opengl 里 state==GLUT_DOWN 这句是什么意思？？

opengl 里 state==GLUT_DOWN 这句是什么意思？？

if(state == GLUT_DOWN) //callback was due to a press (of a mouse button)
//相当于“如果某个鼠标键被按下”
if(state == GLUT_UP) //callback was due to a release (of a mouse button)
//相当于“如果某个鼠标键被放开（弹起）”
if (button== GLUT_LEFT_BUTTON) //callback was due to a release/press of the LEFT button
//相当于“如果鼠标左键被按下或者被放开”
if (button== GLUT_RIGHT_BUTTON) //callback was due to a release/press of the RIGHT button
//相当于“如果鼠标右键被按下或者被放开”
if (button== GLUT_MIDDLE_BUTTON) //callback was due to a release/press of the MIDDLE
//button
//相当于“如果鼠标中键被按下或者被放开”

因此，
if(button == GLUT_LEFT_BUTTON)
{
if(state == GLUT_DOWN)
……
//相当于“如果鼠标左键被按下”

if(button == GLUT_RIGHT_BUTTON)
{
if(state == GLUT_DOWN)
……
//相当于“如果鼠标右键被按下”

这是glut里定义的一个量，表示鼠标左键被按下