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Page 458 OpenGL Super Bible! TerrainWindow = CreateDialog(hInst, MAKEINTRESOURCE(IDD_TERRAIN_DIALOG), SceneWindow, (DLGPROC)TerrainDlgProc); /* * Loop on events until the user quits this application */ while (TRUE) { /* * Process all messages in the queue */ while (!Moving || PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) == TRUE) if (GetMessage(&msg, NULL, 0, 0)) { TranslateMessage(&msg); DispatchMessage(&msg); } else return (1); /* * Handle flying as necessary */ GetCursorPos(&pos); FlyTerrain(pos.x, pos.y); }; return (msg.wParam); } /* * ‘DisplayErrorMessage() - Display an error message dialog. */ void DisplayErrorMessage(char *format, /* I - printf() style format string */ …) /* I - Other arguments as necessary */ { va_list ap; /* Argument pointer */ char s[1024]; /* Output string */ if (format == NULL) return; va_start(ap, format); vsprintf(s, format, ap); va_end(ap);
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OpenGL Super Bible! Page 457 WNDCLASS wc; /* Window class */ POINT pos; /* Current mouse pos */ /* * Initialize the terrain to all grasslands */ InitializeTerrain(); /* * Register main window */ wc.style = 0; wc.lpfnWndProc = (WNDPROC)SceneProc; wc.cbClsExtra = 0; wc.cbWndExtra = 0; wc.hInstance = hInst; wc.hIcon = NULL; wc.hCursor = LoadCursor(NULL, IDC_ARROW); wc.hbrBackground = 0; wc.lpszMenuName = MAKEINTRESOURCE(IDR_MENU1); wc.lpszClassName = ‘Textured Scene ; if (RegisterClass(&wc) == 0) { DisplayErrorMessage(’Unable to register window class! ); return (FALSE); }; /* * Then create it */ SceneWindow = CreateWindow(’Textured Scene , ‘Textured Scene , WS_OVERLAPPEDWINDOW | WS_CLIPCHILDREN | WS_CLIPSIBLINGS, 32, 32, 400, 300, NULL, NULL, hInst, NULL); if (SceneWindow == NULL) { DisplayErrorMessage(’Unable to create window! ); return (FALSE); }; ShowWindow(SceneWindow, nCmdShow); UpdateWindow(SceneWindow); /* * Load the bitmaps for the buttons, and then create the terrain * editing dialog. */ LoadAllBitmaps(hInst);
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Page 456 OpenGL Super Bible! TreesSelectBitmap, MountainsDownBitmap, MountainsUpBitmap, MountainsSelectBitmap; HWND TerrainWindow; /* Terrain dialog */ int TerrainCurrent = IDC_WATER; int TerrainType[TERRAIN_SIZE][TERRAIN_SIZE]; GLfloat TerrainHeight[TERRAIN_SIZE][TERRAIN_SIZE]; GLfloat TerrainNormal[TERRAIN_SIZE][TERRAIN_SIZE][3]; double MoveTime; /* Last update time */ GLboolean Moving = GL_FALSE, /* GL_TRUE if flying */ Drawing = GL_FALSE; /* GL_TRUE if drawing */ POINT CenterMouseXY; /* Initial mouse pos */ GLfloat Position[3] = { 0.0, TERRAIN_SCALE, 0.0 }; /* Viewer position */ GLfloat Heading = 0.0, /* Viewer heading */ Pitch = 0.0, /* Viewer pitch */ Roll = 0.0; /* Viewer roll */ /* * Local functions */ void DisplayErrorMessage(char *, ); void MakePalette(int); LRESULT CALLBACK SceneProc(HWND, UINT, WPARAM, LPARAM); UINT CALLBACK TerrainDlgProc(HWND, UINT, WPARAM, LPARAM); void InitializeTerrain(void); void LoadAllTextures(void); void LoadAllBitmaps(HINSTANCE); void DrawTerrain(int, int); void FlyTerrain(int, int); void RepaintWindow(RECT *); void SaveBitmapFile(void); void PrintBitmap(void); double GetClock(void); /* * ‘WinMain() - Main entry */ int APIENTRY WinMain(HINSTANCE hInst, /* I - Current process instance */ HINSTANCE hPrevInstance, /* I - Parent process instance */ LPSTR lpCmdLine, /* I - Command-line arguments */ int nCmdShow) /* I - Show window at startup? */ { MSG msg; /* Window UI event */
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OpenGL Super Bible! Page 455 Now here is Listing 12-5, the complete terrain viewing program, TEXSCENE.C. Listing 12-5 TEXSCENE.C: The terrain viewing program #include ‘texture.h #include ‘texscene.h #include #include
