I have a two polygon shape draw using HTML5 canvas(GWT). I have all the points for two polygon shape. Means I have the list of points for draw this type of polygon.
Below image showing two polygon intersect or overlapping each other. Now I am searching for a solution how to find two polygon "intersect or not" using java? I am using pure java programming without using any third library.
I have another issue. For explaining this issue I am attaching another image below.
This is the another case when one Polygon inside of another Polygon. In this scenario how to calculate minimum distance between two polygon in negative?
Using Postgis Api Using You get Information That Two Polygon intersect or not . Find Following Link As Reference
http://www.postgis.net/docs/ST_Intersection.html
You can use the sweepline paradigm.
Consider the horizontal lines passing through the vertexes of either polygons. Two consecutive lines cut out slabs (trapezoids).
As the slabs are simple, convex polygons, checking them for intersection is straightforward: it suffices to detect overlap of the bases (mind the case where they form an X cross).
Now the whole process can be decomposed as
sorting the vertices of both polygons by increasing ordinates; for every vertex, remember to what polygon it belongs and what are its neighbors with a higher ordinate (none, one or two);
scanning the vertices by increasing ordinate, while maintaining a list of the edges that are intersected (it is called the active list). Every time you move to another vertex, update the active list;
testing the slabs for intersection.
Related
I'm attempting to render a hemisphere in java. However, I'm wanting to render the slice that is defined by 2 angles - Azimuth and Elevation. Since I'm defining a slice, I cannot (to my knowledge) use any built in primitives. If the azimuth range is defined 0-360 and the elevation range is defined as 0-70, this will be a hemisphere with an upside-down cone-shaped hole in the top.
When rendering this inside "cone", I have chosen to do it as triangles in 5 degree increments. This means that with a 360 degree cone, there are 73 different vertices (if I did the math correctly: 360/5degree slices with the origin or tip of the cone being shared with all sides, and all other vertices shared by adjacent triangle slices)
My question:
Is it more efficient to render these as a single polygon with with many vertices, or many triangles with only 3 vertices each. If I do a single polygon, will I still have to include all three points for each triangle, or if it is a shared vertex, would I only include it once? Sorry, my graphics rendering knowledge is limited. Also sorry for being so verbose; I'm hoping someone may spot something erroneous in my thought process which may clear things up either way.
First - Use Google to find an algorithm to create a sphere that is not a primitive.
Second - Somewhere down the chain - triangles will be used. Most likely by the underlying library. But for you - it depends upon whether or not you plan to chop up the created region. If you are not going to subdivide the region further I would just make it one polygon. Actually, after thinking about it for a second - you can always divide up the polygon afterwards too. So just make it one polygon.
I thought about it some more and decided to amend this answer. There are two ways you can create a polygon in openGL. You can either create it as a triangular mesh or as an outline polygon. So if you were asking "Should I use a triangular mesh or an outline polygon" I would say use the triangular mesh. It is a lot easier to break up the triangular mesh than a polygon outline since, to break the mesh, all you have to do is to just stop at one of the points, include the last two points in the new object, and continue on down the triangular mesh. An outline polygon requires you to go both left and right around the polygon to locate the two points where the break occurs. If that is clear. If not say so.
Update: 12:05pm
When making a polygon you can use a triangular mesh or a polygon outline. The outline is mainly good for 2D whereas the triangular mesh works in both 2D and 3D systems. If you have any kind of a polygon at all bigger than just three points then it is a good idea to put them all into an array. This allows you to use the built-in routines that take an array and simply go through it to build your polygon. By putting everything into an array you also make it easier on yourself to add new points or remove points or adjust points. All you do is to change the array entry and then call the same routine to draw everything again. (Which should be just a single call to a function.)
I'm using a triangulation library to compute the Constrained Delaunay Triangulation of a set of rectangles within some large boundary. The algorithm returns all the edges, but also adds edges inside of the rectangles that define the constraints. I want to be able to find if an edge lies inside of a rectangle in O(1) time.
Here's a more general description of the problem I want to solve. Given a set of nonoverlapping rectangles (the borders of the rectangles may touch) and an edge e with endpoints (x1,y1) and (x2, y2), find in O(1) time if e lies within any of the rectangles (including the border).
Also let me know of any data structures I can use for speedups! I'm also implementing this in java so I have easy access to hash sets, maps and all those nice data structures.
Since the rectangles are completely enclosed, the inside of each rectangle will simply be the CDT of the rectangle itself -- which is to say, two triangles, meeting along a diagonal of the rectangle. So you can just insert all rectangles' diagonals (remember, two possible diagonals per rectangle) into a hashtable, and check which edges exactly match those endpoints.
