用到了glReadPixels glDrawPixels glCopyPixels来创建和拷贝位图-uses glReadPixels glDrawPixels glCopyPix els to create and copy bitmap

用opengl自带的glReadPixels(以及gluUnProject(）函数和点落在三角形内的矢量判断IntersectTriangle然后转成三维坐标的两种方式获取屏幕点选的场景坐标，通过对比发现其值是一样。-Use opengl own glReadPixels (as well as gluUnProject () function and the points fall within the vector triangle to determine IntersectTriangl

The only special thing we need to do for the state tracker s glReadPixels is to validate state (to be sure we have up-to-date framebuffer surfaces) and flush the bitmap cache prior to reading.