GlReadPixels behave weird on QGLWidget

[psyth16]

Hi there,

I'm a beginner of Qt. I'm using Qt 5.2.1.
To make a pickup function of vertices on OpenGL, I used glReadPixels, which is a function of OpenGL.
However, it returns a wrong value, so I cannot obtain correct depth from glReadPixels.
I guess x,y coordinate of openGl does not correspond to those of QGLWidget correctly.

Could anyone help me? My priority is to make a pickup function, so if you have an alternative, it should be fine as well.

Thanks.