multmatrix()

I am trying to write a function that multiplies two 4x4 matrices. The only problem is with indexing. In openGL the matrices are oriented like so

Code:

---

0 4 8 12
1 5 9 13
2 6 10 14
3 7 11 15

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So instead of creating a 2 dimensional array. people create a single array of 16. now when i go to multiply it becomes difficult. I can't seem to come up with a double for loop way to correctly multiply the matrices. Does anyone have any suggestions?

Code:

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m0 m4 m8 m12
m1 m5 m9 m13
m2 m6 m10 m14
m3 m7 m11 m15

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Depending on the order of the matrix this function varies. However you should be able to get it.

Code:

---

for (int i=0;i<4;i++)
{
  for (int j=0;j<4;j++)
  {
    for (int k=0;k<4;k++)
    {
      matOut[i][j]+=mat1[i][k]*mat2[k][j];
    }
  }
}

---

Now unroll the inner loops.

matOut[i][j]=(mat1[i][0]*mat2[0][j])+(mat1[i][1]*mat2[1][j]).....

etc, etc.

You can also unroll the j loop with a bit more code.
Now once you unroll it, use the matrix I provided to eliminate all of the loops.

but my array is only a one dimensional array. Every book that I have read has said to keep it this way as to not confuse with c's implementation of 2 dimensional arrays. Should I declare my array otherwise?

It's a strange advise considering matrixes are traditionally 2D containers. Also, from your own experience it becomes harder to code operations on matrixes built from a one dimensional array.

Regardless, if you insist on a 1D array, the solution is to know what are the bounds of each dimension. A 4x4 1D matrix is of size 16, with the second dimension starting at element 8 and ending at element 15

EDIT: Oops! My bad... Don't know where I had my head...

... with the second dimension starting at element 4 and ending at element 7, third starting at 8 and ending at 11, and so forth. Basically the offset of every dimension starting point is 4 indexes

Just index as (column + size* row) instead of (size* column + row) in your 1D array

Code:

---

size_t location[] =
{
    0, 4, 8, 12,
    1, 5, 9, 13,
    2, 6, 10, 14,
    3, 7, 11, 15,
};

opengl[ location[ x ] ] = whatever;

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Lookup table.


Quzah.

That is a 1D array. Check my diagram. I was stating you could do:

m11*m21*.... etc, etc

Quote:

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Originally Posted by Bubba

That is a 1D array. Check my diagram.

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Yup.

Quote:

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Originally Posted by linuxdude

but my array is only a one dimensional array. Every book that I have read has said to keep it this way as to not confuse with c's implementation of 2 dimensional arrays. Should I declare my array otherwise?

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Quzah.

oh ok. I see now. Thanks a lot.