I currently have an arraylist of points from a freehand drawing on a canvas. I was wondering if there is a simple algorithm to detect if that shape represents a circle.I have already researched this a little and the main items I am pointed at are either the Hough transform or having bitmap images but both of these seem a little over the top for what I need it for. Any pointers to algorithms or implementation would be very helpful.
thanks in advance sansoms,
If I interpret your question correctly, you want to know if all the points are on a circle.
As illustrated in the picture, we pick three points A, B, C from the list and compute the origin O of the presumed circle. By checking the distance between O and each point from the list, we can conclude whether these points are on a circle.
If you do not know what the user wanted to draw (e.g., a circle, an ellipse, a line, or a rectangle), you could use some basic optimization algorithm to find the shape best matching the hand-drawn points.
for each basic shape (oval, rectangle, triangle, line, etc.), create a random instance of that shape and measure the error w.r.t. the given points
optimize each of the shapes (individually) until you have the oval best matching the given points, the rectangle best matching the points, the best triangle, etc.
pick the shape that has the lowest error and draw it
Maybe this answer can give you some ideas: https://stackoverflow.com/a/940041/12860
In short: calculate the second derivative. If it is quite constand, it is probably a circle.
Reading your comment, an easier method to draw a circle is for the user to click the center point, then drag the radius of the circle. It's a lot less calculation, and easier for the user to draw.
You can do the same thing with a rectangle, or any other convex polygon.
Related
I worked on a very simple map editor phase for a game in java. The goal is to put some islands with different shape on the map. But there is some constraints:
islands must not be a specific distance far from another island (lets call it L)
islands must not be a specific distance close from another island (lets call it S)
In the game, the island is place with the mouse. The gamer can see areas where the island can be place or not as you can see.
My problem is that I realize my disalow area is not good. For example, the rectangle island have a rectangle disallow area (my first naive attempt) but in fact I must draw area of S around the rectangle ; that leads to a shape like this:
I'm able to draw these kind of areas as long as my shapes are just composed of lines. But my island can have cubic or quadratic curve (and even though i'll need this kind of area for other shapes later).
The closer I manage to do is that:
In this case, the disallow area around the circle must be ... a circle (simple geometry). But as you can see, I have a weird rounded rectangle.
I currently try to transform each segment of the pathiterator of a Shape to get the area. It's not as simple as scaling a shape (remember the rectangle case). I've allready try many ways to transform the shape and get the area.
Question:
Does someone have information, formula, clues, algorithms, libs to get this area from any java.awt.Shape (or PathIterator) and a distance?
http://www.java2s.com/example/java/java.lang/expand-or-shrink-a-shape-in-all-directions-by-a-defined-offset.html
This site describe how to use stroke to get the offset area.
There is just a single modification to solve my problem ; I have to use BasicStroke.JOIN_ROUND to get the good rectangular Shape.
Then I get:
I'm building a simulator that is based on car collisions on the road. The "cars" are basic rectangles drawn using fillRect and setting random x and y coordinates for each car. The kinematics portion of the simulator work perfect except when cars collide. What I'm trying to do is figure out a way to detect collision without re-inventing the wheel. In essence, is there such implementation in Java that helps with this type of situation?
If not, I have an idea that consists of putting every single x and y point in the area of the square into an array for each car. Then if the other car's "area" overlaps a coordinate with the other, then a collision would occur. Could this be a solution, or is there a simpler way of doing this? Perhaps some advice would be great!
If not, I have an idea that consists of putting every single x and y
point in the area of the square into an array for each car.
No need to reinvent the wheel. Are you using Rectangle objects for your cars underneath? You can call methods such as contains and intersects which are part of the Rectangle api to achieve what you want. You need to make sure you check the next movement of the Rectangles, looking for collisions, before you move them.
Look here.
I'm trying to draw a flat surface out of voxels, the goal is to draw it filled and I'm having a lot of trouble. Everything I try results in holes on the surface. The surface has 4 corners, but I'd like to be able to use the same method for triangles too.
Here's what I've tried:
Draw along from one parallel side to the other
Draw only in one direction (z direction) along a side of the plane
I've had the most success with 2 but it fails when I add any pitch or roll to the plane (any elevation present).
Any tips? There's no code because I'm sure my implementations are all correct, it's just the choice of algorithm that's wrong.
EDIT:
On a side note, though number 2 had less holes, the planes were distorted and didn't appear flat.
