I have a path drawn in OpenGL ES. I can convert it to a CGPath if needed.
How would I check if it intersects itself (If the user created a complete loop)?
Graham Cox has some very interesting thoughts on how to detect the intersection of a CGPathRef and a CGRect, which is similar to your problem and may be educational. The underlying problem is difficult, and most practical solutions are going to be approximations.
You may also want to look at this SO article on CGPathRef intersection, which is also simliar to your problem, and some of the proposed solutions are in the same space as Graham's above.
Note: This answer is to an earlier version of the question, where I thought the problem was to determine if the path was closed or not.
I think a path is considered closed iff the current point == the starting point.
The easiest way I know of to check this is to keep track of these two points on your own, and check for equality. You can also use CGPathGetCurrentPoint, and only track the starting point to compare with this.
Here's a roundabout way to find the starting point, if it's hard to just keep track of it directly:
make a copy of the path
store its current point
call CGPathCloseSubpath
check to see if the current point changed
If it did change, the original path was open; otherwise closed.
This is a way to check if a path composed of a single continuous segment is self-intersecting.
I'm sure that if you wanted a faster implementation, you could get one by using some good thinking and full access to the CGPath internal data. This idea focuses on quick coding, although I suspect it will still be reasonably fast:
Basically, take two copies of the path, and fill it in two different ways. One fill uses CGContextEOFillPath, while the other uses CGContextFillPath. The results will be different iff the path is self-intersecting.
You can check if the result is different by blending the results together in difference blend mode, and testing if the resulting raw image data is all 0 (all black).
Hacky, yes. But also (relatively) easy to code.
** Addendum ** I just realized this won't work 100% of the time - for example, it won't detect a figure eight, although it will detect a pretzel.
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Object recognition system using matlab
I need help to develop an object recognition system. It needs to identify an object in an image by comparing it with an image in an existing database. For example my database may consist of images of cars, buses, cups, etc. If i give a certain image as an input i want the code to check and tell me whether a car(as in the car in the database) can be found to exist in the input image or not. This is strictly to be implemented in matlab. I have tried correlation, image subtraction and a few other algorithms but to no effect. Thanks in advance.
This is a complex subject, that is really on the bleeding edge of technology, but let me give you a few pointers to help start things out.
Somehow, you need to take into account the different sizes, angles, etc that might be around. A car looks very different if photographed from a few feet away as compared to 50 feet, as would it photographed from the front vs the side.
Edge detection algorithms generally work well at pulling the target object's shape away. Take the edges, identify lines in them, and you can try to compare these lines with those from your model.
Range to objects really makes a huge difference in building a successful algorithm. If you know the difference from the front of the car to the back, it can make all of the difference in the world.
Focus, noise, lighting, etc need to somehow be dealt with, to ensure that the system works well.
All in all, I would recommend taking some image analysis classes, reading several papers on the subject, or at least reading the Wikipedia Article, and then starting to work on your project.
The problem you have described is sometimes called object category recognition or object class recognition to emphasize that you are not trying to recognize a particular object, but a member of a category such as "car" or "person".
One popular approach for solving this problem is called Bag of Features of "Bag of Words". If you have access to the Computer Vision System Toolbox for Matlab, it has functions for detecting SURF features, which can be used for this approach.
Also, a better place to ask this question might be Signal and Image Processing stack exchange.
Can someone please explain to me how coordinate transformations work in draw2d?
I have a hierarchical diagram where a figure can contain figures which also contain figures. At first I added internal figures by using the request's getLocation, fetching the host figure of the EditPolicy and appliying hostFigure.translateToRelative(location) but does not work! neither combinations of translateToParent and other things.
At the end I copied the implementation from the Logic example, which uses getConstraintFor, a method provided by the policy which does the translation itself. I checked this could but also could not understand how it works.
I read in a number of threads in the eclipse forums on this subject, but still don't understand why a simple method like translateToAbsolute does not behave as expected. Could anyone please explan? Thanks
Two pieces of information that might shed some light on your problem:
Depending on the request type I would expect the location to already be in absolute coordinates.
