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# Need help with some linear algebra and transforms

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I'm making a 3d software engine and I've come along way so far but I'm getting stuck on concentating matrixes together I think. Here is the synopsis of what happens. * If I make a triangle that has z = 1 on all the coordinates and do a RotationZ, it works fine. Rotates around the z axis like a charm * If I make a triangle that has z = 0 and then do ---------------- Matrix4 matWorld, matTrans, matRot; MatrixTranslation( matTrans, 0, 0, 1 ); MatrixRotationZ( matRot, rad ); MatrixIdentity( matWorld ); MatrixMultiply( matWorld, matWorld, matRot ); MatrixMultiply( matWorld, matWorld, matTrans ); SetWorldTransform( matWorld ); ---------------- This should have the same effect and rotate properly with the triangle 1 on the z axis. Instead it rotates really funky, so I'm guessing that I have a problem with either my Vector * Matrix function or my MatrixMultiply function. I need some help double checking. Here are all the functions that could help. This is my code for a 1x4 Vector * 4x4 Matrix and the code for 4x4 Matrix * 4x4 Matrix
void dmath::VectorMatrixMultiply( dmath::Vector4& lhs, const dmath::Vector4& rhs1, const dmath::Matrix4& rhs2 )
{
lhs.x = (rhs1.x * rhs2.M11) + (rhs1.y * rhs2.M21) + (rhs1.z * rhs2.M31) + (rhs1.w * rhs2.M41);
lhs.y = (rhs1.x * rhs2.M12) + (rhs1.y * rhs2.M22) + (rhs1.z * rhs2.M32) + (rhs1.w * rhs2.M42);
lhs.z = (rhs1.x * rhs2.M13) + (rhs1.y * rhs2.M23) + (rhs1.z * rhs2.M33) + (rhs1.w * rhs2.M43);
lhs.w = (rhs1.x * rhs2.M14) + (rhs1.y * rhs2.M24) + (rhs1.z * rhs2.M34) + (rhs1.w * rhs2.M44);
}

// 4x4 Matrix Multiply

void dmath::MatrixMultiply( dmath::Matrix4& lhs, const dmath::Matrix4& rhs1, const dmath::Matrix4& rhs2 )
{
lhs.M11 = (rhs1.M11 * rhs2.M11) + (rhs1.M12 * rhs2.M21) + (rhs1.M13 * rhs2.M31) + (rhs1.M14 * rhs2.M41);
lhs.M12 = (rhs1.M11 * rhs2.M12) + (rhs1.M12 * rhs2.M22) + (rhs1.M13 * rhs2.M32) + (rhs1.M14 * rhs2.M42);
lhs.M13 = (rhs1.M11 * rhs2.M13) + (rhs1.M12 * rhs2.M23) + (rhs1.M13 * rhs2.M33) + (rhs1.M14 * rhs2.M43);
lhs.M14 = (rhs1.M11 * rhs2.M14) + (rhs1.M12 * rhs2.M24) + (rhs1.M13 * rhs2.M34) + (rhs1.M14 * rhs2.M44);

lhs.M21 = (rhs1.M21 * rhs2.M11) + (rhs1.M22 * rhs2.M21) + (rhs1.M23 * rhs2.M31) + (rhs1.M24 * rhs2.M41);
lhs.M22 = (rhs1.M21 * rhs2.M12) + (rhs1.M22 * rhs2.M22) + (rhs1.M23 * rhs2.M32) + (rhs1.M24 * rhs2.M42);
lhs.M23 = (rhs1.M21 * rhs2.M13) + (rhs1.M22 * rhs2.M23) + (rhs1.M23 * rhs2.M33) + (rhs1.M24 * rhs2.M43);
lhs.M24 = (rhs1.M21 * rhs2.M14) + (rhs1.M22 * rhs2.M24) + (rhs1.M23 * rhs2.M34) + (rhs1.M24 * rhs2.M44);

lhs.M31 = (rhs1.M31 * rhs2.M11) + (rhs1.M32 * rhs2.M21) + (rhs1.M33 * rhs2.M31) + (rhs1.M34 * rhs2.M41);
lhs.M32 = (rhs1.M31 * rhs2.M12) + (rhs1.M32 * rhs2.M22) + (rhs1.M33 * rhs2.M32) + (rhs1.M34 * rhs2.M42);
lhs.M33 = (rhs1.M31 * rhs2.M13) + (rhs1.M32 * rhs2.M23) + (rhs1.M33 * rhs2.M33) + (rhs1.M34 * rhs2.M43);
lhs.M34 = (rhs1.M31 * rhs2.M14) + (rhs1.M32 * rhs2.M24) + (rhs1.M33 * rhs2.M34) + (rhs1.M34 * rhs2.M44);

