GLM multiplication order - c++

Question

I am a little amazed. I have been debugging my code for hours now, and GLM seems to be giving up on me. I am struggling with the following 2 instances:
....
cout << "multiplying A:" << endl;
displayMatrix(node->wMatrix);
cout << "and B:" << endl;
displayMatrix((node->children)[i]->wMatrix);
//switch order!
mat4 temp = (node->children)[i]->wMatrix * node->wMatrix;
cout << "Get result as:" << endl;
displayMatrix(temp);
...
The displayMatrix method is as follows:
void displayMatrix(mat4 &m)
{
cout << m[0][0] << " " << m[0][1] << " " << m[0][2] << " " << m[0][3] << endl;
cout << m[1][0] << " " << m[1][1] << " " << m[1][2] << " " << m[1][3] << endl;
cout << m[2][0] << " " << m[2][1] << " " << m[2][2] << " " << m[2][3] << endl;
cout << m[3][0] << " " << m[3][1] << " " << m[3][2] << " " << m[3][3] << endl;
}
Here is the output I get:
multiplying A:
1 0 0 0
0 1 0 0.5
0 0 1 0
0 0 0 1
and B:
0.540302 -0.841471 0 0
0.841471 0.540302 0 -0.5
0 0 1 0
0 0 0 1
Get result as:
0.540302 -0.841471 0 0
0.841471 0.540302 0 0
0 0 1 0
0 0 0 1
NOTICE that in the code above, the matrix multiplication order is the reverse of what you would write on paper. In other words, the code says B * A. I was very thrown off by this.
The second instance:
cout << "temp:" << endl;
cout << temp.x << " " << temp.y << " " << temp.z << " " << temp.w << endl;
cout << "binding matrix inverse: " << endl;
displayMatrix(bindingInvs.at(jIndex));
temp = bindingInvs.at(jIndex) * temp;
cout << "now temp:" << endl;
cout << temp.x << " " << temp.y << " " << temp.z << " " << temp.w << endl;
cout << "joint world matrix: " << endl;
displayMatrix(joints.at(jIndex)->wMatrix);
temp = (joints.at(jIndex)->wMatrix) * temp;
cout << "now temp:" << endl;
cout << temp.x << " " << temp.y << " " << temp.z << " " << temp.w << endl;
cout << "weight: " << jWeight << endl;
temp = jWeight * temp;
cout << "now temp:" << endl;
cout << temp.x << " " << temp.y << " " << temp.z << " " << temp.w << endl;
The output that I get now is:
temp:
0.087 0 -0.05 1
binding matrix inverse:
1 -0 0 -0
-0 1 -0 0
0 -0 1 -0
-0 0 -0 1
now temp:
0.087 0 -0.05 1
joint world matrix:
1 0 0 0
0 1 0 0.5
0 0 1 0
0 0 0 1
now temp:
0.087 0 -0.05 1
weight: 1
now temp:
0.087 0 -0.05 1
Temp is never getting changed for some reason. I don't know what to do, or why this is happening. My programs compiles and runs (I am pasting from output above). Of course, this is not the entire program. This is only the steps for debugging. But I feel confident that this much should be enough to tell what's going on.

Answer 1

Your displayMatrix function is confusing you, since you print the matrices transposed to what you would expect on paper. GLM uses column major ordering, so the addressing is m[col][row].
Now with that in mind, the operation A*B is actually what you should expect.
For the temp vector, the same problem arises: the first matrix you multiply it by is identity, so it is unchanged. The second matrix is identity, except the last row is 0 0.5 0 1, so x, y and z will be unchanged and the new w' will be 0.5 * y + w. Since y is 0 to begin with, nothing is changed here,too.