Re: [boost] GUI foundations

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-10-04 12:52:35

In-Reply-To: <43426645.30304_at_[hidden]>
john.lists_at_[hidden] (John Torjo) wrote (abridged):
> And, about usability in code. Why would I want in my program,  both:
> gui::rect<pc,double> and gui::rect<pc,int> ?

For my apps I have found there needs to be two levels. There's a 
device/raster level, which wants to use pixels and ints, and there's a 
logical/world level, which uses units like inches or cm and floating 
point.

The device/raster level uses pixels and ints because that's what raster 
hardware uses. This is not something you can really abstract away from. 
For example, it's important to scroll by an integral number of pixels else 
anti-aliasing makes the lines change colour.

The logical/world level needs to be device-independant. It probably needs 
to be scale-invariant too; for example, a rectangle that measures 2x3 
needs to keep its aspect ratio if you halve its size. My employers 
actually use ints and very small units - 1/72000ths of an inch - which are 
also the units used by Postscript and PDF. That decision now seems dated 
to me and more and more of my geometry code is moving to floating point. 
It works better.

It may be that boost doesn't need to tackle the logical/world level 
immediately. The window-manager stuff and the low-level drawing stuff can 
use pixels and ints. However, at least some programs will want a logical 
level too when they come to draw.

> Just think of the many places you'll need to deal with geometry 
> (rectangle/point/size), and converting from one type of unit (px to cm, 
> cm to in, etc) to another.

I'd rather explicit conversion than an attempt to abstract away the 
differences. Typically there is a world-to-device linear transform that 
does the conversion. A single matrix operation provides rotation, 
scrolling and zoom in addition to scaling from inches or centimetres or 
whatever.

-- Dave Harris, Nottingham, UK.