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Building the UFO Program Example In order to really get a feel for how keyboard and mouse input works in games, it’s helpful to work through a complete example. The remainder of this hour focuses on a program example called UFO. Although this program isn’t technically a game, it’s by far the closest thing you’ve seen to a game yet. It involves a flying saucer that you control with the keyboard and mouse. You’re able to fly the flying saucer around a bitmap background image. Perhaps most important is the fact that the UFO program demonstrates how good of a feel you can create for game controls. More specifically, the arrow keys on the keyboard are surprisingly responsive in the UFO program. Although you haven’t really learned about animation yet, the UFO program example makes use of animation to allow you to fly the flying saucer. Fortunately, the program is simple enough that you can get by without knowing the specifics about animation. All you really need to know is that you can alter the position of a bitmap image to simulate movement on the screen. This occurs thanks to the game engine, which redraws the bitmap every game cycle. So, by altering the position of an image and redrawing it repeatedly, you create the effect of movement. The UFO program example reveals how this task is accomplished, as well as how the keyboard and mouse fit into the picture. You learn the details of how animation works in Hour 9, “A Crash Course in Game Animation.” Writing the Program Code The header file for the UFO program example lays the groundwork for the meat of the program, which carries out the details of the flying saucer animation and user input. Listing 6.1 contains the code for the UFO.h header file, which declares global variables used to control the flying saucer. Listing 6.1 The UFO.h Header File Declares Global Variables Used to Keep Track of the Flying Saucer 1: #pragma once 2: 3: //—————————————————————-4: // Include Files 5: //—————————————————————-6: #include 7: #include “Resource.h” 8: #include “GameEngine.h” 9: #include “Bitmap.h” 10: 11: //—————————————————————-12: // Global Variables 13: //—————————————————————-14: HINSTANCE _hInstance; 15: GameEngine* _pGame; 16: const int _iMAXSPEED = 8; 17: Bitmap* _pBackground; 18: Bitmap* _pSaucer; 19: int _iSaucerX, _iSaucerY; 20: int _iSpeedX, _iSpeedY; The first thing of interest in this code is the _iMAXSPEED constant, which establishes the maximum speed of the flying saucer (line 16). The speed of the flying saucer is how many pixels it can travel in a given direction in each game cycle. So, the value of the _iMAXSPEED constant means that the flying saucer can never travel more than 8 pixels in the horizontal or vertical direction in a game cycle. The _pBackground and _pSaucer global variables store the two bitmaps used in the program, which correspond to a night sky background image and the flying saucer image (lines 17 and 18). The remaining variables pertain to the flying saucer, and include its XY position (line 19) and XY speed (line 20). The XY position of the flying saucer is specified relative to the game screen. The XY speed, on the other hand, simply tells the program how many pixels the flying saucer should be moved per game cycle; negative values for the speed variables indicate that the flying saucer is moving in the opposite direction. With the global variables in place, we can move on to the game functions. The first game function to consider is GameInitialize(), which creates the game engine and establishes the frame rate. The frame rate for the program is set to 30 frames per second. This is a relatively high frame rate for games, but it results in much smoother motion for the flying

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