[vtk-developers] Call for Comments: Ascii representation of floats and doubles in files

[Utkarsh Ayachit]

Funny coincidence, I just got bit but this double-string precision
problem in a ParaView issue [1]. Looks forward to having this library
in VTK so we can use it. Bill, if you need help with the 3rd party
stuff, just holler.

Utkarsh


[1] https://gitlab.kitware.com/paraview/paraview/issues/18083

On Wed, Mar 21, 2018 at 8:10 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
> I'm still working in the MR. It works great but I'm having cmake issues on
> Windows. Adding a new third party library is tricky.
>
> On Mar 21, 2018 2:08 PM, "Isaiah Norton" <isaiah.norton at gmail.com> wrote:
>>
>> +1. I needed this recently in Slicer, and ended up using ITK's version
>> because we link it anyway.
>>
>>>
>>> Not sure if we need to add a new third-party library for this, though,
>>> especially one that duplicates so much of C++'s built-in machinery.
>>
>>
>>  Grisu3 (double-conversion) appears to be ~13x faster than the C++ stream
>> functions (and 6x faster than sprintf):
>>
>>   https://github.com/miloyip/dtoa-benchmark
>>
>> Those numbers are on VS2013/Windows, but that code is easy to build
>> locally. The sprintf vs double-conversion conclusion holds on 2015 macbook
>> with 10.12 SDK. The C++ stream functionality is somewhat closer to sprintf.
>>
>> [usual benchmark NaCl disclaimer here]
>>
>> On Thu, Mar 1, 2018 at 11:32 AM, David Gobbi <david.gobbi at gmail.com>
>> wrote:
>>>
>>> I agree that floats should be printed so that conversion is lossless.
>>> Python's console output does this and I find it incredibly useful.
>>>
>>> Not sure if we need to add a new third-party library for this, though,
>>> especially one that duplicates so much of C++'s built-in machinery.
>>> Isn't it just a matter of printing enough digits?  That's basically what
>>> I do in my own code (plus checks for isnan(x) and isinf(x), and a
>>> check of the size of fabs(x) for whether to force scientific notation.)
>>>
>>>  - David
>>>
>>>
>>> On Thu, Mar 1, 2018 at 8:48 AM, Bill Lorensen <bill.lorensen at gmail.com>
>>> wrote:
>>>>
>>>> Will,
>>>>
>>>> It would go in Common/Core I think. The vtk class would be very small.
>>>>
>>>> Bill
>>>>
>>>> On Thu, Mar 1, 2018 at 7:36 AM, Will Schroeder
>>>> <will.schroeder at kitware.com> wrote:
>>>> > If it works why mess with success? Where would this class
>>>> > (vtkNumberToString) be placed, in Common/Core (like vtkIndent) or in
>>>> > some IO
>>>> > directory like VTK/IO/Core?
>>>> >
>>>> > On Thu, Mar 1, 2018 at 10:06 AM, Bill Lorensen
>>>> > <bill.lorensen at gmail.com>
>>>> > wrote:
>>>> >>
>>>> >> Folks,
>>>> >>
>>>> >> I'm working on some examples to save/restore camera, polydata,
>>>> >> property, etc. in XML files. The files can be saved as binary or
>>>> >> ascii. If I save/restore the info in binary, I get the same values
>>>> >> for
>>>> >> the saved info. But, to no surprise, when I save the same data in
>>>> >> ascii, I don't get the same values. This is because the file writing
>>>> >> code does not write enough significant digits.
>>>> >>
>>>> >> In ITK we solved this problem many years ago. ITK uses a reduced
>>>> >> version of the Google double-conversion library,
>>>> >> https://github.com/google/double-conversion.
>>>> >>
>>>> >> We added a small class to ITK that uses the library. The class,
>>>> >> itkNumberToString, has a simple API. To convert floating and fixed
>>>> >> point numbers to ascii without numerical precision erros,
>>>> >>
>>>> >> For example, to convert a float,
>>>> >> #include "itkNumberToString.h:
>>>> >> itk::NumberToString<float> convert;
>>>> >> float a = 1.0f/3.0f;
>>>> >> std::cout << convert(a) << std::endl;
>>>> >>
>>>> >> I propose a similar approach in VTK.
>>>> >>
>>>> >> Comments please,
>>>> >>
>>>> >> Bill
>>>
>>>
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>>
>
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