OpenGL学习笔记2

一、Display Models: (1) drawing points, (2) drawing lines, and (3) drawing triangles and other polygonal patches.
Drawing points corresponds roughly to the model of a graphics image as a rectangular array of pixels.  the model of graphics images as a rectangular array of pixels is only a convenient abstraction and is not entirely accurate. For instance, on a CRT or television screen, each pixel actually consists of three separate points (or dots of phosphor): each dot corresponds to one of the three primary colors (red, blue, and green) and can be independently set to a brightness value. Thus, each pixel is actually formed from three colored dots. If the pixels are small enough, they cannot be seen individually by the human viewer, and the image, although composed of points, can appear as a single smooth image. With a magnifying glass, you can see the colors in the pixel as separate colors (see Figure)


Drawing lines corresponds to vector graphics displays. 
Drawing triangles and polygons corresponds to the methods used by modern graphics display hardware for displaying three-dimensional images.

二、shading means varying the color across the triangle.The shading does a fairly good job of masking the polygonal nature of the object and greatly increases the realism of the image.

三、You can change the convention for which face is the front face by using the glFrontFace command.

To make front or back faces invisible, or to do both, you must use the commands  glCullFace, glEnable( GL_CULL_FACE );

四、By default, OpenGL draws polygons as solid and filled in. It is possible to change this by using the glPolygonMode function, which determines whether to draw solid polygons, wireframe polygons, or just the vertices of polygons.

五、The term“animation” refers to drawing moving objects or scenes.The movement is only a visual illusion, however; in practice, animation is achieved by drawing a uccession of still scenes, called frames, each showing a static snapshot at an instant in time. The illusion of motion is obtained by rapidly displaying successive frames. This technique is used formovies, television, and computer displays.

六、A region of memory where an image is being created or stored is called a buffer. The image being displayed is stored in the front buffer, and the back buffer holds the next frame as it is being created. Note that swapping buffers does not generally require copying from one buffer to the other; instead, one can just update pointers to switch the identities of the front and back buffers.