[Development] Comparing two reals in Qt code

[Sean Harmer]

On 29/11/2012 18:34, BRM wrote:
>> 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.

I have a work in progress patch somewhere that does similar to what you 
suggest with versions for absolute or relative comparisons using ideas 
from the link I posted earlier. I'll tidy it up and see if it's good 
enough to get into 5.1.

Cheers,

Sean


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