[Development] Changing qreal to a float

[lars.knoll at nokia.com]

On 2/15/12 9:26 AM, "ext Thiago Macieira" <thiago.macieira at intel.com>
wrote:

>On quarta-feira, 15 de fevereiro de 2012 07.38.44,
>gunnar.sletta at nokia.com
>wrote:
>> Our initial idea was to change "qreal" to "double" to avoid any
>>potential
>> regression but the impact on size combined with the fact that the added
>> precision is almost never needed changed our minds
>> (https://bugreports.qt-project.org/browse/QTBUG-23758)
>
>There is one reason for choosing double though: it can store any 32-bit
>integer without loss of precision. So doubles can be used to pass around
>pixel 
>coordinates that are 32-bit integers. That's the reason Wayland chose to
>use 
>it for all coordinates.
>
>Float has 24 bits of mantissa precision, which is enough for
>16777216x16777216 
>pixels. Probably good enough I'd say.
>
>I think the most important thing is to unify the definition. Every time I
>met 
>someone from Canonical, they asked me about it because trying to build Qt-
>based software on ARM revealed lots of float-double mistakes.
>
>Note: changing qreal to float is source-incompatible and requires manual
>fixups 
>where it breaks. But it's not silent.

I don't think it'll break too many places though, so I'm not too worried
about the change.

Choosing float has my vote, as it'll use a lot less memory and is the
right thing in the common case. It also directly maps to OpenGL types.
Let's rather use double explicitly where needed.

Cheers,
Lars