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  1. Qt
  2. QTBUG-58083

Be consistent about the value of pi

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    Details

    • Type: Bug
    • Status: Open
    • Priority: P3: Somewhat important
    • Resolution: Unresolved
    • Affects Version/s: 5.6, 5.7, 5.8
    • Fix Version/s: None
    • Component/s: Examples and Demos, Other
    • Labels:
      None

      Description

      We have a qmath.h that, sensibly enough, does

      #ifndef M_PI
      #define M_PI (3.14159265358979323846)
      #endif
      

      Despite this, 302 other places in my Qt-dev source tree supply their own values for pi. There is some diversity of values. Let's start with the correctly-rounded ones:

      • 3.14159
      • 3.14159265 (enough for float - see below)
      • 3.14159265358979
      • 3.141592653589793 (enough for double - see below)
      • 3.1415926535897932
      • 3.141592653589793238
      • 3.14159265358979323846 (same as M_PI in qmath.h)
      • 3.1415926535897932384626433832795
      • 3.14159265358979323846264338327950288
      • 3.14159265358979323846264338327950288419717
      • 3.14159265359
      • 3.1415927

      Then the truncated ones that aren't correct roundings (in each case, the last digit needs +1):

      • 3.1415
      • 3.141592
      • 3.1415926
      • 3.1415926535
      • 3.141592653589

      Finally, the values that look suspiciously like they were meant to be pi, but aren't (inserting a space after the last matching digit):

      • 3.1415 2 (missing 9) In several parameters to sin(...)
      • 3.14159 62 (last two digits swapped) In some JS benchmarks
      • 3.14159265358979 23846 (missing 3) In an ECMA JS test

      I am fairly sure the parameters to sin were meant to use pi; and I suspect changing the JS tests is out-of-scope as they look like imports (i.e. third-party).

      Some of these are in JS (which can use Math.PI), including some in QML.
      The ones in C++ code should surely use M_PI (and #include <qmath.h> to access it).
      Some are in shaders, where we can't use either of those, but can at least be consistent about which literal value we use.

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        For Gerrit Dashboard: QTBUG-58083
        # Subject Branch Project Status CR V

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            • Assignee:
              Eddy Edward Welbourne
              Reporter:
              Eddy Edward Welbourne
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              Dates

              • Created:
                Updated: