[Development] Proposal: Make QPen non-cosmetic by default

[Tony Van Eerd]

I think Windows also uses 0-width to mean the same thing.

On the other hand, I worked on a drawing library that used sub-pixel lines.  Via good anti-aliasing, A 0.5 width line, even if vertical/horizontal, would be drawn semi-transparent.  A 0-width line would thus be invisible.



> -----Original Message-----
> From: development-bounces+tvaneerd=rim.com at qt-project.org
> [mailto:development-bounces+tvaneerd=rim.com at qt-project.org] On Behalf
> Of Samuel Rødal
> Sent: Friday, October 12, 2012 11:02 AM
> To: development at qt-project.org
> Subject: Re: [Development] Proposal: Make QPen non-cosmetic by default
> 
> On 10/12/2012 03:17 PM, Uwe Rathmann wrote:
> > On Fri, 12 Oct 2012 12:21:30 +0000, Bache-Wiig Jens wrote:
> >
> >> After all what is the point of doing a
> >> major version unless we don't even allow ourselves to change broken
> >> defaults.
> >
> > There is nothing broken: it's a well defined API that behaves exactly
> > like it is documented. Your suggestion is about modifying an
> illogical
> > API.
> >
> > I don't know the reasons why the API was decided once the way it is -
> but
> > it is so confusing, that I can't believe that it happened by accident
> > ( being documented later ). My guess is that it was following some
> other
> > system that did it this way.
> 
> Originally a 0-width pen was the only way to get the cosmetic pen
> behavior. Later on setCosmetic() was added to be able to have any pen
> width be cosmetic.
> 
> I guess the whole 0-width thing comes from X11, where 0 is a special
> value with the following documentation: "Thin lines (zero line-width)
> are one-pixel-wide lines drawn using an unspecified, device-dependent
> algorithm."
> 
> http://tronche.com/gui/x/xlib/GC/manipulating.html also contains this
> bit of documentation:
> 
> "A line-width of zero may differ from a line-width of one in which
> pixels are drawn. This permits the use of many manufacturers' line
> drawing hardware, which may run many times faster than the more
> precisely specified wide lines.
> 
> In general, drawing a thin line will be faster than drawing a wide line
> of width one. However, because of their different drawing algorithms,
> thin lines may not mix well aesthetically with wide lines. If it is
> desirable to obtain precise and uniform results across all displays, a
> client should always use a line-width of one rather than a line-width
> of
> zero."
> 
> So 0-width lines in X11 are not only implicitly cosmetic, but also have
> less strict guarantees when it comes to correctness, so that they can
> be
> optimized by custom hardware. I guess originally QPen simply mirrored
> this behavior, which is probably why as you noted 0-width pens are
> faster than 1-width pens with the X11 paint engine in 4.x.
> 
> Still, I agree with Jens that the API of having 0-width pens and
> cosmetic pens as separate concepts is confusing. If we wanted something
> akin to the 0-width concept of X11 it should rather have been a render
> hint saying that it's acceptable for the application to get less
> accurate line drawing in exchange for better performance. I'm not sure
> how valid that use case is these days though.
> 
> --
> Samuel
> 
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