We Keep Coding

Detect interception with line

How can I make a algorithm that detects if a point (x, y) intercepts with a line. (x1, y1, x2, y2)?
I have already tried :
boolean onLine(float a, float b, float c, float d, float x, float y){
boolean answer = false;
float[] p1 = new float[] {a, b};
float[] p2 = new float[] {c, d};
float x_spacing = (p2[0] - p1[0]) / ((a+c)/2 + (b+d));
float y_spacing = (p2[1] - p1[1]) / ((a+c)/2 + (b+d));
List<float[]> line = new ArrayList();
float currentX = 0;
float currentY = 0;
while(currentX+a<c&&currentY+b<d){
currentX += x_spacing;
currentY += y_spacing;
line.add(new float[]{a+currentX, b+currentY});
}
for(int j = 0; j < line.size(); j++){
if(x > line.get(j)[0]-x_spacing && x < line.get(j)[0]+x_spacing && y > line.get(j)[1]-
y_spacing && y < line.get(j)[1]+y_spacing){
answer = true;
println("Hit line!");
break;
}
}
return answer;
}
This works sometimes, but is not always consistent.
I am putting this with a physics game, and I need this so the ball can roll down a line.
What are some ways I can improve it so that it works?.
EDIT: Thanks to Felix Castor I got it working. Here is the final Code:
boolean onLine(float x1, float y1, float x2, float y2, float xt, float yt,
float wid, float hit){
float Y = (y2 - y1)/(x2 - x1)* xt + y1 -(y2 - y1)/(x2 - x1) * x1;
boolean answer = false;
if(abs(Y - yt) < 5) answer = true;
if(abs(Y - yt-hit) < 5) answer = true;
if(abs(Y - yt-(hit/2)) < 5) answer = true;
if(abs(Y - yt+hit) < 5) answer = true;
if(abs(Y - yt+(hit/2)) < 5) answer = true;
return answer;
}

Using slope intercept form you can plug in your x and see if the y's are equal.
y = m*x + b
m = (y2 - y1)/(x2 - x1)
b = y1 - (y2 - y1)/(x2 - x1) * x1
So the equation becomes
Y = (y2 - y1)/(x2 - x1)* X + y1 -(y2 - y1)/(x2 - x1) * x1
given a point (xt, yt) you can plug in the xt into X and evaluate then compare the result to yt. If they are equal then the point is on the line.
if Y == yt given xt then the point is on the line.
You will need to handle the case where you have strictly horizontal lines as edge cases. Those will blow up the equation.
Edit: Conditions Changed
Since you are wanting to determine how far from the line a point is I would say the formula for the distance between a point and a line in cartesian space would be the way to go. See Distance from a point to a line section Line Defined By Two Points. The formula looks ugly but it's straight forward.
double numerator = Math.abs((y2 - y1) * xt - (x2 - x1) * yt + x2 * y1 - y2 * x1);
double denominator = Math.sqrt(Math.pow(y2 - y1, 2) + Math.pow(x2 - x1, 2));
double distance = numerator / denominator;
As before your test point is (xt, yt) and your line is defined by two points (x1, y1) and (x2, y2). Because distance is always >= 0 your test would be:
if( distance <= tolerance) return true
I think this is a better approach if you are interested in a tolerance.

