rounded edges/corners in DirectX (D3D9)

Question

I created my own little 2D-Engine with DirectX (okey, should be more like a GUI in the end) and tried to create rounded edges for a simple Rectangle. Since I never done this with a graphics framework before I had no idea how to supply this. For now, I just overlapped 5 Rectangles and 4 circles (the circles are used for the rounded edges). It does work with opaque colors but if I add alpha into the rectangles the circles are making problems. (Shown in the image below - i should have choose another colors...)

<# Open Image #>

I can't find a solution myself (I googled and whondered I found nothing about rounded edges in DirectX) and I do believe there is a much powerful and faster method doing this. So my final question is, what are the common algorythm to create a rectangle with rounded edges in Direct3D9 ?

Answer 1

The common way to draw rounded quads is the use of textures with an alphachannel. It's very easy and the most of the gui's uses images to achieve a specific look. If you draw only single-colored boxes it may look very generic after a while (even if they have fancy rounded corners ;) ).

But if you want to draw rounded quads directly, I would suppose to generate a custom geometry, which fits the desired area directly without overlapping (need for alphablending). In you case it would be something like this:

The more triangles you're using for the corner the smoother it will look.