Re: [boost] thoughts on a GUI component library

From: Dave Harris (brangdon_at_[hidden])
Date: 2004-12-16 15:48:25

In-Reply-To: <BAY101-F20FDD5860C530B8D6ADE17A0AC0_at_phx.gbl>
msclrhd_at_[hidden] (Reece Dunn) wrote (abridged):
>    struct position{ float x; float y; };
>    struct size{ float dx; float dy; };
>    struct area
>    {
>       float top; float left;
>       float width; float height;
>    };

Why float rather than double?

I'd want to avoid unnecessary conversions, so I'd want to use the same 
units all the way through. At the moment my document model uses integers, 
scaled not to 96 dpi pixels but to logical units which are 72,000 dpi.

With 32-bit ints that gives a drawing area of about 30,000 inches. Using 
floats would give only 23 bits of precision, which reduces the drawing 
area to around 60 inches, which is getting a bit small. I'd prefer to 
avoid using fractional units because of exactness issues with 1/3 etc.

I don't have a strong opinion on floating point generally. Sometimes I 
want to use pixel rounding, and sometimes I want to transform coordinates 
without losing accuracy. Adobe's PDF is an example which uses a floating 
point coordinates, albeit at 72 dpi rather than 72,000.

I do know that most of the time, using integers works fine. You don't lose 
precision because you only ever transform the coordinates once. For 
bounding box transforms you round left/top with floor and right/bottom 
with ceil. For anti-aliasing you transform, eg, from 72,000 dpi to 384 dpi 
to get 1/4 pixel precision.

-- Dave Harris, Nottingham, UK