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

[Bache-Wiig Jens]

> 
> Think about a scatter plot with many points displayed as crosses. On a 
> large zoom level you only see a cloud but when zooming in you can see 
> each cross and its position in detail.

True. There are certainly plenty of valid use cases out there and some of those would require a minor change in Qt5 if we go ahead with this change. Most likely no more than setRenderHint(Qt::CosmeticDefaultPen, true). 

QPen right now is inconsistent and prone to errors. QPen() and QPen(Qt::red) Is not only defaulting to cosmetic, but also (arbitrarily and arguably incorrect on retina) a pen size of 1 pixel. QPen(Qt::black, 2), is non-cosmetic with a width of 2. In fact all non-0 values for pen width are non-cosmetic by default so making only the default constructor a special case with a cosmetic pen of size 1 is arbitrary and confusing regardless if you are using a cosmetic pen or not in your application.

Thinking more about it, I think even maintaining QPen(colour, 0) as a special case with the current behavior is wrong. If a user needs cosmetic pen for a graph, he/she should always use setCosmetic(true) on the pen and be explicit about it so that any pen size he defines is cosmetic. Chances are anyone creating a non-trivial chart or pdf application will already have to do this because you have to be able to deal with more than one line thickness anyway.

Jens