What is a Swap Chain?

A graphics adapter holds a pointer to a surface that represents the image being displayed on the monitor, called a front buffer. As the monitor is refreshed, the graphics card sends the contents of the front buffer to the monitor to be displayed. However, this leads to a problem when rendering real-time graphics. The heart of the problem is that monitor refresh rates are very slow in comparison to the rest of the computer. Common refresh rates range from 60Hz (60 times per second) to 100Hz. If an application is updating the front buffer while the monitor is in the middle of a refresh, the image that is displayed is cut in half, with the upper half of the display containing the old image and the lower half containing the new image. This problem is referred to as tearing.

Microsoft Direct3D implements two ways to avoid tearing: An option that allows updates of the monitor only on the vertical retrace (or vertical sync) operation, or a technique called back buffering. A monitor refreshes its image by sweeping the electron beam of the cathode ray tube (CRT) in a zigzag pattern, starting at the top left of the monitor and ending at the bottom right. When the scanning beam reaches the bottom, the monitor recalibrates the electron beam by moving it back to the upper left so that the process can start again. This recalibration is called a vertical sync. During a vertical sync, the monitor is not drawing anything, so any update to the front buffer will not be seen until the monitor starts to draw again. The vertical sync is relatively slow, but not slow enough to render a complex scene while waiting. What is needed to avoid tearing and to be able to render complex scenes is the process of back buffering.

Back buffering is the process of drawing a scene to an off-screen surface, called a back buffer. Note that any surface other than the front buffer is called an off-screen surface because it is never directly viewed by the monitor. By using a back buffer, an application has the freedom to render a scene whenever the system is idle (that is, no windows messages are waiting) without having to consider the monitor's refresh rate. Back buffering brings an additional complication of how and when to move the back buffer to the front buffer.

The process of moving the back buffer to the front buffer is called surface flipping. Remember that the graphics card simply uses a pointer to a surface to represent the front buffer. Because of this, a simple pointer change is all that is needed to set the back buffer to the front buffer. When an application asks Direct3D to present the back buffer to the front buffer, Direct3D simply flips the two surface pointers. The result is that the back buffer is now the new front buffer, and the old front buffer is the new back buffer. A surface flip is invoked whenever an application asks the Direct3D device to present the back buffer; however, Direct3D can be set up to queue the requests until a vertical sync occurs. This option is referred to as the Direct3D device's presentation interval. Note that the data in the new back buffer might not be reusable, depending on how an application specifies how Direct3D should handle surface flipping. Surface flipping is key in multimedia, animation, and game software. It is equivalent to the way that animation can be done with a pad of paper. On each page, the artist changes the figures slightly, so that when you flip rapidly between sheets, the drawing appears animated.

What is a Swap Chain?

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