I remember being scared of render targets when I first started with XNA. They seemed like weird magic and I didn’t understand them at all. There’s nothing to be frightened of, though, and they are pretty easy to learn how to use.
The first thing you need to know is that when you’re drawing in XNA, you aren’t actually drawing to the screen. Instead you’re drawing to this thing called the “back buffer”. Internally, XNA maintains two sections of graphics memory. Each one is exactly the same size as the other and has all the same properties (such as surface format, whether there’s a depth buffer and/or a stencil buffer, and so on). XNA flips between these two sections of memory every update-draw cycle. So while you are drawing to one, it’s busy drawing the other one on the screen. Then the current update-draw cycle ends, it flips, and the section you were just drawing to gets drawn to the screen while the one that was being drawn to the screen before is now the one you’ll be drawing on. This is what’s meant by “double buffering”. If you drew directly to the screen, the player would see all of those draws taking place as they happened and that would look odd and not very good at all.
Those two sections of graphics memory are render targets. All a render target is, is a section of graphics memory to which things can be drawn. In addition to the two that XNA maintains automatically, you can also create and set your own using RenderTarget2D and GraphicsDevice.SetRenderTarget. Using render targets lets you do all sorts of neat post-processing effects (like ) to make your game look cooler. It also just lets you do things like motion blur and lets you create mirrors in 3D games. There are quite a lot of things that render targets let you do.
To go along with this post, I wrote up a simple sample for how to create and use a RenderTarget2D. It’s available under the terms of the Microsoft Public License and is available for download on my website here:.
Other than the ‘using’ statements, every line is commented in detail so that it should (hopefully) be easy to follow along with and understand. If you have any questions, leave a comment here or drop me a line on Twitter.
One last note. While creating the sample I came across an interesting quirk. If you start by creating a Windows Game, and then make a copy for Windows Phone 7, the drop-down that lets you choose between drawing to a WP7 device and the WP7 emulator stays grayed-out. To resolve this, you need to right click on the Windows Phone 7 version in the Solution Explorer, and choose “Set as StartUp Project”. The bar will then become active, letting you change the target you which to deploy to. If you want another version to be the one that starts up when you press F5 to start debugging, just go and right-click on that version and choose “Set as StartUp Project” for it once you’ve set the WP7 target (device or emulator) that you want.