opengl编程学习笔记（三）（2D绘图）
在opengl中可以绘制二维图像，分为两种，BITMAP和image 1.bitmap和image的区别有： bitmap中对图像的每个像素只存储一位（0或1），用来表示用当前颜色绘制还是不绘制，而image是图像的每个像素存储可能很多位，存储它的各个通道的颜色信息； bitmap是绘制在
在opengl中可以绘制二维图像，分为两种，BITMAP和image
1.bitmap和image的区别有：
bitmap中对图像的每个像素只存储一位（0或1），用来表示用当前颜色绘制还是不绘制，而image是图像的每个像素存储可能很多位，存储它的各个通道的颜色信息；
bitmap是绘制在三位空间中的一群点，其中每个点都受各变换矩阵的影响，等同于一群Gl_begin(GL_POINTS)操作联合在一起 ，而image是直接绘制在帧缓存上，在绘制原点所投影到的屏幕坐标处开始绘制，就是直接在二维屏幕上绘制，就像普通的二维绘制一样，他只有那个绘制的原点受变换矩阵的影响，比如当原点的位置被裁剪掉之后，整张图片也将消失。
bitmap是绘制在三位场景的一些点，也就是每个点都会有法向等信息，回收光照等影响，拉近看还会发现点间的距离，image是直接写入帧缓存的，绘制的颜色就是最终看到的颜色， 像素点间不会有距离
2.bitmap的绘制步骤
glColor3f制定当前绘制的点的颜色
glRasterPos2f定义当前的绘制原点（OPENGL绘制的原点是在左下角的，即定义之后，都是从这点开始按照从左至右，从下至上绘制的）
只有当调用了 glRasterPos2f之后，他前面的glColor3f才会起作用，所以要先设置颜色，再定义原点
void glBitmap( GLsizei width, 绘制宽度的点数
GLsizei height, 绘制高度 的点数
GLfloat xorig, 在以原点的二维坐标系中的X坐标
GLfloat yorig, 在以原点的二维坐标系中的Y坐标
GLfloat xmove,绘制后原点的横向位移
GLfloat ymove,绘制后原点的纵向位移
const GLubyte * bitmap 存储的数据);
绘制存储在内存中的数据
其中bitmap中存储的数据按照从左下角起8位对其（一个GLUBYTE），例如下图的数据就应该因该存储为
0xc0, 0x00, 0xc0, 0x00, 0xc0,0x00, 0xc0, 0x00, 0xc0, 0x00,0xff, 0x00, 0xff, 0x00, 0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00,0xff, 0xc0, 0xff, 0xc0。如果width不是八的倍数，比如是30个点，那么这一行仍然要占8的倍数位，应占用4个字节，最后面的两位将被自动舍弃。
bitmap更适合绘制颜色较单一，且需要能进行三维观察的图
他也可以绘制彩色图，只是要每个点绘制前改变当前绘制颜色，如图
2.image的绘制步骤
2.1glPixelStorei（）设置数据的存储和解读方式，这里可以进行的常用设置有
GL_PACK_SWAP_BYTES改变一个像素各个成分（不是颜色通道的顺序）的顺序，比如原先的存储顺序为（颜色、深度、索引），颠倒后为（索引、深度、颜色），而一个颜色的通常都存储为RGB。这个设置通常不用改变
剪裁图像：比如我们只要绘制在内存中图像数据中的一个矩形区域，原图为640*480，我们需要绘制从（10，10） 开始长100，宽100的区域到屏幕。我们需要先用glPixelStorei(GL_UNPACK_ROW_LENGTH,640)设置图像的原宽为640，再用glPixelStorei(GL_UNPACK_SKIP_ROWS,10)设置我们跳过了10行，然后用glPixelStorei(GL_UNPACK_SKIP_PIXELS,10)设置我们又把起点跳过了10个像素（列），最后调用glDrawPixels(100,100...）来绘图。
2.2glPixelTransferf设置像素的传递和映射，用这个操作，可以在存储或读取时将特定的颜色映射到一种颜色，或者将特定颜色的值扩大缩小等
2.3glPixelZoom（x，y）设置几何变换，这将设置最后图像的一个像素在X/Y方向上被放大的倍数，都是2则放大四倍，如果为负数在读取时进行镜像
2.4调用glDrawPixels绘制存储在内存中的图像到帧缓存上
或调用glReadPixels读取帧缓存上的数据到内存中（可以做截图用）
或调用glCopyPixels拷贝帧缓存的一处数据到帧缓存的另一处（觉得很适合做赛车游戏的后视镜 ^^）
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