Re: Select a NSBezierPath
Re: Select a NSBezierPath
- Subject: Re: Select a NSBezierPath
- From: Graham Cox <email@hidden>
- Date: Tue, 21 Sep 2004 08:54:15 +1000
Detecting hits on the path itself is a moderately hard problem, but
certainly not impossible. If you're doing a lot of this type of thing
with bezier curves NSBezierPath might be too opaque. I'm working on a
vector drawing program and found exactly this, so in the end I decided
to create my own bezier class that handles all the stuff that
NSBezierPath doesn't. A bit of hunting around on the net soon turned up
literally thousands of sites explaining the maths involved, which is
actually very straightforward. By combining my own bezier class with
the drawing features provided by NSBezierPath, I can do all this and
much more. Also search for "graphics gems".
One thing to watch out for though is that the very popular
deCasteljau's recursive algorithm which is frequently put forward as a
"good" way to build beziers is in practice slow - there are much faster
practical methods.
Essentially detecting a hit on the path involves walking the list of
curve points, treating pairs of successive points as defining a
straight line, and calculating the distance from this line of the hit
point. If this distance drops below some defined minimum, it counts as
a hit (note that you have to allow some tolerance, since the path is
infinitely thin, hitting it exactly is nigh on impossible). My class
does some optimisations to cut down on the sheer number of calculations
needed, so in practice the hit testing and other stuff is fast. This
list of points can be quite coarse, since you are not using it to draw
a curve - you can use NSBezierPath for that - which also helps make it
fast.
If you're interested in this approach contact me off list and I'll
happily share the code with you, though at the moment it's rather
specific to my application.
--Graham
On 21 Sep 2004, at 12:28 am, Lorenzo wrote:
Hi,
I have 2 NSBezierPath with lines and curves only (not closed) and I
try to
select one of them at mouseDown
if([bp1 containsPoint:thePoint]){
selected = 1;
}
else if([bp2 containsPoint:thePoint]){
selected = 2;
}
but even if I don't click on the lines, the API "containsPoint"
returns YES.
And this is wrong.
This happens because I click in a zone which, I presume, if the path
was
closed/filled, should appear coloured.
But I click on an empty zone, and the path is not closed, so, since I
don't
click on the lines of the path I should not select it.
This sometimes causes I select the path1 instead of the path2, which
really
is the path I clicked on. Thus this is wrong.
- Q1: How to fix this?
- Q2: How to get the coordinates x and y of "any" point of the path?
As I wrote here earlier, I am trying to build a Sound-Volume/Time
control such a way I can set different volume levels in different
time-points of the song. It's something like the Sound-Volume
control
in iMovie.
- Q3: Am I on the right way?
- Q4: If not, how to create such a control?
Best Regards
--
Lorenzo
email: email@hidden
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