Debugging Java/Processing output of an object patrolling screen

I am a first year computer programming student working on a project of Robots and Robot testing environment.
Although I'm new to Stackoverflow, I'm pretty aware that we are not supposed to help with assignments or homework.
I'm struggling to understand why such behavior is happening, so it would be really helpful just to understand why it's happening.
We are using Processing libraries to program in Java.
Our task is to create a robot testing environment and create three robots performing different movements.
The movement I'm trying to program is Patrol, which means just go around the edges of the screen.
The expected behavior is to go forward until it reaches a wall, turn left (90 degrees counter-clock wise), and go forward again.
This project is in conjunction with our Math class. That being said, we can't use methods like rotate(), translate(), pushMatrix() and popMatrix() to help rotating the forms (each robot is a triangle).
So, the steps I followed to rotate the triangles were:
1) translate its center point to the origin (0,0) and the vertices using the same translation;
2) rotate all points, where (x, y) become (y, -x);
3) translate back to correct position (inverse of the step 1 translation).
I set some boundaries if statements to put the triangle back to screen if, after rotation, anything went off screen.
My problem
After turning, the triangle is appearing in strange position, like teleporting.
I added a few lines so we can get each vertex's coordinates, the direction it's going and detect when direction is changed.
Code
There are two files: TestRobots, Robot.
Robot:
import processing.core.PApplet;
public class Robot{
int colour;
String name;
float width;
float height;
float x1;
float y1;
float x2;
float y2;
float x3;
float y3;
int direction;
int speed;
float centralPointX = width/2;
float centralPointY = height/2;
PApplet parent;
public Robot(PApplet parent, String name, int colour, float x1, float y1, float x2, float y2, float x3, float y3, int speed){
this.parent = parent;
this.name = name;
this.colour = colour;
this.x1 = x1;
this.y1 = y1;
this.x2 = x2;
this.y2 = y2;
this.x3 = x3;
this.y3 = y3;
this.speed = speed;
direction=4;
width = x2-x3;
if (width < 0){
width *= -1;
}
if (y2 > y3){
height = y2-y1;
}else{
height = y3-y1;
}
if (height < 0){
height *= -1;
}
if (y1<y2 && y1<y3){
direction=4;
}
}
public void drawRobot(){
parent.fill(colour);
parent.triangle(x1, y1, x2, y2, x3, y3);
parent.ellipseMode(parent.CENTER);
parent.ellipse(x1, y1, 3, 3);
}
public void moveForward(){
if(x1<parent.width || y1 <parent.height || x1 > 0 || y1 > 0){
switch (direction){
case 1:
x1 += speed;
x2 += speed;
x3 += speed;
break;
case 2:
y1 += speed;
y2 += speed;
y3 += speed;
break;
case 3:
x1 -= speed;
x2 -= speed;
x3 -= speed;
break;
case 4:
y1 -= speed;
y2 -= speed;
y3 -= speed;
break;
}
}
}
public void turnLeft(){
//Store original coordinates.
float tempX1 = x1;
float tempY1 = y1;
float tempX2 = x2;
float tempY2 = y2;
float tempX3 = x3;
float tempY3 = y3;
//Calculate translation of the central point of triangle to the origin.
float xTranslation = 0 - centralPointX;
float yTranslation = 0 - centralPointY;
//Translate all points by the translation calculated.
float translatedX1 = tempX1 + xTranslation;
float translatedY1 = tempY1 + yTranslation;
float translatedX2 = tempX2 + xTranslation;
float translatedY2 = tempY2 + yTranslation;
