Oddly drawn GraphicsPath with Graphics.FillPath

Question

I have written some code which creates a rounded rectangle GraphicsPath, based on a custom structure, BorderRadius (which allows me to define the t

Answer 1

I'd suggest explicitly adding a line from the end of each arc to the beginning of the next one.

You could also try using the Flatten method to approximate all curves in your path with lines. That should remove any ambiguity.

The result you're getting from FillPath looks similar to an issue I had where the points on a Path were interpreted incorrectly, essentially leading to a quadratic instead of a cubic bezier spline.

You can examine the points on your path using the GetPathData function: http://msdn.microsoft.com/en-us/library/ms535534%28v=vs.85%29.aspx

Bezier curves (which GDI+ uses to approximate arcs) are represented by 4 points. The first is an end point and can be any type. The second and third are control points and have type PathPointBezier. The last is the other end point and has type PathPointBezier. In other words, when GDI+ sees PathPointBezier, it uses the path's current position and the 3 Bezier points to draw the curve. Bezier curves can be strung together but the number of bezier points should always be divisible by 3.

What you're doing is a bit strange, in that you are drawing curves in different places without explicit lines to join them. I'd guess it creates a pattern like this:

PathPointStart - end point of first arc
PathPointBezier - control point of first arc
PathPointBezier - control point of first arc
PathPointBezier - end point of first arc
PathPointLine - end point of second arc
PathPointBezier - control point of second arc
PathPointBezier - control point of second arc
PathPointBezier - end point of second arc
PathPointLine - end point of third arc
PathPointBezier - control point of third arc
PathPointBezier - control point of third arc
PathPointBezier - end point of third arc

That looks reasonable. GDI+ should be drawing a line from the last endpoint of each curve to the first endpoint of the next one. DrawPath clearly does this, but I think FillPath is interpreting the points differently. Some end points are being treated as control points and vice versa.