转载，我看到的这篇博客也是转载别人的，我都不知道原始在哪儿^_^
glReadPixels (GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels);
format和type取值如下
/* PixelCopyType */
#define GL_COLOR 0x1800
#define GL_DEPTH 0x1801
#define GL_STENCIL 0x1802
/* PixelFormat */
#define GL_COLOR_INDEX 0x1900
#define GL_STENCIL_INDEX 0x1901
#define GL_DEPTH_COMPONENT 0x1902
#define GL_RED 0x1903
#define GL_GREEN 0x1904
#define GL_BLUE 0x1905
#define GL_ALPHA 0x1906
#define GL_RGB 0x1907
#define GL_RGBA 0x1908
#define GL_LUMINANCE 0x1909
#define GL_LUMINANCE_ALPHA 0x190A
/* DataType */
#define GL_BYTE 0x1400
#define GL_UNSIGNED_BYTE 0x1401
#define GL_SHORT 0x1402
#define GL_UNSIGNED_SHORT 0x1403
#define GL_INT 0x1404
#define GL_UNSIGNED_INT 0x1405
#define GL_FLOAT 0x1406
#define GL_2_BYTES 0x1407
#define GL_3_BYTES 0x1408
#define GL_4_BYTES 0x1409
#define GL_DOUBLE 0x140A