Re: [boost] GUI coordinate units (Was: GUI foundations)

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-10-09 12:45:17

In-Reply-To: <434426EF.5030808_at_[hidden]>
mw8329_at_[hidden] (Martin Wille) wrote (abridged):
> > But you aren't going to draw a PCB on screen with boost.GUI widgets.
> 
> Why shouldn't a good GUI library be prepared for that use case?

I think he's saying that the use case would be handled by a different 
component of the library.

I'm coming around to the idea that the library should use ints and device 
pixels for its core graphics units. Or rather, some fraction of pixels: 
probably 100 or 256 units per pixel. All other units eventually get 
converted to this. The conversion is dependant on the device, and if a 
component needs to be device-independant it uses its own units and defers 
conversion until it knows what device it is using.

Many widget calculations won't care what the units are at all. Some sizing 
happens top-down. The user drags out a window, the app gives half the 
space to one widget and half to the other. It doesn't care what the units 
are, it is just dividing by two.

Some sizing happens bottom-up. Often this is driven by text. A button 
needs enough room to display the word "Cancel" in 12-point text.

    int height = 12 * device.points_to_gu();
    Font font( "Arial", height );
    int width = font.width( "Cancel" );

I don't think there is any need to force the width to be in absolute 
units. Both millimetres and inches would be wrong for someone.

A PCB would be laid out with a CAD app, which would measure in 
millimetres. Part of its rendering pipeline would be a conversion from mm 
to graphic units.

A dialog editor is a different kind of app. It might measure in points and 
have a different conversion. The Microsoft one uses what it calls "dialog 
units", which are scaled according to dialog's chosen font, which can in 
principle be changed by the end-user.

We don't need to force the dialog editor to use the same internal units as 
the CAD app. When they need to communicate, they can use pixels units * 
256 a lingua-franc. I do not think the need for a device-independant 
linga-franca is compelling enough for it to dominate the core GUI.

-- Dave Harris, Nottingham, UK.