[Qt-interest] Keep Same QGraphicsItem's Pen Width Across Scaling

[anonymous anonymous]

Hi again.
I dont know if I have a problem with my mailing list account, but for the
3rd time: I think what you are looking for is exactly
QPen::setCosmetic(true). No more, no less. No matrix or other hack involved.
Cosmetic pen do not have their width impacted by any item transformation.



2009/10/2 Josiah Bryan <jbryan at productiveconcepts.com>

> Ben Swerts wrote:
> > Hi Josiah,
> >
> > In the paint function of your custom QGraphicsItem you can retrieve the
> > transformation matrix of the painter (QPainter::transform). You can then
> > multiply the pen width with the inverse of the scaling value to obtain a
> > fixed width.
> >
> Hey Ben -
>
> Thanks! Worked perfectly. After reading
> http://doc.trolltech.com/4.3/qtransform.html#basic-matrix-operations, I
>  found I could use m11 and m22 for scaling. For the record, here's the
> code that I finally came up with:
>
> QPen p = ...;
> QTransform tx = painter->transform();
> qreal scale = qMax(tx.m11(),tx.m22());
> p.setWidthF(1/scale * p.widthF());
> painter->setPen(p);
>
> Not sure if qMax or qMin would be more appropriate to extract a 1-D
> scale value for our use, so I rolled the dice and decided to use qMax.
> Looks good enough for my use.
>
> Thanks again for your help, Ben!
>
> Cheers!
> -josiah
>
>
> >
> >> I've got a custom QGraphicsItem (simple box) that draws an outline
> >> around itself, say 3px wide. Great for "normal" live view. However, my
> >> app also renders an "icon" view of the scene that the box is in - this
> >> "icon" is about 96x96. Then there's also the "preview" view of the scene
> >> before it goes live, this is rendered about 320x240. The live scene is
> >> scaled approx 1:1, so its about 1024x768.
> >>
> >> What I'm wondering is if there's any way for my custom graphics item to
> >> find out the scale it's being rendered at so that it can adjust it's pen
> >> width accordingly inorder to render at approx 3px width regardless of
> >> scale? (The outline isn't always 3px - it can vary depending on various
> >> other factors [type of box, datapoint the box represents, etc]).
> >>
> >> Any ideas on how to find the scale of the scene from inside the paint
> >> method of my graphics item?
> >>
> >> Thanks!
> >> -josiah
> >
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>
> --
>
> -=-=-=-=-=-=-=-=-=-=-=-=-
> Josiah Bryan
> Productive Concepts, Inc.
> jbryan at pciint.com
> (765) 964-6009, ext. 224
>
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