Unless explicitly implemented otherwise, Figures don't have a local coordinate system for their children. So converting a location up and down the Figure hierarchy does not necessarily change the coordinates.
My Question is something similar to this.
I have 2 CGPathRef and 1 will be moved by finger touch. I want to find that whether the 2 CGPathRef are intersected? That question was asked almost 2 years ago and I want to know whether something has been found in the mean time.
This is fairly old, but I found it looking for a similar solution, in my problem I wanted to find when a circle overlapped with a path (a special case of your question).
I solved this by using CGPathCreateCopyByStrokingPath to create a stroked version of the original path using the radius of the circle as the stroke width. If the center point of the circle overlaps the stroked path then the original path overlaps the circle.
BOOL CGPathIntersectsCircle(CGPathRef path, CGPoint center, CGFloat radius)
{
CGPathRef fuzzyPath;
fuzzyPath = CGPathCreateCopyByStrokingPath(path, NULL, radius,
kCGLineCapRound,
kCGLineJoinRound, 0.0);
if (CGPathContainsPoint(fuzzyPath, NULL, center, NO))
{
CGPathRelease(fuzzyPath);
return YES;
}
CGPathRelease(fuzzyPath);
return NO;
}
Edit: A minor bug where the fuzzyPath was not released.
I have written a small pixel based path collision detection API for CGPathRefs. It requires that you add a few source directories to your project, and it only works with ARC, but it should at least show you how one might do something like this. It basically draws the two paths on two separate contexts, and then does pixel-by-pixel checks to see if any pixels are on both paths. Obviously this would be slow to run every time the user drags their finger, but it certainly could be done once every half second or so, maybe not even on the main thread.
This is the easiest way I've found of doing something like this, and it may easily be that there's no better way, besides using lots of math.
The source on Github
A quick Youtube demo.
Generally speaking, finding the intersection of two arbitrary CGPaths is going to be very complex.
There are ways to do approximations. Checking the intersections of the bounding boxes is a good first step. You can also subdivide the curve and repeat the process to get better approximations. Another option is to flatten the paths and see if any of the line segments of the flattened paths intersect.
For the general case, however, things get very nasty very fast. Consider, for example, the fact that two cubic bezier segments (never mind an entire path... just one segment) can intersect with another segment at up to 6 points. The more segments in your path, the more potential intersections. There is also the problem of degenerate bezier curves where a segment has a cusp that just touches one point of another segment. Does that count as an intersection? (sometimes yes, sometimes no)
It's not clear from your question, but you might also want to consider the intersections of the strokes that are applied to the curves, and correctly account for line joins and miters. That that gets even harder. Macromedia FreeHand (a drawing program similar to Adobe Illustrator) had a very large, complex, intensely mathematical library for discovering arbitrary bezier curve intersections. The problem is not easily solved.
To find the intersection of two CAShapeLayers, we can use below method, CAShapeLayer won't return frame. But we can get the refPath frame using CGPathGetBoundingBox. But this one will give the frame in rectangle.I thing you may understand.
if (CGRectIntersectsRect(CGPathGetBoundingBox(layer.path), CGPathGetBoundingBox(layer.path)))
I need to create an algorithm to layout some hierarchical data but have never done this kind of thing before and need some broad tips.
Basically I need to recreate this diagram (with dynamic data):
diagram http://dl.dropbox.com/u/15126868/diagram.png
bigger
I don't have a problem with most of it but need help with two things:
How do I approach writing a layout algorithm?
Should I use UIView subclasses for all discs or use quartz (I do need interaction)
Any suggestions most welcome. I don't need too much detail.
A bit more detail:
I'm currently thinking I should use UIView subclasses and layoutSubviews. Trouble is I need to know the size (at least roughly) of all nodes before I can start to position them. Then, as the positioning involves rotation, I may need to adjust child positioning again - and I can't add labels until after any rotation.
