如何通过两次调用gluUnproject从场景坐标中获取深度值（不使用gluUnProject）

Question

In this thread. The second answer suggest that :

Solution 2 call the gluUnproject two time, one with clipZ = -1 and another one with clipZ = 1 , you get two point (in world space). With these two point you get a ray and you can use some raycast algorithm to compute the mouse coordinate.

But unfortunately it is not explained there after the suggestion. There is also this thread which explains a direction can be get by calling gluUnproject twice. Getting a direction makes sense to me but my question is how can I get the depth info by calling gluUnProject twice?

Answer 1

You don't get depth info. You only have a X and a Y, but to unproject you need a Z. And no magic is going to give it. Instead try to deduce it.

After you undo the viewport transform you have X,Y in NDC. What Z? Use z1= 1 and X, Y. Unproject. Now you have a point P1 in world coordinates. Repeat Unproject but this time with z2= -1 . You get P2. Likely you have to deal with forth 'w' coordinate. Remember you can extract 3D coordinates after dividing by w. In homogeneous coordinates w is a scale factor.

With P1 and P2 perhaps you can find on your own the intersection between line P1P2 and your model.

z= -1, 1 are most separated coordinates in NDC, so more accurate the calculation results.