- What Is a Game Engine?
- Pondering the Role of a Game Engine
- Developing a Game Engine
- Building the Game Skeleton Example Program
- Summary
- Q&A
- Workshop
This chapter is from the book
This chapter is from the book
This chapter is from the book
Building the Game Skeleton Example Program
The Skeleton program from the previous lesson demonstrated the basics of Windows programming, and it represents a minimal Windows program. The game engine you created in this lesson includes much of the code found in the Skeleton program, which allows you to create games without having to repeat any of that code again. In this section, you use the game engine to create a new program example called Game Skeleton that is somewhat of a revamped Skeleton program. However, you'll quickly realize that the Game Skeleton program is much easier to follow and understand because the game engine hides most of the mess associated with Windows programs.
You'll be glad to know that the Game Skeleton program isn't just a remake of the Skeleton program with the game engine thrown in. Because the game engine includes support for establishing a game loop complete with timed game cycles, it only makes sense to take advantage of that feature. So, the Game Skeleton program demonstrates the power of game cycles and how they make it possible to get interesting graphical effects with little effort. More specifically, you find out how to rapidly draw skeleton icons at random locations on the game screen and see firsthand how the speed of the game loop impacts the performance of a game.
Writing the Program Code
The Game Skeleton program example is divided into two source files: the Skeleton.h header file and the Skeleton.cpp source code file. Listing 3.6 contains the code for the Skeleton.h header file, which is relatively simple. Keep in mind that all of the code for the Game Skeleton example program is available on the accompanying CD-ROM.
Listing 3.6 The Skeleton.h Header File Simply Imports a Few Header Files and Declares the Important Global Game Engine Pointer
1: #pragma once 2: 3: //----------------------------------------------------------------- 4: // Include Files 5: //----------------------------------------------------------------- 6: #include <windows.h> 7: #include "Resource.h" 8: #include "GameEngine.h" 9: 10: //----------------------------------------------------------------- 11: // Global Variables 12: //----------------------------------------------------------------- 13: GameEngine* _pGame;
This header file includes the familiar windows.h, as well as Resource.h and GameEngine.h. After importing these header files, a global game engine pointer, pGame, is defined. This pointer is very important because it will provide the Game Skeleton program access to the game engine.
The Game Skeleton program is fleshed out in the Skeleton.cpp source code file, which is shown in Listing 3.7
Listing 3.7 The Skeleton.cpp Source Code File Reveals How Straightforward the Program Code for a Minimal Windows Program Becomes When a Game Engine Is Used
1: //-----------------------------------------------------------------
2: // Include Files
3: //-----------------------------------------------------------------
4: #include "Skeleton.h"
5:
6: //-----------------------------------------------------------------
7: // Game Engine Functions
8: //-----------------------------------------------------------------
9: BOOL GameInitialize(HINSTANCE hInstance)
10: {
11: // Create the game engine
12: _pGame = new GameEngine(hInstance, TEXT("Game Skeleton"),
13: TEXT("Game Skeleton"), IDI_SKELETON, IDI_SKELETON_SM);
14: if (_pGame == NULL)
15: return FALSE;
16:
17: // Set the frame rate
18: _pGame->SetFrameRate(15);
19:
20: return TRUE;
21: }
22:
23: void GameStart(HWND hWindow)
24: {
25: // Seed the random number generator
26: srand(GetTickCount());
27: }
28:
29: void GameEnd()
30: {
31: // Cleanup the game engine
32: delete _pGame;
33: }
34:
35: void GameActivate(HWND hWindow)
36: {
37: HDC hDC;
38: RECT rect;
39:
40: // Draw activation text on the game screen
41: GetClientRect(hWindow, &rect);
42: hDC = GetDC(hWindow);
43: DrawText(hDC, TEXT("Activated!"), -1, &rect,
44: DT_SINGLELINE | DT_CENTER | DT_VCENTER);
45: ReleaseDC(hWindow, hDC);
46: }
47:
48: void GameDeactivate(HWND hWindow)
49: {
50: HDC hDC;
51: RECT rect;
52:
53: // Draw deactivation text on the game screen
54: GetClientRect(hWindow, &rect);
55: hDC = GetDC(hWindow);
56: DrawText(hDC, TEXT("Deactivated!"), -1, &rect,
57: DT_SINGLELINE | DT_CENTER | DT_VCENTER);
58: ReleaseDC(hWindow, hDC);
59: }
60:
61: void GamePaint(HDC hDC)
62: {
63: }
64:
65: void GameCycle()
66: {
67: HDC hDC;
68: HWND hWindow = _pGame->GetWindow();
69:
70: // Draw the skeleton icon at random positions on the game screen
71: hDC = GetDC(hWindow);
72: DrawIcon(hDC, rand() % _pGame->GetWidth(), rand() % _pGame->GetHeight(),
73: (HICON)(WORD)GetClassLong(hWindow, GCL_HICON));
74: ReleaseDC(hWindow, hDC);
75: }
The really interesting thing about the code for the Game Skeleton program is how the only functions present in the code are the game event functions described in GameEngine.h. The first of these functions is GameInitialize(), whose responsibility is to get the program started off on the right foot. More specifically, the GameInitialize() function creates a GameEngine object and assigns it to the _pGame global variable (lines 12–15). The GameInitialize() function then sets the frame rate for the game to 15 frames per second, which is a little slower than the default setting of 20 frames per second (line 18). This change is primarily to demonstrate how you will often change the default frame rate for games depending on their specific needs.
