

If that is sufficient for your needs you can simply set the pen's .Alignment property to Inset like so:
g.DrawPath(New Pen(New SolidBrush(Color.Black), 20) With {.Alignment = PenAlignment.Inset}, gp)
This basically tells the pen to draw itself on the inside of the path at all times. Default is that it draws itself on the middle which leads to the situation that the outside width at edges is dependant on the angle of the edge.
Result is:

Answer 2

Just in case anyone else struggles like i did trying to find a solution then this is what I did in the end. I gave up trying to do using some form of getbounds method.
My solution in the was to scan the rendered bitmap instead to calculate the bounds based on pixels that were non-transparent (the background of my rendered image(s)). In fact I only need to scan once on the combined image and it takes a fraction of a second to calculate the "true" bounds now.
I used this article Fit a PictureBox to its non-transparent pixels to achieve a starting point that proved the concept and gave me a region that I could do a "getbounds" on. Then I swapped out the slow getpixel method for the lockbits example at Work with bitmaps faster in C# and voila!
Just ran all the code through an online c# to vb convertor and pasted it together to get completely working in less than an hour.
Took a fraction of the time I wasted trying to find a solution on the internet that would work directly on the vector image.
Julian