I'm having some problems implementing a function that'll multiply two matrices. I've been testing it with the identity matrix, since I've heard that any matrix multiplied with the identity matrix won't change (which makes it good for testing my function ). Here's my matrix struct: typedef struct { float v[4][4]; } MATRIX; Here's how I construct my identity matrix: const MATRIX identity_matrix(void) { MATRIX result; memset(&result, 0, sizeof(result)); result.v[0][0] = result.v[1][1] = result.v[2][2] = result.v[3][3] = 1; return result; } And this is the function that should multiply two matrices: MATRIX matrix_mul(MATRIX m1, MATRIX m2) { MATRIX result; int col, row; float sum; for(col = 0; col < 4; col++) { sum = 0; for (row = 0; row < 4; row++) { sum += m1.v[row][col] * m2.v[row][col]; result.v[row][col] = sum; } } return result; } And this is the function I use to print matrices to screen (but I don't think that there's anything wrong with it): void matrix_print(MATRIX m) { printf("[%.0f, %.0f, %.0f, %.0f]\n", m.v[0][0], m.v[0][1], m.v[0][2], m.v[0][3]); printf("[%.0f, %.0f, %.0f, %.0f]\n", m.v[1][0], m.v[1][1], m.v[1][2], m.v[1][3]); printf("[%.0f, %.0f, %.0f, %.0f]\n", m.v[2][0], m.v[2][1], m.v[2][2], m.v[2][3]); printf("[%.0f, %.0f, %.0f, %.0f]\n", m.v[3][0], m.v[3][1], m.v[3][2], m.v[3][3]); } I'm sorry if the code looks weird, but it's not my fault, it's the forums fault I can add and subtract matrices, but this multiplying stuff really makes my head hurt Can anyone help me figure out what's wrong with the matrix_mul() function? /. Muzzafarath Mad House Software Edited by - Muzzafarath on 6/14/00 12:37:23 PM Edited by - Muzzafarath on 6/14/00 12:38:02 PM Edited by - Muzzafarath on 6/14/00 12:39:05 PM

The solution:

sum += m1.v[row][col] * m2.v[col][row];

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Thanks for the replies, but I don't think it's working. I've changed the matrix_mul() function to this:

MATRIX matrix_mul(MATRIX m1, MATRIX m2)
{
MATRIX result;

int col, row;
long sum;

for(col = 0; col < 4; col++)
{
sum = 0;

for (row = 0; row < 4; row++)
{
sum += m1.v[row][col] * m2.v[col][row];

result.v[row][col] = sum;

}
}

return result;
}

When I multiply the matrix

[1, 0, 0, 0]
[0, 1, 0, 0]
[0, 0, 1, 0]
[0, 0, 0, 1]

with

[5, 0, 0, 3]
[0, 0, 0, 0]
[0, 0, 0, 0]
[0, 0, 7, 0]

I get

[5, 0, 0, 0]
[5, 0, 0, 0]
[5, 0, 0, 0]
[5, 0, 0, 0]

Shouldn't I get

[5, 0, 0, 3]
[0, 0, 0, 0]
[0, 0, 0, 0]
[0, 0, 7, 0]

since the first matrix is the identity matrix, and identity matrix multiplied with some other matrix returns "the other matrix"?

I've got to get a book on this subject! It's way too hard to learn this stuff from tutorials on the internet

/. Muzzafarath
Mad House Software

Edited by - Muzzafarath on June 14, 2000 4:16:53 PM

Nevermind, I figured it out my self (well, not on my own, someone helped me, but it solved my problem ):

MATRIX matrix_mul(MATRIX m1, MATRIX m2)
{
MATRIX result;
int i, j;

for (j = 0; j < 4; j++)
{
for (i = 0; i < 4; i++)
{
result.v[j] = m1.v[0] * m2.v[0][j] + m1.v[1] * m2.v[1][j] +<br> m1.v[2] * m2.v[2][j] + m1.v[3] * m2.v[3][j];<br> }<br> }<br><br> return result;<br>}<br></code><br>/. <a href=mailto:de@goteborg.mail.teila.com>Muzzafarath</a><br><a href=http://w1.315.telia.com/~u31504183/>Mad House Software</a><br><br>Edited by - Muzzafarath on June 14, 2000 4:29:34 PM<br><br>Edited by - Muzzafarath on June 14, 2000 4:30:36 PM

Yes I know that D3D has premade matrix operators and such, but I''ll learn a lot more about matrices if I write my own

/. Muzzafarath
Mad House Software