EDIT2:
I'm sticking with my first decision, but now the question is, how do I detect when there will be a hole? By observation I notice there's the same amount of holes per plane regardless of pitch and roll. Yaw is the culprit here.
EDIT3:
I'm leaving this question up but I decided to just test a nearby block to see if it's empty. I didn't want to do it, but yeah. If you have a more elegant solution I'm all ears.
A plane, being infinite, does not have corners. Are you talking about a four-sided polygon? Does it have square corners?
For a polygon, I would certainly start off with a triangle, since you can construct any other polygon out of triangles, not the other way around.
Then, a good start for filling a triangle would probably be to come up with an accurate test of whether a given voxel should be filled or not. Here's an example of two different point-in-triangle tests.
After you have that you can proceed in different ways. For example, although not the most efficient, you could region-grow from the center, testing each neighboring voxel and recursing with a stack.
I'm trying to draw a 5 point star in AWT.
Each point in the 2d grid is 72 degrees apart - so I thought I could draw the polygon using only 5 points by ordering the points 144 degrees apart, so the polygon gets fed the points in order 1,3,5,2,4
Unfortunately, this involves a lot of intersecting lines, and the end result is that there are 5 triangles with my desired colour, circling a pentagon that has not been coloured.
Looking through, it has something to do with an even-odd rule, that intersecting points will not be filled.
I need my star to be drawn dynamically, and using the specific shape described (for scaling and such).
If I manually plot the points where it intersects, I get some human error in my star's shape.
Is there any way to just turn this feature off, or failing that, is there a way to get the polygon to return an array of x[] and y[] where lines intersect so I can just draw another one inside it?
Thanks.
Draw it with ten points, 36 degrees apart, at two alternating radii.
Establish the 10-point Polygon in cartesian coordinates, as suggested by relet and as shown in this example. Note how the coordinate system is centered on the origin for convenience in rotating, scaling and translating. Because Polygon implements the Shape interface, the createTransformedShape() method of AffineTransform may be applied. A more advanced shape library may be found here.
Is there a way to get the polygon to return an array of x[] and y[] where lines intersect?
Although typically unnecessary, you can examine the component coordinates using a Shape's PathIterator. I found it instructive to examine the coordinates before and after invoking createTransformedShape().
I'm currently writing an application that actually acts as a "cut" tool for 3D meshes. Well, I had some problems with it now which I am clueless on how to solve, since it is my first application.
I have loaded a model from an object file onto the canvas, then on the same canvas, I use the mouse drag event to draw lines to define the cutting point.
Let us say I want to cut a ball into half and I draw the line in the middle. How do I detect the vertices of the ball under the line.
Secondly, if I rotate/translate the ball, would all the the vertices information change?
Think of what you'd do in the real world: You can't cut a ball with a line, you must use a knife (a line has no volume). To cut the ball, you must move the knife through the ball.
So what you're looking after is a plane, not a line. To get such a plane, you must use some 3D math. What you have is the canvas orientation and the "side view" of the plane (which looks like a line).
So the plane you're looking for is perpendicular to the canvas. A simple way to get such a plane is to take the canvas orientation and create a plane which has the same orientation and then rotate the plane around the line by 90°.
After that, you can visit all edges of your model and determine on which side of the plane they are. For this, determine on which side of the plane the end points of the edge are. Use the cross product. If they are on the same side (both results of the cross products will have the same sign), you can ignore the edge. Otherwise, you need to determine the intersection point of the edge and plane. Create new edges and connect them accordingly.
See this page for some background on the math. But you should find some helper methods for all this in your opengl library.
if I rotate / translate the ball, would all the the vertices information change
Of course.
It's not going to be that easy.
I assume the line you are drawing induces a plane which then cuts the sphere.
To do so, you have to calculate the intersecting area of the sphere and the plane.
This is not a trivial task and I suggest using an existing framework for this or if you really want to do this yourself, read about basic intersection problems to get a feeling for this kind of problem. This paper offers a good introduction to various intersection tests.
In general boundary represended volumes, as in your case, are difficult to handle when it comes to more advanced manipulations. Cutting a sphere in half is easy compared to burring a small hole into it. Sometimes it's better to use a volume representation, like tetrahedral meshes or CSG.
Regarding your second question, you shouldn't rotate or translate the sphere, rotate and translate the camera.