I'm making an ipad game using cocos2d and box2d.
Among other elements, there's a fast-moving player object and a bunch of static line objects. I want the lines to detect when the player crosses them but not to act like a wall to the player object or any other moving objects in the game. So I've got the lines set to be sensors.
However, the nifty anti-tunneling code that Box2D has for fast-moving object collision detection doesn't seem to apply to bodies that are set as sensors. So now my player object passes right through the lines and only gets detected maybe one time in five.
How can I get box2d to detect the sprite crossing the line every time, no matter how fast it's going?
Edit: I found this post on the box2D forums where someone had a similar issue and found a possible solution. However I don't follow how to implement the solution. Maybe it'll help someone else, or maybe someone can explain what this person did more clearly. Here's what they said:
OK I got it working. Someone responded in the Box2D forums with a solution, which is to use a ray cast instead of relying on the built-in collision detection. I was able to find instructions on how to do this in this excellent tutorial on RayWenderlich.com
For my purposes, I simply calculated the sprite's velocity from the last frame, then performed a ray cast to see if it crossed any lines. The callback gives the x,y coordinate of where it crossed.
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Actually, I'm currently working on a simple project to detect collision between 2 specific objects in a surgery scene. The problem is that I don't have background on such problems so I'm really newbie to such things and I don't know yet what to do. After a little bit of research, I found Bullet library which can be used as a collision detection tool but not sure yet if it suits my case. I already checked some examples where the developer create the objects of interest manually which led me to think that I should detect first the objects of interest then launch the collision detection process.
In my case, I have 2 types of data:
Video shooting the operating room
Cloud points representing the room in 3D
I need to detect the collision between two objects in the scene. Is there any way to use Bullet to achieve such thing? Is it common to use a video as input for a detection collision problem(I'm wondering since I could find too much resources on it)?
I'm just starting so it might be a fuzzy question so sorry in advance for any inconveniences.
EDITED:
I already checked it but my point was to understand what options can be used before digging into the details. For me, a collision detection problem should have 2 parts: the objects of interest (The 2 or more objects that we're trying to detect their collision) and the scene in which we will be trying to detect the collision of the objects of interest. For the scene, the data I have is presented in 2 types mentioned above. So, I was asking about which type of data should be used as input for bullet collision process. Should it be an image taken from the video or should it be a list of 3D points? Or something else?
I have used Bullet half a year ago. I remember, that you need to register objects to Bullet with a collision shape. In simplistic case of your points, it could probably be small spheres. In case of your video, you need to have a 3d representation. I do not understand a 100% what you mean by detecting a "video" for collisions. However, to use Bullet, you need to have a collision shape associated with the object.
Further, you register a Collision Callback. This is one function called for each collision detected. All callbacks are listed here: http://www.bulletphysics.org/mediawiki-1.5.8/index.php?title=Collision_Callbacks_and_Triggers
As the wiki says - and I implemented it this way - to detect a specific collision, you need to iterate over allr esulting manifolds from Bullet manually. A little bit painful and performance wise strange approach. So you cannot register a specific callback for a specific object with another specific object!
Once the objects are registered, you run the algorithm and then you can check all manifolds in the callback.
To get started with Bullet, I used Bullet Physics Simplest Collision Example with the answers at that time.
Hello recently i started to mess around with SDL. Since i was interested in some 2D/2.5D games.So i started messing around with SDL in C++, I was looking to recreate something similar to Original Zelda.
So as far as i understand those game work with some kind of isometric prespective, or standard Orthogonal view but one thing i do not understand is how can you generate 3D-like Collisions between those objects on the map (tiles, sprites etc which are in 2D). Have a look at the video link below. Is this created purely in SDL, is it PerPixel collision or rectangular ? Or it might involve OpenGL as well ?
Link: https://www.youtube.com/watch?v=wFvAByqAuk0
The original was probably a simple Rectangular collision.
I believe that your "3D collision" is the partial collision present in some objects. For example, Link can go through the leaves, but not through the trunk.
You can do it easily in 2 ways:
Layers of rendering and collision. The trunk is located in one layer and is covered by some collision boxes. Link is present in a intermediary layer. And the leaves are in another layer, on top of Link. Then you can check collision between Link's Layer and the layer with the trunk and other objects, for example.
Additionally you can create a property for your tiles in which you can store the type of collision you hope to obtain. For example, 'box' collision will tell your engine that the object is collidable on every side. Or 'bottom' collision will tell your engine that Link will collide with this object only if he is walking down into the object (this is the effect of you will see on some 2D sidescrollers: jump through a tile but then fall into it solid.
Per pixel collision in those simple cases is not worth it. I find it much better to personalize the collision ourselves, using creativity, masks and layers.