float translatedX3 = tempX3 + xTranslation;
float translatedY3 = tempY3 + yTranslation;
//Rotate all points 90 degrees counterclockwise, (x, y) --> (y, -x).
float rotatedX1 = translatedY1;
float rotatedY1 = -translatedX1;
float rotatedX2 = translatedY2;
float rotatedY2 = -translatedX2;
float rotatedX3 = translatedY3;
float rotatedY3 = -translatedX3;
//Translate all points back.
x1 = rotatedX1 - xTranslation;
y1 = rotatedY1 - yTranslation;
x2 = rotatedX2 - xTranslation;
y2 = rotatedY2 - yTranslation;
x3 = rotatedX3 - xTranslation;
y3 = rotatedY3 - yTranslation;
//Check which y and which x are the smallest, in order to correct any negative numbers.
float minX;
float minY;
if (y1<y2 && y1<y3){
minY = y1;
} else if (y2<y1 && y2<y3){
minY = y2;
} else {
minY = y3;
}
if (x1<x2 && x1<x3){
minX = x1;
} else if (x2<x1 && x2<x3){
minX = x2;
} else {
minX = x3;
}
//Check which y and which x are the biggest, in order to correct any out-of-screen draws.
float maxX;
float maxY;
if (y1>y2 && y1>y3){
maxY = y1;
} else if (y2>y1 && y2>y3){
maxY = y2;
} else {
maxY = y3;
}
if (x1>x2 && x1>x3){
maxX = x1;
} else if (x2>x1 && x2>x3){
maxX = x2;
} else {
maxX = x3;
}
//Correct position if any coordinate is negative.
if((minY-speed)<=minY){
float differenceY = -minY + 10;
y1 += differenceY;
y2 += differenceY;
y3 += differenceY;
}
if(x1<=(x1-speed)){
float differenceX = -minX + 10;
x1 += differenceX;
x2 += differenceX;
x3 += differenceX;
}
//Correct position if any coordinate is bigger than the screen size.
if((parent.height<=(maxY+speed))){
float differenceY = (-maxY+parent.height) + 10;
y1 -= differenceY;
y2 -= differenceY;
y3 -= differenceY;
}
if((x1+speed)>=parent.width){
float differenceX = (-maxX+parent.width) + 10;
x1 -= differenceX;
x2 -= differenceX;
x3 -= differenceX;
}
//Change direction variable and adjust it between 0 and 4.
direction -=1;
if (direction == 0){
direction = 4;
}
}
public void patrol(){
System.out.println("Direction is: "+ direction);
if(((y1-speed)<= 0)||((y1+speed)>= parent.height) || ((x1+speed)>=parent.width)||((x1-speed)<=0)){
turnLeft();
System.out.println("The NEW direction is: "+ direction);
}
moveForward();
}
}
TestRobots:
import processing.core.PApplet;
public class TestRobots extends PApplet{
Robot alice = new Robot(this, "Alice", 255, 257f, 389f, 309f, 450f, 209f, 450f, 3);
public static void main(String[] args){
PApplet.main("TestRobots");
}
public void settings(){
size(1000, 500);
}
public void setup(){
frameRate(30);
}
public void draw(){
background(255);
alice.patrol();
System.out.println("x1 = "+ alice.x1);
System.out.println("y1 = "+ alice.y1);
System.out.println("x2 = "+ alice.x2);
System.out.println("y2 = "+ alice.y2);
System.out.println("x3 = "+ alice.x3);
System.out.println("y3 = "+ alice.y3);
alice.drawRobot();
}
}
Here is a print of the generated output. I snipped the part where it changes direction:
Direction is: 2
x1 = 62.0
y1 = 497.0
x2 = 10.0
y2 = 436.0
x3 = 110.0
y3 = 436.0
The NEW direction is: 1
x1 = 500.0
y1 = 58.0
x2 = 439.0
y2 = 110.0
x3 = 439.0
y3 = 10.0
Same strange behavior here:
Direction is: 4
x1 = 257.0
y1 = 2.0
x2 = 309.0
y2 = 63.0
x3 = 209.0
y3 = 63.0
The NEW direction is: 3
x1 = -1.0
y1 = 62.0
x2 = 60.0
y2 = 10.0
x3 = 60.0
y3 = 110.0
The NEW direction is: 2
x1 = 62.0
y1 = 74.0
x2 = 10.0
y2 = 13.0
x3 = 110.0
y3 = 13.0
Thank you very much for reading until here and sorry in advance if I wasn't clear enough. It's my first question!
Gustavo

