# What is a plane?

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Hi there, I'm having troubles in understanding a plane, lol. It has just a normal and a distance in DirectX and it breaks my head when I ask myself: What is the position of the plane? I've read it's in the distance? ... but the distance is a float and not a vector. Ok, maybe it's the distance to the world origin but distance in which direction? Or is the normal the direction? But then how do you specify the plane's orientation? Is the orientation bound to the direction? I searched the web and I'm confused. Could someone explain it to me in simple words? Please. Best Regards, sick.

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A plane is an infinite sized quad. Its direction is determined by the normal (Which is a unit vector perpendicular to the plane), and the offset is it's distance from the origin along the normal.

If you have a plane along the XZ axes (A "ground plane" in D3D coordinates), which sits on the Y origin, its normal will be (0, 1, 0) and D will be 0 (normal points vertically up, distance to origin is zero).

If the same plane is moved down 10 units, so it still sits on the XZ plane, but intersects the Y axis at y=-10, then the normal is still (0, 1, 0), but D=-10.

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A plane is given by a point (in the plane of course), and its normal.
The given point can be any point, that's in the plane. The normal is a direction, which is perpendicular to the plane. So it can point in two directions (since two direction vectors are perpendicular, and they point in the opposite direction). The orientation of the plane depends on the orientation of the normal. This thing is mainly used to determine what "side" of the plane a point is (that's not in the plane).

see wiki

Hope that helps.

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Hmm .. I get it now.

It doesn't need a position because it is infinite (d'uh!) and if I want it at position x= 0, y= -10, z= 0 facing z ... then it still can be simply defined as distance= 0 and normal x= 0, y= 0, z= 1 ... *gg ...

Ok ok ... I get it.

Thanks!

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Quote:
 Original post by sickbatteryWhat is the position of the plane?

Quote:
 Original post by Evil SteveA plane is an infinite sized quad.

Because of that a plane has no such thing as a position. As Evil Steve explained a plane has an infinite number of points. Hence you specify its "location" in 3D space by giving its distance from the origin along the plane's normal vector.

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Quote:
 Original post by sickbatteryHmm .. I get it now.It doesn't need a position because it is infinite (d'uh!) and if I want it at position x= 0, y= -10, z= 0 facing z ... then it still can be simply defined as distance= 0 and normal x= 0, y= 0, z= 1 ... *gg ...Ok ok ... I get it.Thanks!
If the plane is facing +Z, on the XY axis, then it won't have a particular Y coordinate. (0, -10, 0) will be on the plane, though but so will (0, 34535345, 0).

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