# Tangents and Bi-Normals

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I use this code to create the normals for my terrain model.  How would I create the Tangents and Bi-normals?  I can't use the surface->UpdateTangentsAndBinormals() function, as I want the code to able to update normals for specific vertices only.

How exactly do the tangents and bi-normals relate to the normal and how do they effect normal mapping?

```Vec3 _cPos = _getTerrainVertexPosition(x, z);
Vec3 _lPos = _getTerrainVertexPosition(x - 1, z);
Vec3 _rPos = _getTerrainVertexPosition(x + 1, z);
Vec3 _uPos = _getTerrainVertexPosition(x, z + 1);
Vec3 _dPos = _getTerrainVertexPosition(x, z - 1);

//==================================================================================================//
//											TOP LEFT												//
//==================================================================================================//
_U.x = _uPos.x - _lPos.x;
_U.y = _uPos.y - _lPos.y;
_U.z = _uPos.z - _lPos.z;

_V.x = _cPos.x - _lPos.x;
_V.y = _cPos.y - _lPos.y;
_V.z = _cPos.z - _lPos.z;

_topLeft.x = (_U.y * _V.z) - (_U.z * _V.y);
_topLeft.y = (_U.z * _V.x) - (_U.x * _V.z);
_topLeft.z = (_U.x * _V.y) - (_U.y * _V.x);

//==================================================================================================//
//											TOP RIGHT												//
//==================================================================================================//
_U.x = _cPos.x - _rPos.x;
_U.y = _cPos.y - _rPos.y;
_U.z = _cPos.z - _rPos.z;

_V.x = _uPos.x - _rPos.x;
_V.y = _uPos.y - _rPos.y;
_V.z = _uPos.z - _rPos.z;

_topRight.x = (_U.y * _V.z) - (_U.z * _V.y);
_topRight.y = (_U.z * _V.x) - (_U.x * _V.z);
_topRight.z = (_U.x * _V.y) - (_U.y * _V.x);

//==================================================================================================//
//										BOTTOM LEFT													//
//==================================================================================================//
_U.x = _dPos.x - _cPos.x;
_U.y = _dPos.y - _cPos.y;
_U.z = _dPos.z - _cPos.z;

_V.x = _lPos.x - _cPos.x;
_V.y = _lPos.y - _cPos.y;
_V.z = _lPos.z - _cPos.z;

_bottomLeft.x = (_U.y * _V.z) - (_U.z * _V.y);
_bottomLeft.y = (_U.z * _V.x) - (_U.x * _V.z);
_bottomLeft.z = (_U.x * _V.y) - (_U.y * _V.x);

//==================================================================================================//
//										BOTTOM RIGHT												//
//==================================================================================================//
_U.x = _rPos.x - _cPos.x;
_U.y = _rPos.y - _cPos.y;
_U.z = _rPos.z - _cPos.z;

_V.x = _dPos.x - _cPos.x;
_V.y = _dPos.y - _cPos.y;
_V.z = _dPos.z - _cPos.z;

_bottomRight.x = (_U.y * _V.z) - (_U.z * _V.y);
_bottomRight.y = (_U.z * _V.x) - (_U.x * _V.z);
_bottomRight.z = (_U.x * _V.y) - (_U.y * _V.x);

_normal.x = (_topLeft.x + _topRight.x + _bottomLeft.x + _bottomRight.x) / 4.0f;
_normal.y = (_topLeft.y + _topRight.y + _bottomLeft.y + _bottomRight.y) / 4.0f;
_normal.z = (_topLeft.z + _topRight.z + _bottomLeft.z + _bottomRight.z) / 4.0f;

//==================================================================================================//
//										NORMALIZE													//
//==================================================================================================//
float _vectorLength = sqrt((_normal.x * _normal.x) + (_normal.y * _normal.y) + (_normal.z * _normal.z));
_normal.x = _normal.x / _vectorLength;
_normal.y = _normal.y / _vectorLength;
_normal.z = _normal.z / _vectorLength;```

Tangents and binormals are a vector describing the way the texture mapping lies. You can call UpdateTangentsAndBinormals() once and thereafter you can change the normals and the tangents / binormals will be unaffected.

Will they remain unaffected even if I change the position of the vertices in the terrain patch, and then just update the normal?

Are they 90 degrees to the normal and each other?

The position is another input. I will send you the equation in a private message.

Okay thank you.

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