I think your problem relates to the variables centralPointX and centralPointY. Look at this code you wrote:
//Calculate translation of the central point of triangle to the origin.
float xTranslation = 0 - centralPointX;
float yTranslation = 0 - centralPointY;
//Translate all points by the translation calculated.
float translatedX1 = tempX1 + xTranslation;
float translatedY1 = tempY1 + yTranslation;
float translatedX2 = tempX2 + xTranslation;
float translatedY2 = tempY2 + yTranslation;
float translatedX3 = tempX3 + xTranslation;
float translatedY3 = tempY3 + yTranslation;
It seems from this that you think centralPointX and centralPointY indicate the center of the triangle... However, look at where they are defined:
float centralPointX = width/2;
float centralPointY = height/2;
So they don't actually represent the center xy co-ordinates. What you need to do is fix this part:
//Calculate translation of the central point of triangle to the origin.
float xTranslation = 0 - centralPointX;
float yTranslation = 0 - centralPointY;
So that it actually does what the comment says it should do, i.e., calculates the x and y co-ordinates of the center of your triangle.
I won't implement this for you, because as you said, it is homework. However, I think this should definitely point you in the right direction. Also, keep up the good work. I wish every first question was as well thought out as this.

Processing sketch glitchs when using values depending upon values of x

My program contains main shapes i.e square, rect, circle they work good ( for x2,y2 in those shapes I used absolute values e.g square( x, y, 5, 5) ), but when working with triangle shape,
Triangles in my program glitches.
below is the code of my program modules,
if (vehicleStyle.getVehicleShape().equals(VehicleShape.TRIANGLE)) {
processingVisualizer
.fill(vehicleStyle.getColor().red,
vehicleStyle.getColor().green,
vehicleStyle.getColor().blue);
processingVisualizer.strokeWeight(1 * vehicleSize);
//System.out.println(x + "-" + y);
//## to place face toward movement direction
/*
* -----<|----
* | |
* \/ /\
* | |
* -----|>----
*/
float x2, y2, x3, y3;
if (x == 100) {
System.out.println( "x==100");
x2 = x - 5;
y2 = y + 5;
x3 = x + 5;
y3 = y + 5;
processingVisualizer.triangle(x, y, x2, y2, x3, y3);
} else if (x == 20) {
System.out.println( "x==20");
x2 = x - 2;
y2 = y - 2;
x3 = x + 2;
y3 = y - 2;
processingVisualizer.triangle(x, y, x2, y2, x3, y3);
} else if (y == 100) {
System.out.println( "y ==100");
x2 = x - 2;
y2 = y - 2;
x3 = x - 2;
y3 = y + 2;
processingVisualizer.triangle(x, y, x2, y2, x3, y3);
} else if (y == 20) {
System.out.println( "y ==20");
x2 = x+5;//x - 2;
y2 = y-5;
x3 = x+5 ;//- 2;
y3 = y+5;
processingVisualizer.triangle(x, y, x2, y2, x3, y3);
}
}
processingVisualizer.strokeWeight(1);
}

Can't test your code, but my assumption is the "glitch" happens because you are rendering a triangle only when your 4 conditions are met. You should update the triangle's corner positions based on your conditions if you like, but render the triangle all time, even when the positions are out of date:
if (vehicleStyle.getVehicleShape().equals(VehicleShape.TRIANGLE)) {
processingVisualizer
.fill(vehicleStyle.getColor().red,
vehicleStyle.getColor().green,
vehicleStyle.getColor().blue);
processingVisualizer.strokeWeight(1 * vehicleSize);
//System.out.println(x + "-" + y);
//## to place face toward movement direction
/*
* -----<|----
* | |
* \/ /\
* | |
* -----|>----
*/
float x2, y2, x3, y3;
if (x == 100) {
System.out.println( "x==100");
x2 = x - 5;
y2 = y + 5;
x3 = x + 5;
y3 = y + 5;
} else if (x == 20) {
System.out.println( "x==20");
x2 = x - 2;
y2 = y - 2;
x3 = x + 2;
y3 = y - 2;
} else if (y == 100) {
System.out.println( "y ==100");
x2 = x - 2;
y2 = y - 2;
x3 = x - 2;
y3 = y + 2;
} else if (y == 20) {
System.out.println( "y ==20");
x2 = x+5;//x - 2;
y2 = y-5;
x3 = x+5 ;//- 2;
y3 = y+5;
}
processingVisualizer.triangle(x, y, x2, y2, x3, y3);
}
processingVisualizer.strokeWeight(1);
}
Also, you can easily compute the direction of motion, if you store the previous position, using the arc tangent function (atan2()):
float angle = atan2(currentY-previousY,currentX-previousX);
Here's a quick example:
float cx,cy,px,py;//current x,y, previous x,y
float len = 15;
void setup(){
size(200,200);
background(255);
}
void draw(){
//update position - chase mouse with a bit of easing
cx -= (cx - mouseX) * .035;
cy -= (cy - mouseY) * .035;
//find direction of movement based on the current position
float angle = atan2(cy-py,cx-px);
//store previous position
px = cx;
py = cy;
//render
fill(255,10);noStroke();
rect(0,0,width,height);
fill(127,32);stroke(0);
pushMatrix();
translate(cx,cy);
pushMatrix();
rotate(angle);
triangle(len,0,-len,-len,-len,len);
line(0,0,len,0);
popMatrix();
popMatrix();
}

