Thread: Reverse of sine table (back to code)

  1. #1
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    Reverse of sine table (back to code)

    Hi Guys
    I have this sine table for various things in a program such as sine
    text and plasma that Iíve been using for the last million years.
    Now programming for a micro, and with code running low,
    I wonder if anyone can tell me how to generate it at run time (or pretty close).

    Basically, if I start with the same array not explicitly defined like this:
    Code:
    WORD sin_table[91];
    and then populate it with code, I assume Iíll save some program memory space.
    Any help appreciated (itís not homework just hobby).
    Cheers, Art.


    Code:
    WORD sin_table[91] ={
        0,   285,   571,   857,  1142,  1427,  1712,  1996,  2280,  2563,
        2845,  3126,  3406,  3685,  3963,  4240,  4516,  4790,  5062,  5334,
        5603,  5871,  6137,  6401,  6663,  6924,  7182,  7438,  7691,  7943,
        8191,  8438,  8682,  8923,  9161,  9397,  9630,  9860, 10086, 10310,
        10531, 10748, 10963, 11173, 11381, 11585, 11785, 11982, 12175, 12365,
        12550, 12732, 12910, 13084, 13254, 13420, 13582, 13740, 13894, 14043,
        14188, 14329, 14466, 14598, 14725, 14848, 14967, 15081, 15190, 15295,
        15395, 15491, 15582, 15668, 15749, 15825, 15897, 15964, 16025, 16082,
        16135, 16182, 16224, 16261, 16294, 16321, 16344, 16361, 16374, 16381,
        16384
    };

  2. #2
    and the hat of int overfl Salem's Avatar
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    So what are you trying to optimise?

    How fast?
    How small?
    How accurate?

    If you can't spare 180 bytes for a table, then perhaps compute it at run time.
    Bhaskara I's sine approximation formula - Wikipedia
    Maclaurin Series -- from Wolfram MathWorld
    Though if your micro lacks mul (and especially div), that could be pretty slow. But if you're looking for small code to create a table in RAM it might be worth it.

    With only 90 elements, do you really need 16 bits of precision to go with it?

    Are you performing any interpolation on the results?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Slow, Very, The closer the better.

    Something I realised, I have a function implemented to rotate a point about an origin for graphics x/y coordinates.
    So if the point starts 16384 units under the origin, I should be able to to a for loop with 90 steps incrementing the angle 1 degree each iteration,
    and only look at the y offset for the result.
    Does that sound right?
    That would cost me almost no code.

    Thatís way off the screen, but I donít have to draw it.

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