Unit Size in 3D space = ?

[MrDoomMaster]

I was wondering how Units are measured in 3D space in Direct3D 9.

I read in a tutorial somewhere that units in 2D space are simply 1 pixel for every 1 unit. Yet, in 3D space, a unit can be any measurement. How are units measured in 3D space? How do you set the unit size?

Thanks!

[Marc G]

There is not single/simple answer.
In 3D you can do whatever you want with your units. The units are not fixed. Also, because you can freely move in your 3D world, units are actually something relative.

[MrDoomMaster]

Again, I need to know how spaced out 1 unit is. I don't know how large a cube would be if I gave it a size of 3 units. There *has* to be a logical measurement for a unit in 3D space as it is used in Direct3D 9.

Can anyone help? Thanks!

[philkr]

Marc is right. It is all relative. Imagine you have a cube of 3 units and the camera is 100 units away. The cube looks very small. If you move the camera to 10 units before it is of course bigger. It's like in the real world. You need something of which you know the size to compare it. For example if a tree is next to the cube you can guess how big the cube is because you know the tree height more or less.

[MrDoomMaster]

ah! Yes I see now. That makes a little more sense. Sorry Marc, I didn't quite understand what you were saying.

Thank you both!