It is possible to break up the area that all of the rectangles cover into a N by M grid of boxes. By labeling each box with the rectangle it is in or the rectangles it overlaps. It is possible to obtain O(1) queries, with O(N*M) pre-processing.
However, in order for it to work, the grid has to created based on an algorithm that allows for calculating which box a point lies in in O(1). It also requires that the number of rectangles a box overlaps be very small (ideally no more than 2 or 3) as otherwise the average query time could be O(log N) or worst. This means that the number of boxes can get very large.
So, I'd like to figure out a function that allows you to determine if two cubes of arbitrary rotation and size intersect.
If the cubes are not arbitrary in their rotation (but locked to a particular axis) the intersection is simple; you check if they intersect in all three dimensions by checking their bounds to see if they cross or are within one another in all three dimensions. If they cross or are within in only two, they do not intersect. This method can be used to determine if the arbitrary cubes are even candidates for intersection, using their highest/lowest x, y, and z to create an outer bounds.
That's the first step. In theory, from that information we can tell which 'side' they are on from each other, which means we can eliminate some of the quads (sides) from our intersection. However, I can't assume that we have that information, since the rotation of the cubes may make it difficult to determine simply.
My thought is to take each pair of quads, find the intersection of their planes, then determine if that line intersects with at least one edge of each of the pairs of sides. If any pair of sides has a line of intersection that intersects with any of their edges, the quads intersect. If none intersect, the two cubes do not intersect.
We can then determine the depth of the intersection on the second cube by where the plane-intersection line intersects with its edge(s).
This is simply speculative, however. Is there a better, more efficient way to determine the intersection of these two cubes? I can think of a number of different ways to do this, and I can also tell that they could be very different in terms of amount of computation required.
I'm working in Java at the moment, but C/C++ solutions are cool too (I can port them); even psuedocode since it is perhaps a big question.
To find the intersection (contact) points of two arbitrary cubes in three dimensions, you have to do it in two phases:
Detect collisions. This is usually two phases itself, but for simplicity, let's just call it "collision detection".
The algorithm will be either SAT (Separating axis theorem), or some variant of polytope expansion/reduction. Again, for simplicity, let's assume you will be using SAT.
I won't explain in detail, as others have already done so many times, and better than I could. The "take-home" from this, is that collision detection is not designed to tell you where a collision has taken place; only that it has taken place.
Upon detection of an intersection, you need to compute the contact points. This is done via a polygon clipping algorithm. In this case, let's use https://en.wikipedia.org/wiki/Sutherland%E2%80%93Hodgman_algorithm
There are easier, and better ways to do this, but SAT is easy to grasp in 3d, and SH clipping is also easy to get your head around, so is a good starting point for you.
You should take a look at the field of computer graphics. They have many means. E.g. Weiler–Atherton clipping algorithm. There are also many datastructures that could ease up the process for you. To mention AABBs (Axis-aligned bounding boxes).
Try using the separating axis theorem. it should apply in 3d as it does in 2d.
If you create polygons from the sides of the cubes then another approach is to use Constructive Space Geometry (CSG) operations on them. By building a Binary Space Partitioning (BSP) tree of each cube you can perform an intersection on them. The result of the intersection is a set of polygons representing the intersection. In your case if the number of polygons is zero then the cubes don't intersect.
I would add that this approach is probably not a good real time solution, but you didn't indicate if this needed to happen in frame refresh time or not.
Since porting is an option you can look at the Javascript library that does CSG located at
http://evanw.github.io/csg.js/docs/
I've ported this library to C# at
https://github.com/johnmott59/CGSinCSharp
I have a series of Polygons, created using the JTS Topology Suite. Each polygon is a set of points, longitudes and latitudes that form a shape, as shown below.
((lat1,lon1),(lat2,lon2),....(lat1,lon1))
I want to find each of these polygons neighbours, the other shapes that are physically next to them. I have thought of looking for matching points, but obviously that won't work in all cases. I was wondering if there is any package that will check to see if polygons share the same edge that I could use in this situation? Or if not then another way to do this in Java.
Thanks.
Since you are looking for shared edges, you can create a hash table uses the edge as the key and a list of polygons that have that edge as its item.
Then you go through each polygon and fill up the table. Which is linear in the number of edges.
Then you go through the table and look for edges that are tied to multiple polygons, which is what you are looking for. This is linear in the number of unique edges.
