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EDIT3:
Ok, I can calculate the tangent and binormal from the normal on a terrain. This works on a model terrain, but when I use it on a terrain, I get the SAME tangents as when I use the tangent flag.. Seems like the problem is related to the normals then..
worldToTangentSpace[0] = cross(InNorm, float3(0.0f, 0.0f, 1.0f));
worldToTangentSpace[1] = cross(worldToTangentSpace[0], InNorm);
worldToTangentSpace[2] = InNorm;

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Ok, I can calculate the tangent and binormal from the normal on a terrain. This works on a model terrain, but when I use it on a terrain, I get the SAME tangents as when I use the tangent flag.. Seems like the problem is related to the normals then..
worldToTangentSpace[0] = cross(InNorm, float3(0.0f, 0.0f, 1.0f));
worldToTangentSpace[1] = cross(worldToTangentSpace[0], InNorm);
worldToTangentSpace[2] = InNorm;

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I would advise you to use the above way of calculating the tangent and not use the tangent flag, because it is faster on the GPU. Ofcourse it only works for terrains.

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This is most certainly not faster than using the tangents calculated by the engine.. why would you think that? Tangents are calculated once only, not every frame...otherwise it would be super slow.

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On animated models the tangents have to be recalculated every frame, affecting the frame rate, so don't use this flag unnecessarily.

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Haven't looked at it thouroughly, but I think this is wrong:
float3 inTangent : TANGENT,

The engine only passes tangents on TEXCOORD2, AFAIK.

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struct VS_INPUT_STRUCT

{

float4 position : POSITION;

float3 normal : NORMAL;

float3 texcoord0 : TEXCOORD0;

float3 tangent : TEXCOORD2;

};

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