[Development] Comparing two reals in Qt code

[BRM]

> From: Alan Alpert <416365416c at gmail.com>
> Sent: Thursday, November 29, 2012 11:52 AM
> Subject: Re: [Development] Comparing two reals in Qt code
> 
> On Thu, Nov 29, 2012 at 6:53 AM, Sean Harmer <sean.harmer at kdab.com> wrote:
>>  On Thursday 29 November 2012 15:01:05 Dominik Holland wrote:
>>>  Hi all,
>>> 
>>>  last month i had some performance problems on a QML animation running 
> on
>>>  low end hardware (imx233).
>>>  I debugged that problem and it was caused because of rounding errors
>>>  during the comparison of two reals.
>>> 
>>>  I fixed that problem by replacing the comparison by a qFuzzyCompare() 
> in
>>>  the Qt4 declarative source code.
>>>  Afterwards i checked whether qt5 has the same problem... and it has
>>>  exactly the same problem.
>>> 
>>>  Now i pushed my patch to gerrit
>>>  (https://codereview.qt-project.org/#change,40655) and looked again at
>>>  the code
>>>  for any other occurrences.
>>> 
>>>  What i found is, that this happens in many places and it would be a
>>>  really huge effort to change the code to compare the qreals in the 
> right
>>>  way.
>>> 
>>>  Is this already a known problem and do you think that it is worth to 
> try
>>>  to fix the code ?
>> 
>>  Also please be aware that qFuzzyCompare() is nto the right way to compare 
> two
>>  arbitrary floating point values. If one of them is zero or close to zero 
> the
>>  condition it tests for becomes impossible to satisfy.
>> 
>>  Something along the lines of
>> 
>>  fuzzyCompare(float p1, float p2)
>>  {
>>      if (qFuzzyIsNull(p1))
>>          return qFuzzyIsNull(p2);
>>      else if (qFuzzyIsNull(p2))
>>          return false;
>>      else
>>          return qFuzzyCompare(p1, p2);
>>  }
>> 
>>  is slightly better but still not fantastic. Comparing two floats that we do
>>  not know a priori is actually very difficult
>> 
>>  http://randomascii.wordpress.com/2012/02/25/comparing-floating-point-
>>  numbers-2012-edition/
>> 
>>  I would love to see a better qFuzzyCompare() implementation in Qt but the
>>  existing one is used in so many places it would be a nightmare to introduce
>>  without unwanted side effects.
> 
> What about a second qFuzzyCompare using an absolute instead of
> relative epsilon? Something like 1e-5 should give a decent answer for
> values between 1e5 and -1e5, which could be used for values like on
> screen positions.
> 
> By having two functions for developers to choose from we'd mitigate
> the a priori knowledge problem, because some developers will know
> roughly which floats they're going to be testing and they can pick the
> right function for that.
> 

How about allow the epsilon to be specified as an argument?
Then developers who need more resolution can get it, and it still mitigates the a priori issue
as it is (i) better documented, and (ii) documented in user code for what the developer requires
or thought they were getting.

Ben