How to integrate camera position in custom shader?

[Bill-XU]

Hi,

I am using a custom shader to make gradient colorization.

The gradient colorization is worked as I expected, but it is not somehow rendered completely like default built-in shaders.
I tried to use light source methods and material methods to create diffuse and ambient effects,
but failed to integrate camera position into it, as I don't know how to get it inside a shader program.

Is there anybody know about this? Could you give some hints?
Thank you in advance.

Bill