Other considerations seem to be: that the presentation area is rectangular, not square; that I can't spill off the page; and that I will need to animate changes to the sizes of the discs.
Any pointers would be great, thanks.
This sort of thing is very difficult.
Interestingly the perhaps main actual initial constraint here is the size of the typography.
In the example given: Observe they could have chosen a different SCPT** somewhat larger (perhaps, 10%-15% larger) or somewhat smaller and it would have still worked. They made an aesthetic decision on the SCPT.
White space is critical to design. Their particular graphic designer happened to like the particular feel of white space which you see. But it would have by no means been "wrong" with a smaller SCTP. Further, observe they could have used an even larger SCPT ... IF ... they used a smaller point size on the typography.
Note that in any event you simply won't be able to display that much type that small on an iPad (or Fone4).
So straight away you have to make decisions about how the type will appear, popup, audio or whatever. Even the white type ("on the discs" type) will give you trouble.
You will have to do lots of tests with photoshop first on to your iPad before even proceeding with an algorithm. So purely for what it's worth...
Here's how I personally would do this sort of thing. In general plan: I would try to do a squishy algorithm that retries itself until it finds a result it is happy with.
IMHO, based on previously doing this type of thing: this problem is too hard to get it done in one go with some particularly smart-ass heuristic. Since there is no one smart-ass heuristic that will save the day, I'd do this:
1) calculate the total trillions to display. (it looks like about 2.5 is the total in the example image)
2) guess a SCPT value to begin with. what about for example "18" based on the actual image at the screen size we see above as posted inside your question.
3) put the big one (sun) in the dead center, and for the middle ones (planets) -- just choose a very easy heuristic, what about from biggest to smallest going anticlockwise srtaing at the top left (don't try to get clever than that with that part of the problem - which indeed could be a huge research project purely on it's own) .. and do the same with the small ones (moons).
4) for the sticks between planets and moons - adopt a trivial solution (like "always 0.5 cm"!!) and that's that. with AI you gotta cut your losses .. everywhere! :) Fix the moons to the planets and forget about them.
5) Now a hard part .. run some sort of heuristic over them that evenly balances what you have so far. treat color as mass and no color as no mass and move the "sun" until it is balancedish. (to be clear, as an example that would be likely downwards if you followed the "planet" layout mentioned in 3.) maybe also move all the planet/moon systems in-out to try to balance it.
6) next the iteration. look at that result and decide if you like it! go back to (2) and pick a new value. (maybe "16!" for example)
(7) there are two possible outcomes here. it might be that during development, there is one magic value for SCPT that always works. perhaps "14.3" or "18.2" or whatever. if you find such a value, never tell anyone. keep it as your own secret information!!!! milk it for everything it is worth with clients. conversely and more difficultly, you might find you need a different value each time. in that case: your AI will have to on it's own iterate through values until it finds one it likes. (for example, by determining whether all your labels fit or not .. and obvious things like "are they touching" "all on screen" etc.)
Anyway FWIW (perhaps nothing) that is what I would do - an iterative approach based on a first guess for the SCPT.
Incidentally: you may well want to buy and study the classic and brilliant book on this sort of display of information!!! Everyone should have a copy.
Tufte's The visual display of Quantatative Information
by Edward R. Tufte
ISBN 0961392142
Regarding the mechanics of laying out the image. You should use quartz or any other low-level drawing - forget about UIViews and the like. You should surely completely separate the logic from the drawing layer, so (even if you do want to change to UIViews, OpenGLES, or whatever) it's only changing a few lines of code.
Hope it helps somehow.
Notes...
** SCPT .. square centimeters per trillion
Followup...
"To keep the logic separate would you use a manager-type pattern?"
To be honest: if I was doing it, I would just start a whole new app purely for the "research" of getting this part, this challenge, working right. In that app (to be honest!) I would make bugger all effort to do anything in any tidy manner whatsoever! :-/ Globals everywhere! :) Unfortunately for me I can only think of the one thing at a time, so at that stage I would only be thinking about the algorithm, per se.
