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I wonder what is the reason that when z increase, the objects appear far away and when z is negative, the object is behind the camera?

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    $\begingroup$ To answer the title question: it is not. $\endgroup$
    – lightxbulb
    Commented Dec 9, 2019 at 21:15

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If you want to use the traditional x is towards right and y is up you are used to in math and physics classes. Now if you follow a right handed coordinate space, which is also commonly used. Then the direction of the z follows from the x and y direction. In this sceme positive z is pointing out from the screen.

Now you can also choose a left handed coordinate system in which case the z is reversed. Its just a matter of choosing which convention you want to use.

Likewise if you choose y to be down, ala traditional windowing math then z is reversed even if you use right hand rule.

None of these conventions are set in stone you can do whatever you want. A bit like you can choose row major or column major matrixes. You just need to know what comvention you follow.

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