Introduction
Occasionally, I get tired of plain old rectangular shaped WinForms applications and decide to add some simple rounded-rectangular-ly shaped elements to change it up. The problem is I have to remember how to make them. And, because it's the holiday season, why not keep it fun and light? For your programming pleasure and amusement, in the spirit of the holidays, here is a simple control that rounds the edges of a panel.
Understanding the Coordinate System for Drawing Arcs with GDI+
Generally, in math classes or if you are a pilot or use maps, you think of the coordinate system with North (or 360° or 0°) as being situated at the top, rotation clockwise through 360°. In a Cartesian system this would situate 0° at the top and 180° at the bottom, representing the y-axis and 90° and 270° situated left and right representing the x-axis. GDI+ does not treat the coordinate system that way, at least as far as drawing arcs is concerned. The coordinate system is rotated 90° to the right, as shown in Figure 1.
Figure 1: The Cartesian coordinate system is oriented titled to the right, so 0° is on the right or x-axis.
Knowing the orientation of significant reference points is important if you are going to draw anything manually with GDI+.
Technically, to draw a rounded corner you must know the radius of your circle. The radius will be the offset distance from any of the rectangle's four edges. For example, a rounded upper-right corner would be start at width-d and 0 where d represents the arcs radius, width-d is the x-coordinate and 0 is the y-coordinate. Because the arc starts in the up position, the starting angle would be 270°; to obtain a quarter-circle, the arc would sweep 90°. (Four corners mean 360° divided by 4, yields 90° for each corner arc.) See Figure 2 for an illustration.
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