OBD:PLEA

PLEA : Plane Equation Array

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Overview @ Oni Stuff

Offset	Type	Raw Hex	Value	Description
0x00	res_id	01 41 02 00	577	00577-.PLEA
0x04	lev_id	01 00 00 06	3	level 3
0x08	char[20]	AD DE	dead	unused
0x1C	int32	03 00 00 00	13554	array size
First element (black outline)
0x00	float	00 00 00 00	0.000000	1st coefficient of the plane equation (x-part of normal vector)
0x04	float	00 00 00 00	0.000000	2nd coefficient of the plane equation (y-part of normal vector)
0x08	float	00 00 80 3F	1.000000	3rd coefficient of the plane equation (z-part of normal vector)
0x0C	float	00 00 19 44	612.000000	4th coefficient of the plane equation

Plane equation

The canonical equation for a plane in 3D is :

a * x + b * y + c * z + d = 0

where (a, b, c, d) are 4 real numbers. The plane is the set of points (x, y, z) verifying that equation.

It is likely that the four floats in one of a PLEA's packages are exactly the (a, b, c, d) of the above definition.

In that case, the first package of the above example defines the plane (0 * x + 0 * y + 1 * z + 612 = 0), i.e., the vertical plane z = -612.

It is possible, however, that the definition used is different, e.g. : a * x + b * y + c * z = d .

In that case, the first package of the above example defines the plane (0 * x + 0 * y + 1 * z = 612), i.e., the vertical plane z = 612.

Front/back

The 4 numbers (a, b, c, d) can also serve to define whether a 3D point (x, y, z) is on the "front" side of the plane or in the "back".

"points are in front of the plane if a * x + b * y + c * z >= d" (from commented Myth source)

That would mean the plane equation is a * x + b * y + c * z = d , not a * x + b * y + c * z + d = 0 .

It's rather easy to check (just flip the "normal" of the plane equation for a known axis-aligned quad).

The oriented "plane inequation" can be used for: culling; collision.

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