Good EPSILON value

This is a discussion on Good EPSILON value within the C Programming forums, part of the General Programming Boards category; Hello everybody. I'm currently working on a simple raytracer. No the time has come to adjust my EPSILON value (eg. ...

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    Good EPSILON value

    Hello everybody. I'm currently working on a simple raytracer. No the time has come to adjust my EPSILON value (eg. for comparing doubles). What is a good value for it? At the momant I have 1e-10 but I'm not long enought confirm with C to say that this value is optimal. Thank you for response.

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    It does depend a little bit on what you're trying to do and what range your numbers are, but if your numbers are relatively small (+/-1.0..100.0 or so) then your value will be fine.

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    and the hat of wrongness Salem's Avatar
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    Somewhere between 12 to 15 decimal places to the right of the smallest values you care about.

    You need to look at the range and precision of the values you're storing to begin with.
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    If you wanted to, you could calculate an epsilon at runtime. Using something like pow(10, log10(number)-15), assuming that your doubles have 15 digits, might work. Of course this would be quite slow, so unless you have wildly variable values, use a hard-coded value.

    I think there may be a constant called DBL_DIGITS in <float.h> which tells you how many digits of precision a double has, but I'm not sure how portable it is.
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    I think there may be a constant called DBL_DIGITS in <float.h> which tells you how many digits of precision a double has, but I'm not sure how portable it is.
    I think it is DBL_DIG, according to C99.
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    Thank you for this good answers!

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    Frequently Quite Prolix dwks's Avatar
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    Yes, that's it. There's also DBL_DIGITS in some implementations, but it's less portable.
    dwk

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