OpenGL ES units

I am developing an application for iPhone.

I am using OpenGL to display a 3D object in the screen, wit开发者_开发技巧h the camera view as background.

I'd like to know how can i change the OpenGL ES unit to centimeters/meters.

How can i do that?

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You dont.

Thought experiment time!

First imagine you have sphere 1 centimeter in diameter. And you have a camera 10 centimeters away. You would see a small sphere in the center of the frame.

Now imagine you have a sphere 1 kilometer in diameter and a camera 10 kilometers away. How would you expect the image to be different?

The correct answer is you would not expect the image to change at all. All that really matters is the relative sizes of things. So the unit type you attribute to the the numbers only matters to the programmer, and not to the program.

So you simply mentally declare that one unit is equal to one centimeter and create your objects and world according to that scale. There is no code level change to make this happen. It's merely a convention that you use to help you build things in correct dimensions relative to each other.

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OpenGL does not have a notion for units. It just uses unit-less values. What these values mean is up to you. They just have to be consistent. So if your objects coordinates and viewing parameters are all specified with meters in mind and you have an object whose coordinates are in centimeters, just scale that object by a factor of 0.1.