GraphicsPath.AddArc gives the wrong angle

Question

I'm running into the problem that my rounded corners method returns non-uniform corners.
Method:

GraphicsPath GetRoundedPath(Rectangle r, int radius, bool topRight = true, bool topLeft = true, bool bottomLeft = true, bool bottomRight = true)
{
    GraphicsPath path = new GraphicsPath();

    if (topLeft)
        path.AddArc(r.X, r.Y, radius, radius, 180, 90);
    else path.AddLine(r.X, r.Y, r.Right, r.Y);

    if (topRight)
        path.AddArc(r.Right - radius, r.Y, radius, radius, 270, 90);
    else path.AddLine(r.Right, r.Y, r.Right, r.Height);

    if (bottomRight)
        path.AddArc(r.Right - radius, r.Bottom - radius, radius, radius, 0, 90);
    else path.AddLine(r.Right, r.Bottom, r.Right, r.Height);

    if (bottomLeft)
        path.AddArc(r.X, r.Bottom - radius, radius, radius, 90, 90);
    else path.AddLine(r.X, r.Bottom, r.X, r.Y);

    return path;
}

When invoke:

Panel p = new Panel();
p.BackColor = Color.Red;
p.Size = new Size(200, 200);
Controls.Add(p);
p.Region = new Region(GetRoundedPath(new Rectangle(p.ClientRectangle, 30, true, true, true, true));
p.Location = new Point(200, 200);

Return:

It is noticeable that the upper left corner is drawn normally, the upper right and lower left are crooked, and the lower left is even crooked.

After, I noticed that exactly one pixel was not visible on the right and bottom, then I opened the Paint(program) and painted it myself and got even corners:

What I tried:
Passed less Rectangle to GetRoundedPath
GetRoundedPath(new Rectangle(p.ClientRectangle.X, p.ClientRectangle.Y, p.ClientRectangle.Width - 1, p.ClientRectangle.Height - 1), 30, true, true, true, true)
The control just became a pixel smaller in width and height.
Don't to change Control.Region, to use Graphics.FillPath instead

p.Paint += (s, e) =>
{
    e.Graphics.FillPath(Brushes.Red, GetRoundedPath(p.ClientRectangle, 30, true, true, true, true));
};

Nothing has changed, I also tried to use it in conjunction with the first
Increase radius for individual corners

int oppositeRadius = (int)Math.Round(radius * 1.5)
...

The corners became stretched and looked more like slanted lines

UPD: I found similar post - Strange GraphicsPath.AddArc() behaviour but it work only with Region and all corners, when I have problem with GraphicsPath

score 0 · Answer 1 · answered Jan 11 '23 at 06:52

It looks like off by 1 pixel for center of arc error. Maybe try something like:

GraphicsPath GetRoundedPath(Rectangle r, int radius, bool topRight = true, bool topLeft = true, bool bottomLeft = true, bool bottomRight = true)
{
    GraphicsPath path = new GraphicsPath();

    if (topLeft)
        path.AddArc(r.X, r.Y, radius, radius, 180, 90);
    else path.AddLine(r.X, r.Y, r.Right, r.Y);

    if (topRight)
        path.AddArc(r.Right - radius - 1, r.Y, radius, radius, 270, 90);
    else path.AddLine(r.Right, r.Y, r.Right, r.Height);

    if (bottomRight)
        path.AddArc(r.Right - radius - 1, r.Bottom - radius - 1, radius, radius, 0, 90);
    else path.AddLine(r.Right, r.Bottom, r.Right, r.Height);

    if (bottomLeft)
        path.AddArc(r.X, r.Bottom - radius - 1, radius, radius, 90, 90);
    else path.AddLine(r.X, r.Bottom, r.X, r.Y);

    return path;
}

The corners are still crooked, the figure has only decreased by 1 pixel. Comparison - https://imgur.com/a/iBIAEsk — Yotic, Jan 12 '23 at 02:53
I was reading docs for addArc method you use (https://learn.microsoft.com/en-us/windows/win32/api/gdipluspath/nf-gdipluspath-graphicspath-addarc(int_int_int_int_real_real) and noticed, that in this signature your radius param is not a radius, but full width or height of arc. So maybe radius/2 — Nikolay, Jan 12 '23 at 06:26

GraphicsPath.AddArc gives the wrong angle

1 Answers1