>>123
実装を見て考えたら？

Vector3 Vector3::Unproject( Vector3 vector, float x, float y, float width, float height, float minZ, float maxZ, Matrix worldViewProjection )
{
　Vector3 v;
　Matrix matrix;
　Matrix::Invert( worldViewProjection, matrix );

　v.X = ( ( ( vector.X - x ) / width ) * 2.0f ) - 1.0f;
　v.Y = -( ( ( ( vector.Y - y ) / height ) * 2.0f ) - 1.0f );
　v.Z = ( vector.Z - minZ ) / ( maxZ - minZ );

　Vector3::TransformCoordinate( v, matrix, v );
　return v;
}