Structuring each frame

All games start up, initialize, and run in a loop until it is time to close down and clean up. The big difference that games have over other applications is they will often load, reinitialize, and destroy content multiple times over the life of the process, as the player progresses through different levels or stages of the game.

Another difference lies in the interactive element of games. To create an immersive and responsive experience, games need to iterate through all of the subsystems, processing input and logic before presenting video and audio to the player at a high rate. Each video iteration presented to the player is called a Frame. The performance of games can be drawn from the number of frames that appear on the monitor in a second, which leads to the term Frames Per Second or FPS (not to be confused with First Person Shooter). Modern games need to process an incredible amount of data and repeatedly draw highly detailed objects 30-60 times per second, which means that they need to do all of the work in a short span of time. For modern games that claim to run at 60 FPS, this means that they need to complete all of the processing and rendering in under 1/60th of a second. Some games spread the processing across multiple frames, or make use of multithreading to allow intensive calculations, while still maintaining the desired frame rate. The key thing to ensure here is that the latency from user input to the result appearing on screen is minimized, as this can impact the player experience, depending on the type of game being developed.

The loop that operates at the frame rate of the game is called the game loop. In the days of Win32 games, there was a lot of discussion about the best way to structure the game loop so that the system would have enough time to process the operating system messages. Now that we have shifted from polling operating system messages to an event-based system, we no longer need to worry, and can instead just create a simple while loop to handle everything.

A modern game loop would look like the following:

while the game is running
{
  Update the timer
  Dispatch Events
  Update the Game State
  Draw the Frame
  Present the Frame to the Screen
}

Although we will later look at this loop in detail, there are two items that you might not expect. WinRT provides a method dispatch system that allows for the code to run on specific threads. This is important because now the operating system can generate events from different OS threads and we will only receive them on the main game thread (the one that runs the game loop). If you're using XAML, this becomes even more important as certain actions will crash if they are not run on the UI thread. By providing the dispatcher, we now have an easy way to fire off non-thread safe actions and ensure that they run on the thread we want them to.

The final item you may not have seen before involves presenting the frame to the screen. This will be covered in detail later in the chapter, but briefly, this is where we signal that we are done with drawing and the graphics API can take the final rendered frame and display it on the monitor.