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

[Josiah Bryan]

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