#ifndef M_PI # define M_PI (double)3.14159265358979323846 #endif /* !M_PI */ /* * Constants */ #define TERRAIN_SIZE 21 #define TERRAIN_EDGE ((TERRAIN_SIZE - 1) / 2) #define TERRAIN_SCALE (500.0 / TERRAIN_EDGE) #define GRASS_HEIGHT 0.0 #define WATER_HEIGHT 0.0 #define TREES_HEIGHT 0.0 #define ROCKS_HEIGHT 0.5 #define MOUNTAINS_HEIGHT 1.0 /* * Globals */ HWND SceneWindow; /* Scene window */ HPALETTE ScenePalette; /* Color palette (if necessary) */ HDC SceneDC; /* Drawing context */ HGLRC SceneRC; /* OpenGL rendering context */ GLuint SkyTexture, /* Sky texture image */ GrassTexture, /* Grass */ RocksTexture, /* Rock */ WaterTexture, /* Water */ TreesTexture, /* Trees */ MountainsTexture; /* Mountains */ HBITMAP GrassDownBitmap, /* Grass button down image */ GrassUpBitmap, /* Grass button up image */ GrassSelectBitmap, /* Grass button selected image */ RocksDownBitmap, /* */ RocksUpBitmap, RocksSelectBitmap, WaterDownBitmap, WaterUpBitmap, WaterSelectBitmap, TreesDownBitmap, TreesUpBitmap,
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Page 454 OpenGL Super Bible! Summary In this chapter you ve learned how to texture-map images onto polygons and other primitives using OpenGL. Texturing can provide that extra measure of realism that makes computer graphics so exciting to work with. The OpenGL glTexParameter functions provide many ways to improve the quality of texture images when they are drawn. Mipmapped texture images provide multiple levels of detail that improve rendering quality and speed. Linear interpolation of texture images can improve certain types of textures, such as the sky texture used in the example project. The glTexGen functions can simplify generation of texture coordinates by removing unnecessary or tedious calculations. By removing large amounts of conditional glTexCoord calls, automatic coordinate generation also simplifies programs that must display both textured and nontextured scenes For games and other interactive, animated displays, you may want to support both textured and nontextured displays until accelerated OpenGL graphics boards become more widely available.
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OpenGL Super Bible! Page 453 void glTexGeniv(GLenum coord, GLenum pname, GLint *params) where the coord parameter specifies which texture image coordinate to generate, GL_S or GL_T, and the pname parameter specifies the vector to define; in this case GL_OBJECT_PLANE. Finally, the params array specifies the object plane vector that is used to compute the texture coordinate. The previous code for our terrain would generate these coordinates: S = 2 * X T = 2 * Z To make OpenGL use these generated coordinates, you must enable texture coordinate generation, as follows: glEnable(GL_TEXTURE_GEN_S); glEnable(GL_TEXTURE_GEN_T); The file TEXSCENE.C contains a version of our terrain viewing program that uses generated texture coordinates. The same techniques can be used with a 1D texture image. For the 1D image, you d probably generate the S coordinate from the height (Y) to color the terrain based upon the height of the terrain. Generating texture coordinates is usually faster than specifying them manually in immediate mode, but is slower when using display lists. Flying Through the Terrain When the user is flying through the terrain, we need to regulate the flying speed based on the update rate of our scene. Rather than trying to maintain a fixed update rate which can vary depending on the graphics card and CPU being used we will measure the elapsed time from the last update to the current time. The FlyTerrain function manages this by measuring the time in milliseconds between each call, and moving the viewer forward at a fixed speed relative to the elapsed time.
