SwapChains are a concept often used in post processing where two render targets (which are sometimes referred to as "buffers") are swapped back and forth to apply a chain of effects. SwapChains can be automatically managed by FlatRedBall or can be created and updated manually when resolutions change.
The following code creates an automatically managed SwapChain containing two render targets:
Alternatively, SwapChains can be created explicitly. Typically the internal RenderTargets must be resized when the game's resolution changes, so the game's resize event can be subscribed to with a handler that resizes the SwapChain as shown in the following code.
Once you have created a SwapChain, you can use it to apply shader effects. For more information about adding post processing to your game via shaders, see the Effect (.fx, .fbx) documentation.
To understand the purpose of a SwapChain, we must first discuss how rendering works without a swap chain. Whenever graphics are rendered to the screen (also known as the back buffer), the following are required:
A set of vertices that define a surface that can display a texture. In FlatRedBall, these vertices usually define a Sprite, a tile in a TileMap, or a Gum UI object. This is also called the source.
A shader, which in this context is responsible for processing each pixel on the sprite
A destination (by default the back buffer)
Even if you do not explicitly use shaders in your game, FlatRedBall uses shaders to draw every visual object, including Sprites which draw to the screen with no visual modification.
Of course, the shader may make additional modifications to the Sprite such as changing its color.
Although shaders are always used when rendering individual objects, we can also use shaders to modify an entire scene after it has finished rendering. This is referred to as post processing. To perform post processing, all FlatRedBall objects must first be drawn to a render target which is a temporary storage of all graphics (sometimes called a buffer). Effects can be applied to this render target using shaders before it is drawn to the back buffer that is shown on the display device.
The following diagram shows what the process might look like if we were to render a sprite to a render target first, and then render that to the final screen with additional processing (blurring). Note that we can render multiple sprites to the render target, and then perform one final pass to apply post processing.
The example above applies a single post processing effect to the entire screen to produce a blurred effect. The render target becomes the source or input for the post processing shader which renders to the screen.
However, if we have multiple effects, then the output of the post processing shader must become the input to the next shader. For simplicity the next diagram omits the rendering of the sprites and focuses on how multiple render targets are used:
The example above shows a situation where four render targets are used. The first is the render target that the default FlatRedBall rendering process uses. Each subsequent render target is used for a separate shader, supporting three post processing effects: grayscale, pixelate, and blur.
Notice that Render Target 1, which is used as the destination for regular FlatRedBall rendering, is only used when rendering the Grayscale Shader to Render Target 2. After that, Render Target 1 is no longer needed in the series of shader rendering. Similarly, Render Target 2 is only needed as the destination when rendering the Grayscale shader and the source for Pixelate shader. We can reduce the number of render targets needed by reusing these render targets. Since one render target will be the source and one will be the destination, we need two render targets. Each time a shader is drawn the input and source are swapped, which is where the term SwapChain comes from.
Since a render target typically holds uncompressed pixel data for the entire rendered screen, reusing render targets saves memory and processing time!
The example above uses a SwapChain to perform three post processing passes. Notice that if we were to add additional passes we could still use two render targets since each render target is reused either as a source or destination.
Now that you understand the use case and how to set up a SwapChain, see the Effect (.fx, .fbx) documentation to apply post processing effects.