I believe, once you cracked the problem per se, once you came to implement it in a bigger project ... really, FWIW, if it was me, I'd simply make it a class (let's say AmazingClass) nothing more complicated than that. Personally I would set the data somewhere separately (whether in a DB or just an array or whatever) and I would just let the AmazingClass take care of getting the data, even. (My thinking - you never know how the hell you're going to need the data and when, at what point in the process of AmazingClass. So, just give up and let AmazingClass take it as and when it wants it.)
If you are familiar with these awesome-sounding manager-patterns of which you speak - yeah, why not! In short I would heavily separate it out as much as possible. I'm not good enough to speak on the best way to do that - but just completely separate it out somewhere. Sorry I can't help on that one.
I am trying to make an iPhone application which can draw a path between two points (similar to Google Maps) but instead of the map i want to use any other image as a background, this path between the two points might not be straight and there might be multiple paths to get from one point to another then I want to draw the shortest path between the two points.
I tried using the CGContext & CGPath but I got stacked.
Can you help me plz.
Thanx,
Ghaith
I think you're looking for UIBezierPath. You can add simple lines/polygons with something like:
UIBezierPath* path = [UIBezierPath bezierPath];
[aPath moveToPoint:CGPointMake(50.0, 50.0)];
[aPath addLineToPoint:CGPointMake(10.0, 10.0)];
[aPath addLineToPoint:CGPointMake(10.0, 50.0)];
[aPath closePath];
You can also, of course, add curves (bezier ones!) and other shapes. Then to draw it use the [aPath stroke] call in your view's drawRect method.
For more information see the iPad Programming Guide
This seems like a problem that's not really related to drawing the route.
You want to find the shortest path from one point to another, given certain criteria - where you can and cannot move, for example. I don't see this problem as something you can solve with drawing, but with actually calculating the different possible ways and then compare them. When you have decided which is the best route. Drawing is pretty simple.
How you would go by deciding I'm actually not sure - sorry 'bout that. But you should probably have a look at some shortest path algorithms. But that probably means you have to represent the underlying image as a pattern, or a series of nodes but graphical problems are not my cup of tea, so I'm not really sure how.
Just a side note - If the number of possible ways of getting from point A to point B are great, this can become a computational problem, and you have to make sure that the iPhone can manage.
(this should probably be a comment somewhere, but since I can't yet and I still wanted to share my two cents, it became an answer.)
Edit:
I just thought of really naive aproach! - for fun mostly, but I couldn't keep myself from posting.
Suppose you have a representation of the image. What parts can't be traveled on and what parts can be. Each pixel that can be travelled on is represented by a 1, and every other pixel is represented by a 0. Thus the pixels represented by 1s can be seen as nodes on which we can travel.
Each node can reach, at most, 8 other nodes - the adjacent pixels. And the weight of travelling between any two nodes could be set as 1. But we have to account that travelling in a diagonal is a greater distance so that weight should be sqrt(2).
Now we have a great bunch of nodes - each with weights in between them. From here we can apply a djikstra-algorithm to find the best route. (maybe some other algorithm is more beneficial at this point - but djikstras is the only one I'm familiar with).
hum, wonder how bad of a solution this would be. ... again, you probably don't want this solution...
EDIT 2:
I will say this again that this is probably not the best way to do this! You should seriously ask someone with more experience in algorithms and in graphical problems. - This was something I thought of at 3am and was mostly for laughs.
If your question is about calculating routes instead of drawing routes, that's a whole different problem. The standard algorithm for finding efficient routes through a given space are the "A*" (pronounced A-star) algorithms, which are typically what real-time strategy games use when you click a unit and tell it to "go there". It's also got many uses in AI when searching for a transition through a space.
It's not easy to get right, though. It might be easier to find a good game engine that already includes an A* implementation and integrate that into your software.