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Page 452 OpenGL Super Bible! Figure 12-6 The terrain-drawing algorithm As you can see, this algorithm won t track the terrain exactly, but it is fast and simple to implement. It scans the terrain from left to right and from bottom to top, and starts a new GL_QUAD_STRIP primitive whenever the terrain type changes. Along the way it assigns lighting normals and texture coordinates for each point on the terrain. Automatically Generating Texture Coordinates Generating all those texture coordinates can be tedious. Fortunately, OpenGL has an answer that we can use! In the current drawing code, we issue glTexCoord2i calls glTexCoord2i(x * 2, y * 2); for each and every point in the terrain. But instead of doing this for each point, we can use the glTexGen functions to define the S and T coordinates in terms of the X and Z position in the scene (Y is used for the height). To generate coordinates for our terrain, then, we can use the following: static GLint s_vector[4] = { 2, 0, 0, 0 }; static GLint t_vector[4] = { 0, 0, 2, 0 }; glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR); glTexGeniv(GL_S, GL_OBJECT_PLANE, s_vector); glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR); glTexGeniv(GL_T, GL_OBJECT_PLANE, t_vector); Here the GL_OBJECT_LINEAR mapping mode maps the texture coordinates from object coordinates: coordinate = X * vector[0] + Y * vector[1] + Z * vector[2] + W * vector[3] The vector array is specified with glTexGen function:
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OpenGL Super Bible! Page 451 basically because texture mapping on a standard PC is too slow for animation. When the user isn t flying around (or drawing, for that matter), we want to draw with the textures. We will take care of this with a little conditional code and a few lighting parameters. Also, because drawing the textured scene will be slower than the fly-through scene, we need to provide some feedback to the user that our program is doing something. For simplicity, we ll just draw to the front buffer (the visible one) when texturing, and to the back buffer (the invisible one for animation) when flying or drawing. This way, when the program updates the textured scene, the user will see the image being drawn. You ll learn more about buffers in Chapter 15. The RepaintWindow function handles redrawing the terrain for the user. It starts off by selecting the front or back buffer (as described just above). Then it clears the color and depth bits, as follows: glViewport(0, 0, rect->right, rect->bottom); glClearColor(0.5, 0.5, 1.0, 1.0); glEnable(GL_DEPTH_TEST); if (Moving || Drawing) { glDisable(GL_TEXTURE_2D); glDrawBuffer(GL_BACK); } else { glEnable(GL_TEXTURE_2D); glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL); glDrawBuffer(GL_FRONT); }; glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); After this, RepaintWindow draws in the sky. For performance reasons, the sky is only drawn when the user is not flying over or drawing the terrain. Since the background is cleared to a light blue, this isn t really a problem. The sky is shaped like a pyramid and has the SKY.BMP texture image mapped to it for a nice, cloudy blue sky. Once the sky is drawn, RepaintWindow starts drawing the terrain. The algorithm used is quite simple and basically generates strips of quadrilaterals (squares) along the terrain points. Each strip uses a different texture or lighting material color, so we have to issue glBegin/glEnd calls for each one. See Figure 12-6 for a graphical depiction of the algorithm.
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Page 450 OpenGL Super Bible! /* * Set the height of the terrain ‘cell . For water, the * height is constant at WATER_HEIGHT. Other other types, * we add a random pertubation to make the terrain more * interesting/realistic. */ switch (TerrainCurrent) { case IDC_WATER : TerrainHeight[y][x] = WATER_HEIGHT; break; case IDC_GRASS : TerrainHeight[y][x] = GRASS_HEIGHT + 0.1 * (rand() % 5); break; case IDC_TREES : TerrainHeight[y][x] = TREES_HEIGHT + 0.1 * (rand() % 5); break; case IDC_ROCKS : TerrainHeight[y][x] = ROCKS_HEIGHT + 0.1 * (rand() % 5); break; case IDC_MOUNTAINS : TerrainHeight[y][x] = MOUNTAINS_HEIGHT + 0.15 * (rand() % 5); break; }; } For the IDC_WATER terrain type, the point height is just set to WATER_HEIGHT (0.0). For other types, we add a small amount of random jitter to make the terrain look more realistic. Once the selected cell is drawn, we recompute the lighting normals using the new height values in UpdateNormals. Each lighting normal is calculated using the points above and to the right of the current point with the following formula: N = lighting normal H = height of current point Hu = height of point above Hr = height of point to the right Nx = (Hr - H) / |N| Ny = 1 / |N| Nz = (Hu - H) / |N| This is just a simplification of the cross product of adjacent terrain grid- cells. Once all the normals are recalculated, the scene is redrawn. Drawing the Scene Now that we ve taken care of the drudge work, we can concentrate on displaying the terrain. You ll remember that besides displaying a pretty textured image, we also want to fly through this terrain. To accomplish this, we need to draw the terrain without textures
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OpenGL Super Bible! Page 449 The heart of the drawing interface is in the DrawTerrain function. It uses the OpenGL selection mechanism to determine which terrain points are under the mouse pointer. Instead of drawing the terrain to the screen, selection rendering records hits inside the selection area (in this case, the mouse pointer) to a buffer you provide. In DrawTerrain, we record the (x,y) location of the terrain in the selection buffer, as in a paint-by-numbers book (see Figure 12-5). OpenGL selection is covered in more detail in Chapter 19. Figure 12-5 Picking a terrain cell Once we have the (x,y) terrain locations, we then reset the height and type of these points in the draw_cell function (Listing 12-4). Listing 12-4 The draw_cell function void draw_cell(int x, /* I - Terrain X location */ int y) /* I - Terrain Y location */ { /* * Range check the terrain location */ if (x < 0 || x >= TERRAIN_SIZE || y < 0 || y >= TERRAIN_SIZE) return; if (TerrainType[y][x] == TerrainCurrent) return; /* Already the right type */ TerrainType[y][x] = TerrainCurrent; /* * Force a redraw */ InvalidateRect(SceneWindow, NULL, TRUE);
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