# OpenGL rotation matrix, [math] rotates in different direction

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First of all i would like you to see this:

Image shows a tank (not rotated, static) with 4 collision spheres (white, red, green, blue)

/ note: three lines what you see (white,red,green) show the positive side of an axis, so for red its +x, for green its +y and for white its +z,

My problem is whenever i try to rotate a tank spheres are rotated backwards. like on this video:

To rotate a tank i use exactly the same rotation matrix i use to rotate these spheres.

now to explain it more deeply: sphere base position (in relation to unrotated tank) is calculated like this:

front_left sphere which is white (span_dist, 0, front_dist) [xyz]

front_right (red) (-span_dist, 0, front_dist) [xyz]

rear_left (green) (span_dist, 0, -front_dist)

rear_right (blue) (-span_dist, 0, -front_dist)

i draw the tank like this:



glPushMatrix();

gluLookAt(eyepos.x,eyepos.y,eyepos.z,lookat.x,lookat.y,lookat.z,up.x,up.y,up.z);

glTranslatef(pos.x,pos.y,pos.z);
glMultMatrixf(hull.rotation.AIR_MATRIX);

hull.model->DrawSimpleModel();

glPopMatrix();


hull.rotation.AIR_MATRIX <- this is my rotation matrix (it only has rotation data, - no translation and scale data)

now i calculate spheres positions like:



float rotm[16];    int i;
for (i=0; i<16;i++) rotm[i] = hull.rotation.AIR_MATRIX[i];

col_front_left = front_left * rotm;
col_front_left = col_front_left + pos;

col_front_right = front_right * rotm;
col_front_right = col_front_right + pos;

col_rear_left = rear_left * rotm;
col_rear_left = col_rear_left + pos;

col_rear_right = rear_right * rotm;
col_rear_right = col_rear_right + pos;


where rotm is a copied hull.rotation.AIR_MATRIX (ofc same values)

a code for multiplying a vertex by a matrix is:



t3dpoint operator*(const float in[16]) const
{     //since its an opengl engine i use this order
//note that i do not add translation data and do not use scaling
return t3dpoint(
in[0]*x+in[1]*y+in[2]*z,

in[4]*x+in[5]*y+in[6]*z,

in[8]*x+in[9]*y+in[10]*z);
}


No i should say something about how do i calculate rotation matrix:

lets say i have three vectors front, right and up vector

i rotate them by calling pitch, yaw, roll funcitons

and then i build a matrix like this:



AIR_MATRIX[0] = rr.x; //right
AIR_MATRIX[1] = rr.y;
AIR_MATRIX[2] = rr.z;
AIR_MATRIX[3] = 0.0f;
AIR_MATRIX[4] = ru.x; //up
AIR_MATRIX[5] = ru.y;
AIR_MATRIX[6] = ru.z;
AIR_MATRIX[7] = 0.0f;
AIR_MATRIX[8]  = -rf.x; //front
AIR_MATRIX[9]  = -rf.y;
AIR_MATRIX[10] = -rf.z;
AIR_MATRIX[11] = 0.0f;
AIR_MATRIX[12] =  0.0f;  //point
AIR_MATRIX[13] =  0.0f;
AIR_MATRIX[14] =  0.0f;
AIR_MATRIX[15] = 1.0f;


Hopefully its enough info for someone that can help me ;x

oh i forgot to add that whenever i try to pitch the rotation matrix (opengl model draws properly) but i get stranger spheres pos.

Edited by WiredCat

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When calculating the sphere positions you are multiplying on the wrong side of the matrix

float rotm[16]; int i;
for (i=0; i<16;i++) rotm[i] = hull.rotation.AIR_MATRIX[i];

// switch the order of multiplication here
col_front_left = rotm * front_left;
col_front_left = col_front_left + pos;

col_front_right = rotm * front_right;
col_front_right = col_front_right + pos;

col_rear_left = rotm * rear_left;
col_rear_left = col_rear_left + pos;

col_rear_right = rotm * rear_right;
col_rear_right = col_rear_right + pos;

EDIT: Thought I should explain why this is.

For a rotation matrix, the transpose is the same as the inverse

RT = R-1

Changing the order of multiplication is the same as multiplying by the transpose

R * vector = vector * RT

But since the transpose is the same as the inverse you were basically multiplying the points by the inverse of the rotation.

Edited by HappyCoder

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And that worked, thanks alot!

Edited by WiredCat

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First of all i would like you to see this:

tank01.jpg

...cannot be unseen. You still don't have sufficient buttons. ;)

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