The GameStart() function is next, and its job is to initialize game data and start a game. In the case of the Game Skeleton program, there really isn't any game data, so the only code in GameStart() is code to seed a random number generator. I mentioned earlier that the Game Skeleton program draws skeleton icons at random positions on the screen. In order to successfully generate random numbers for these positions, you have to seed the random number generator. This is accomplished with a call to the standard C library function, srand() (line 26).
Similar to the GameStart() function, the GameEnd() function is designed to clean up game data once a game is over. In this case the GameEnd() function is only required to cleanup the game engine (lines 29–33).
The GameActivate() and GameDeactivate() functions are very similar to each other in the Game Skeleton program. Both are here just to demonstrate how you can respond to game activations and deactivations, and they do so by drawing text on the game screen. For example, the GameActivate() function obtains the client rectangle for the game window (line 41), and then uses it as the basis for drawing a line of text centered on the game screen (lines 43 and 44). I realize that some of this graphics code probably looks a little strange, but don't worry too much about it because the next lesson gives you the whole scoop on how to draw graphics in Windows. Speaking of strange graphics code, the GamePaint() function is responsible for painting the game screen, but in this case all the painting takes place in the GameCycle() function, so GamePaint() does nothing.
The GameCycle() function is the last function in the Game Skeleton program, and without a doubt the most interesting. The job of this function is to draw a skeleton icon at a random location on the game screen. This might not seem like a big deal, but keep in mind that you set the frame rate to 15 frames per second, which means that the GameCycle() function is getting called 15 times every second; that means 15 icons get drawn in random locations every second! The first step in the GameCycle() function is to obtain a window handle for the main game window (line 68); this window handle is important because it allows you to draw on the game screen. The drawing actually takes place on lines 72 and 73 when the Win32 DrawIcon() function is called to draw the skeleton icon. The standard rand() function is called to determine a random location on the game screen, and the icon is extracted from the game window class using the Win32 GetClassLong() function.
Although I admittedly threw you a few curves with the graphics code in the Game Skeleton program, you've got to admit that it's considerably easier to follow than the original Skeleton program. This is the benefit of relying on the game engine to take care of a lot of the dirty work associated with Windows game programming.
Testing the Finished Product
You'll be glad to know that the Game Skeleton program is much more fun to tinker with than the Skeleton program—thanks to the game engine. When you run Game Skeleton, you are presented with a game screen that rapidly fills up with skeleton icons, as shown in Figure 3.2.
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Figure 3.2 The Game Skeleton program example demonstrates how the game engine makes it possible to focus solely on the game-specific aspects of a Windows program.
It doesn't take too long for the Game Skeleton screen to fill up with skeleton icons, as shown in Figure 3.3. This has a lot to do with the fact that you have the game set up so that it runs through 15 game cycles per second. You could dramatically slow down or speed up the icons being drawn by altering the frame rate of the game in the GameInitialize() function.
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Figure 3.3 The timing aspect of the game engine causes the Game Skeleton program to fill up quite quickly with randomly placed skeleton icons.
Another interesting point to make about the Game Skeleton program is how it isn't smart enough to repaint the skeleton icons. In other words, if you minimize the program or activate another window in front of it, the game screen will get cleared. This happens because the GamePaint() function doesn't have any code to redraw the skeleton icons in response to the game screen needing a repaint.
The repaint problem in the Game Skeleton program is addressed in Hour 4, "Drawing Basic Graphics," when you explore Windows graphics in more detail.