On Thursday 28 August 2008 15:43:34 Alexis Ménard wrote:
> I'm quite interrested by understand the difference of the two types QPen, i
> mean uses cases and so
>
> On Thu, Aug 28, 2008 at 9:30 PM, Diego Iastrubni <elcuco@kde.org> wrote:
> > can any one of you graphics ninjas explain to us n00bz the difference
> > between a cosmetic pen and a non cosmetic pen...?

I think there's a lot of documentation for them in the Qt documentation of 
QPen.
Instead of paraphrasing it here, I'm going  to add some thing about it here. 
So 1) check the QPen docs, 2) read the below :)

In vector graphics cosmetic pens are known as the "non-scaling stroke" (in the 
SVG spec and in the OpenVG spec).

When stroking (or in Qt terms "when drawing the path with a QPen") the 
rendering engine actually generates a second path. The path is essentially an 
outline of the path that we want to stroke. The generation of the outline is 
governed by some parameters of the stroke/QPen, in particular those are end 
caps, join style and the width.

When rendering a path the path is decomposed and each point is transformed by 
a transformation matrix. So if the stroke is not transformable that means that 
for all vector graphics api, you'll avoid a matrix multiplication for every 
element in a path (which for larger paths will be visible).

More over the cosmetic pen of width 0 has a special meaning in Qt because it 
means that as long as round join and caps aren't used instead of a complex 
path the outline can be imitated with a set of line (a polyline). That means 
that rendering a stroke equals rendering a few lines, versus rendering a very 
complex polygon (e.g. on X11 Qt uses XDrawLine instead of tessellating the 
path and using XRenderCompositeTrapezoids when it's rendering a opaque 
cosmetic pen).

Hope that helps.

z


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