GDI+ RoundedRect

• Download source files - 98 Kb
• Download demo executable - 91 Kb

Introduction

Although GDI+ is a very exciting extension to GDI, for some reason, the RoundedRect function is not provided. Although it is obviously possible to use GDI functions with GDI+, this does not give us the ability to use new features such as texturing and alpha transparency with our rounded rectangles. Luckily, GDI+ offers the GraphicsPath class, which we can use to create any shape and then draw using any brush. I've provided a function which can be added to any project, and used as shown to draw a rounded rect, and provided a demo application that allows setting the rounding percentage via a slider. It's quick and dirty ( it's late here ), but it is adequate to show off what the function does. The function is as follows:

GraphicsPath* MakeRoundRect(Point topLeft, Point bottomRight, INT percentageRounded)
{
          ASSERT (percentageRounded >= 1 && percentageRounded <= 100);

          INT left  = min(topLeft.X, bottomRight.X);
          INT right = max(topLeft.X, bottomRight.X);

          INT top    = min(topLeft.Y, bottomRight.Y);
          INT bottom = max(topLeft.Y, bottomRight.Y);

          INT offsetX = (right-left)*percentageRounded/100; 
          INT offsetY = (bottom-top)*percentageRounded/100;

          GraphicsPath pt;
          GraphicsPath * path = pt.Clone();

          path->AddArc(right-offsetX, top, offsetX, offsetY, 270, 90);

          path->AddArc(right-offsetX, bottom-offsetY, offsetX, offsetY, 0, 90);

          path->AddArc(left, bottom - offsetY, offsetX, offsetY, 90, 90);

          path->AddArc(left, top, offsetX, offsetY, 180, 90);

          path->AddLine(left + offsetX, top, right - offsetX/2, top);

          return path;
}

The interesting thing to note is that since I first wrote this code, the ability to call new for GraphicsPath has been removed in some cunning manner ( try it and see ). Instead I must make a local object, and call it's 'Clone' method. It's entirely possible I am just missing something at this late hour, but it seems in line with other classes in GDI+, that you need to create new objects via pointers using GDI+ methods instead of new, and I presume this is because the other .NET languages do not offer pointers.

That's about all there is to it. This code has one flaw - it is poor design that the function creates an object, and the caller must delete it. However, GDI+ kindly does not allow us to return the path as an object, only as a pointer. I'm guessing this has something to do with GDI+ returning every class as a reference, but I am not sure. For some fun, I recommend you play with other things you can add to this path – the Warp method looks like it could especially be a lot of fun.