In-Reply-To: <434426EF.5030808@yahoo.com.au> mw8329@yahoo.com.au (Martin Wille) wrote (abridged):

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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.