BTW: This topic would fit better on https://gamedev.stackexchange.com/
I am working on Opencv application that need to count any object which motion can be detected by the camera. The camera is still and I did the object tracking with opencv and cvblob by referring many tutorials.
I found some similar question:
Object counting
And i found this was similar
http://labs.globant.com/uncategorized/peopletracker-people-and-object-tracking/
I am new to OpenCV and I've gone through the opencv documentation but I couldn't find anything which is related to count moving objects in video.
Can any one please give me a idea how to do this specially the counting part. As I read in article above, they count people who crosses the virtual line.Is there a special algorithm to detect the object crossing the line?
Your question might be to broad when you are asking about general technique that count moving objects in video sequences. I would give some hints that might help you:
As usual in computer vision, there does not exist one specific way to solve your problem. Try do do some research about people detection, background extraction and motion detection to have a wider point of view
State more clearly user requirements of your system, namely how many people can occur in the image frame? The things get complicated when you would like to track more than one person. Furthermore, can other moving objects appear on an image (e.g. animals)? If no and only one person are supposed to be track, the answer to your problem is pretty easy, see an explanation below. If yes, you will have to do more research.
Usually you cannot find in OpenCV API direct solution to computer vision problem, namely there is not such method that solve directly problem of people counting. But for sure there exists some paper, reference (usually some scientific stuff) which can be adopted to solve your problem. So there is no method that "count people crossing vertical line". You have to solve problem my merging some algorithms together.
In the link you have provided one can see that they use some algorithm for background extraction which determined what is a non-moving background and moving foreground (in our case, a walking person). We are not sure if they use something more (or sophisticated), but information about background extraction is sufficient to start with problem solving.
And here is my contribution to the solution. Assuming only one person walks in front of the stable placed camera and no other objects motion can be observed, do as following:
Save frame when no person is moving in front of the camera, which will be used later as a reference for background
In a loop, apply some background detector to extract parts in the image representing motion (MOG or even you can just calculate difference between background and current frame, followed by binary threshold and blob counting, see my answer here)
From the assumption, only one blob should be detected (if not, use some metrics the chooses "the best one". for example choose the one with maximum area). That blob is the person we would like to track. Knowing its position on an image, compare to the position of the "vertical line". Objects moving from left to right are exiting and from right to left entering.
Remember that this solution will only work in case of the assumption we stated.
I have integrated box2d in my engine, ( Debug Draw, etc. ) and with a world I can throw in some 2d squares/rectangles etc.
I saw this post, where the user is basically not using a world for collision detection, however the user doesn't explain anything about how he's using the manifold (b2Manifold), etc.
Another post, is in the cocos2d forum, ( scroll down to the user Lam in the third reply )
Could anyone help me a bit with this?, basically want to add collision detection without the need of using b2World ,etc etc.
Thanks a lot!
Is there any reason you can't use a b2World? Just because you use it, doesn't mean you have to use the physics simulations, unless you're severely performance limited.
See this example on using Box2D for collision only in Cocos2d. Maybe you can apply something similar to your project: http://www.raywenderlich.com/606/how-to-use-box2d-for-just-collision-detection-with-cocos2d-iphone
The code in both of those posts seems complete to me. You begin with two polygon shapes and their corresponding transforms and from b2CollidePolygons you get back a contact manifold.
The b2Manifold is just a collection of points at which the two shapes' boundaries intersect. In the posted code the author uses that if the manifold's point count is greater than zero there is an overlap. Depending on the nature of the shapes there can be different number of intersection points when they overlap.
Note that the collision handling of the b2World is quite robust. Beyond simple collision queries it has broad-phase culling, ray-casting, a spatial tree, begin-contact and end-contact events, efficient memory management to name some things. There are classes in the API for most of these tasks but I can't say I'm familiar with using them manually. Could be worth a look.
I am working on a simple 2D openGL project. It contains a main actor you can control with the keyboard arrows. I got that to work okay. What I am wanting is something that can help explain how to make another actor object follow the main actor. Maybe a tutorial on openGL. The three main things I need to learn are the actor following, collision detection, and some kind of way to create gravity. Any good books or tutorials to help get me in the right direction would be great.
You could use a physics library like Chipmunk Physics, which lets you attach springs and things between the two objects and detect when they hit each other and other things.
A pre-rolled library would be good, but the concepts you describe are ones you need to know if you are going to do any sort of game programming anyways:
A simple way to make one actor follow behind another is to have the lead actor store its position every time it moves. Feed these positions to a trailing actor with a delay of a few values - the longer the delay, the further behind they travel. Simple, but doesn't handle dynamic collision (other actors moving the block collision.)
Collision detection in 2D can simply be axis aligned (AA) bounding boxes. Search for this and you'll see the 4 ifs or so that are needed.
Gravity is just adding a fixed velocity (usually down) to every object every game loop. This is constant acceleration which is exactly how gravity works.