multmatrix()

This is a discussion on multmatrix() within the Game Programming forums, part of the General Programming Boards category; I am trying to write a function that multiplies two 4x4 matrices. The only problem is with indexing. In openGL ...

  1. #1
    Registered User linuxdude's Avatar
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    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
    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?

  2. #2
    Super Moderator VirtualAce's Avatar
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    Code:
    m0 m4 m8 m12
    m1 m5 m9 m13
    m2 m6 m10 m14
    m3 m7 m11 m15
    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.
    Last edited by VirtualAce; 08-17-2006 at 03:36 AM.

  3. #3
    Registered User linuxdude's Avatar
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    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?

  4. #4
    (?<!re)tired Mario F.'s Avatar
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    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
    Last edited by Mario F.; 08-17-2006 at 04:25 PM.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
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  5. #5
    Crazy Fool Perspective's Avatar
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    Just index as (column + size* row) instead of (size* column + row) in your 1D array

  6. #6
    ATH0 quzah's Avatar
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    Code:
    size_t location[] =
    {
        0, 4, 8, 12,
        1, 5, 9, 13,
        2, 6, 10, 14,
        3, 7, 11, 15,
    };
    
    opengl[ location[ x ] ] = whatever;
    Lookup table.


    Quzah.
    Hope is the first step on the road to disappointment.

  7. #7
    Super Moderator VirtualAce's Avatar
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    That is a 1D array. Check my diagram. I was stating you could do:

    m11*m21*.... etc, etc

  8. #8
    ATH0 quzah's Avatar
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    Quote Originally Posted by Bubba
    That is a 1D array. Check my diagram.
    Yup.

    Quote 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?

    Quzah.
    Hope is the first step on the road to disappointment.

  9. #9
    Registered User linuxdude's Avatar
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    oh ok. I see now. Thanks a lot.

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