I'm not sure how Java implement hash tables, so it might be a bit of work to get this setup. Make sure the edges are sorted to prevent the edge (A,B) being different from the edge (B,A), which will mess up the algorithm.
As for libraries, seems like what you're trying to do might be a bit specialized, and thus not sure you'll find library implementations, hence why I outlined an algorithm.
There was in fact a very simple way to do this. I created an arraylist containing an array of objects. The object referenced the name/id of the polygon, the actual polygon coordinates and then a string that would contain ids of any neighbours.
In JTS Topology suite there is a method that you can call on polygons called touches that returns a boolean; so I had a double for loop, going through my arraylist twice and calling the method touches on polygon(i) so:
arraylist<object[]>..
//where in the array the objects are
object[0] = id
object[1] = polygon
object[2] = neighbours
for (int i=0;i<arraylist;i++)
for (int j=0;j<arraylist;j++)
if (i)[1].touches(j)[1]
update i[2]..
It's probably not the best method but it seems to work.
If you are using JTS geometries anyway you can use the spatial relations that can be used with any Geometry object.
Use touches if you can ensure that at least one point between neighbours is in common and their interiors do not intersect.
Use intersects if you cannot ensure that the interiors do not intersect. Depending on the datasource this is probably the better choice.
Given the polygons a method to get the neighbours can look like this:
public ArrayList<Polygon> getNeighbourList(Polygon center, ArrayList<Polygon> possibleNeighbourList){
// result list
ArrayList realNeighbourList = new ArrayList();
for(Polygon p : possibleNeighbourList){
// check if current polygon is a neighbour of the center polygon by using the spatial relation touches or intersects
if(center.intersects(p)){
realNeighbourList.add(p);
}
}
return realNeighbourList;
}
I have two geo-spatial simple convex polygon areas, in latitudes and longitudes (decimal degrees) that I want to compute using Javas Polygon class. I basically want to see if these two lat,long polygons intersect, and to calculate the areas (in sq meters) of both.
Now java.awt.Polygon class only uses x,y co-ordinate system, but I need to use lats,longs.
How can I do this, and/or is there any other classes/libraries available?
Why not use Polygon class multiplying coordinates for 10^n making them integers? Polygon accepts arrays of int as points.
Just an Idea
All you need to do is convert from spherical to rectangular coordinates if you think that really matters.
I'm not sure that it does, because the java.awt.Polygon is merely a data structure holding for pairs of values. If you read the javadocs, they say
The Polygon class encapsulates a description of a closed, two-dimensional region within a coordinate space. This region is bounded by an arbitrary number of line segments, each of which is one side of the polygon. Internally, a polygon comprises of a list of (x,y) coordinate pairs, where each pair defines a vertex of the polygon, and two successive pairs are the endpoints of a line that is a side of the polygon. The first and final pairs of (x,y) points are joined by a line segment that closes the polygon.
They happen to label those points as x and y, which makes all of us think rectangular coordinates, but they need not be from your point of view. A bug on the service of your sphere would view it as a 2D space. Your radius is large and constant. You just can't count on using any of Polygon's methods (e.g., contains(), getBounds2D(), etc.)
In this case, the important thing is your area calculation. You can use a Polygon for your area calculation, storing lats and longs, as long as you view Polygon as a data structure.
You might also thing of abandoning this idea and writing your own. It's not too hard to create your own Point and Polygon for spherical coordinates and doing it all with that coordinate system in mind from the start. Better abstraction, less guessing for clients. Your attempt to reuse java.awt.Polygon is admirable, but not necessary.
You can perform the area calculation easily by converting it to a contour integral and using Gaussian quadrature to integrate along each straight line boundary segment. You can even include the curvature of each segment at each integration point, since you know the formula.
Going off my intuition here.
If the polygons are small in ratio to the size of the planet you can treat them as flat polygons. The steps involved would be converting the lat/long into absolute x/y/z, taking any three of the points and finding the normal of the plane the polygons lie on and then using this to project the points into two dimensions. Once you have the 2D points it's easy to calculate the area or if they intersect.
Probably not the best answer but hopefully it will motivate some people to make better ones because it's a good question.
Maybe you could use GeoTools for this. It allows you to create Geometry objects, and check wether they intersect (see: Geometry Relationships)
Here is a solution that works like a charm:
Class PolygonArea from net.sf.geographiclib
Refer link below with sample code:
https://geographiclib.sourceforge.io/html/java/net/sf/geographiclib/PolygonArea.html
gradle dependency:
compile group: 'net.sf.geographiclib', name: 'GeographicLib-Java', version: '1.42'