Re: Select a NSBezierPath
Re: Select a NSBezierPath
- Subject: Re: Select a NSBezierPath
- From: Graham Cox <email@hidden>
- Date: Tue, 21 Sep 2004 18:11:34 +1000
It sounds simpler, but it's much slower.
When you draw a Bezier curve, you have to first compute the curve, then
rasterize it into a buffer, then hit test that. The mathematical
approach involves only the first of these steps, and it can bail early
if a hit is found. The drawing step is the killer, dwarfing the time
the rest takes. I'm not sure if there are ways to get around this with
NSBezierPath - I guess turning off antialiasing and setting the
flatness to a coarser value would help.
I'm not dismissing the approach, but I found that for 2D drawing and
hit testing, the calculation approach was much faster. If the graphics
don't change much, I guess the drawing step can be avoided most of the
time, so it may become a better technique for some situations. In a
drawing editor, by definition the graphics tend to change quite often.
--Graham
On 21 Sep 2004, at 9:50 am, John C. Randolph wrote:
There is a much simpler way to do this, which Michael Johnson at Pixar
explained to me a couple of years ago. The approach he described is
to use an off-screen drawing destination, and draw each object that
you want to check for hits in a distinct color. Then, given a
location, look at the color at that point, and use the color as an
index to the object that you've hit. This technique has been used
for many years in the 3D animation industry.
-jcr
On Sep 20, 2004, at 3:54 PM, Graham Cox wrote:
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
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Cocoa-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
email@hidden
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Cocoa-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Cocoa-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden