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Ngoài ra nó còn dùng trong các ứng dụng CAD, thực tế ảo, mô phỏng khoa học, mô phỏng thông tin, phát triển trò chơi. OpenGL còn có một đối thủ cạnh tranh là DirectX của Microsoft. | 52 Chapter 3 OpenGL States and Primitives Now that you have a handle on lines let s move on to the heart and soul of almost every 3D game in existence the all-mighty polygon. Drawing Polygons in 3D Although you can and will do some interesting things with points and lines there s no doubt that polygons give you the most power to create immersive 3D worlds so that s what we ll spend the rest of the chapter on. Before we get into specific polygon types supported by OpenGL that is triangles quadrilaterals and polygons we need to discuss a few things that pertain to all polygon types. You draw all polygons by specifying several points in 3D space. These points specify a region that is then filled with color. At least that s the default behavior. However as you d probably expect by now the state machine controls the way in which the polygon is drawn and you re free to change the default behavior. To change the way polygons are drawn you use void glPolygonMode GLenum face GLenum mode As you will learn in the next subsection OpenGL handles the front and back faces of polygons separately as a result when you call glPolygonMode you need to specify the face to which the change should be applied. You do this by setting the face parameter to GL_FRONT for front-facing polygons GL_BACK for back-facing polygons or GL_FRONT_AND_BACK for both. The mode parameter can take on any of the values in Table 3.7. If for example you want to set the front-facing polygons to be drawn filled and the back-facing ones to be rendered as a wire frame as lines you could use the following code glPolygonMode GL_FRONT GL_FILL glPolýgonMode GL_BACK GL_LINE Table 3.7 Value Polygon Modes Definition GL_POINT Each vertex specified is rendered as a single point the rendering of which can be controlled by the point states discussed earlier. This basically produces the same effect as calling glBegin with GL_POINTS. GL_LINE This will draw the edges of the polygon as a set of lines. Any of the line states .
