Calculating accurate bounds of stroked paths
Calculating accurate bounds of stroked paths
- Subject: Calculating accurate bounds of stroked paths
- From: Graham Cox <email@hidden>
- Date: Tue, 8 Jul 2008 22:16:06 +1000
I need to know a rect within which all pixels will be painted for a
given path and stroke width. I'd prefer not to make this rect any
larger than it really needs to be. To compute this, I have to factor
in all sorts of bits and pieces such as the angles of the line joins,
miter limits and so on. It's getting so complicated I feel there has
to be an easier way. Surely there's something in Quartz that can do
this?
A close second best would be a rect that was the worst case bounds for
a given stroke width (it wouldn't need to know the actual path, just
its basic bounds), which thus assumed that all angles were acute
enough to trigger the mitre conversion. Even that is proving hard to
figure, as I'm probably not really understand the miter limit
calculation. The docs state:
"The miter limit helps you avoid spikes at the junction of two line
segments connected by a miter join (NSMiterLineJoinStyle). If the
ratio of the miter length—the diagonal length of the miter join—to the
line thickness exceeds the miter limit, the joint is converted to a
bevel join. The default miter limit value is 10, which converts miters
whose angle at the joint is less than 11 degrees."
So what *is it*? A value in degrees? Or a ratio of two sides of a
triangle? What triangle, exactly? How can I tell just how far out I
need to outset the rect to make sure I'm always just outside the edge
of any mitred corner?
thanks for any insight into this, it's getting frustrating. I'm pretty
OK at trig generally, but the definition is a bit vague, so I'm not
sure what I should be using as my starting points (I possibly mean
that literally!).
cheers, Graham_______________________________________________
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