lhs.M41 = (rhs1.M41 * rhs2.M11) + (rhs1.M42 * rhs2.M21) + (rhs1.M43 * rhs2.M31) + (rhs1.M44 * rhs2.M41);
lhs.M42 = (rhs1.M41 * rhs2.M12) + (rhs1.M42 * rhs2.M22) + (rhs1.M43 * rhs2.M32) + (rhs1.M44 * rhs2.M42);
lhs.M43 = (rhs1.M41 * rhs2.M13) + (rhs1.M42 * rhs2.M23) + (rhs1.M43 * rhs2.M33) + (rhs1.M44 * rhs2.M43);
lhs.M44 = (rhs1.M41 * rhs2.M14) + (rhs1.M42 * rhs2.M24) + (rhs1.M43 * rhs2.M34) + (rhs1.M44 * rhs2.M44);
}

This is the code for MatrixRotation, MatrixIdentity, MatrixTranslation
// 4x4 Identity Matrix

void dmath::MatrixIdentity( dmath::Matrix4& m )
{
m.M11 = 1; m.M12 = 0; m.M13 = 0; m.M14 = 0;
m.M21 = 0; m.M22 = 1; m.M23 = 0; m.M24 = 0;
m.M31 = 0; m.M32 = 0; m.M33 = 1; m.M34 = 0;
m.M41 = 0; m.M42 = 0; m.M43 = 0; m.M44 = 1;
}

// -- MATRIX TRANSLATION -- //

// 4x4 Translation Matrix

void dmath::MatrixTranslation( dmath::Matrix4& m, const float x, const float y, const float z )
{
m.M11 = 1; m.M12 = 0; m.M13 = 0; m.M14 = 0;
m.M21 = 0; m.M22 = 1; m.M23 = 0; m.M24 = 0;
m.M31 = 0; m.M32 = 0; m.M33 = 1; m.M34 = 0;
m.M41 = x; m.M42 = y; m.M43 = z; m.M44 = 1;
}

// 4x4 Matrix Rotation Z

void dmath::MatrixRotationZ( dmath::Matrix4& m, const float rad )
{
m.M11 = cosf(rad);  m.M12 = sinf(rad);  m.M13 = 0;  m.M14 = 0;
m.M21 = -sinf(rad); m.M22 = cosf(rad);  m.M23 = 0;  m.M24 = 0;
m.M31 = 0;		    m.M32 = 0;	        m.M33 = 1;  m.M34 = 0;
m.M41 = 0;		    m.M42 = 0;	        m.M43 = 0;  m.M44 = 1;
}

// -- MATRIX PROJECTION -- //

// 4x4 Projection Matrix

void dmath::MatrixProjection( dmath::Matrix4& m )
{
m.M11 = 1; m.M12 = 0; m.M13 = 0; m.M14 = 0;
m.M21 = 0; m.M22 = 1; m.M23 = 0; m.M24 = 0;
m.M31 = 0; m.M32 = 0; m.M33 = 1; m.M34 = 1;
m.M41 = 0; m.M42 = 0; m.M43 = 0; m.M44 = 0;
} 
I'm sorry I posted so much code I just don't know exactly where the problems lies. Hope someone will take the time to read this. -ph33r [edited by - ph33r on December 14, 2003 11:57:11 AM]

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I notice that you're passing the same variables by reference...

so that you do something like this:

void func(int & a, const int & b);int main(int argc, char ** argv){  int a=1;  func (a, a);  return 0;}void func(int & a, const int & b){  a=b*2;  printf("%d\n", b);}

Note that on my box, it produces this output:

scout@b0rked:~/bob$make asdf g++ asdf.C -o asdf scout@b0rked:~/bob$ ./asdf
2

Also, remember that in matrix multiplication, order matters.
However, I think that the code thing may be your issue. This would result in something pretty strange, because the matrix on the right would be updating while you were still multiplying!

I hope this helps.

Mark (Scout)

All polynomials are funny - some to a higher degree.
Furthermore, polynomials of degree zero are constantly funny.

[edited by - MrScout on December 14, 2003 12:55:57 PM]

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i think you''re gay name might also have something to do with why it might not be working correctly.

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