Float calculation keeps returning 0.0

I'm writing a method to return a List of Points between 2 Points. Somehow the slope (y2-y1)/(x2-x1) keeps giving me 0.0 all the time regardless startPoint and endPoint positions.
Here is the method:
public ArrayList<Point> calculatePath(Point startPoint, Point endPoint) {
ArrayList<Point> calculatedPath = new ArrayList<>();
int x1 = startPoint.x;
int y1 = startPoint.y;
int x2 = endPoint.x;
int y2 = endPoint.y;
System.out.println("Run");
if ((x2 - x1) != 0) {
float ratio = ((y2 - y1) / (x2 - x1));
System.out.println(ratio);
int width = x2 - x1;
for (int i = 0; i < width; i++) {
int x = Math.round(x1 + i);
int y = Math.round(y1 + (ratio * i));
calculatedPath.add(new Point(x, y));
}
} else {
if (y1 < y2) {
while (y1 == y2) {
calculatedPath.add(new Point(x1, y1));
y1++;
}
} else {
while (y1 == y2) {
calculatedPath.add(new Point(x1, y1));
y1--;
}
}
}
return calculatedPath;
}
Can anyone point out what i'm doing wrong? Thanks

Try casting your ints into floats as well
During your caluclation you need to cast at least one element to float:
float ratio = ((float)(y2 - y1) / (float)(x2 - x1));
That is because:
float a = integer / integer
^^^^^^^^^^^^^^^^^ - The result will be an integer.
Therefore u need to cast at least one of the
to float
This examples shows it easly:
public static void main(String[] args)
{
float resultWithoutCast = 5 / 3;
float resultWithCast = (float)5 /3 ;
System.out.println(resultWithoutCast);
System.out.println(resultWithCast);
}
It will print
1.0
1.6666666

You forgot to cast your int during division. Try something like this:-
float ratio = ((float)(y2 - y1) / (x2 - x1));

When performing arithmetic you need to make sure you use types which allow for the expected result. For example, the issue with your code is you are looking for a floating point result but using int - the problem here is int will simply truncate any floating point.
There are a couple of ways to solving this problem - as already suggested you could use a cast
float ratio = ((float)(y2 - y1) / (x2 - x1));
Or you could use float variables, it makes for more readable code e.g.
float x1 = (float)startPoint.X;
float y1 = (float)startPoint.Y;
...
float ratio = (y2 - y1) / (x2 - x1);
However, this results in more casting.
Alternatively, you could swap out Point for PointF and eliminate casting completely.

Check for intersection between line segments in path [duplicate]

I have a path stored as points in an arraylist and I want to check if the line segments are intersecting. For some unknown reason it's not working! I don't get any message in the LogCat despite that I'm drawing a shape that intersects. Preciate if someone could see what I have done wrong or have suggestions how to improve code.
// Intersection control
if(touchActionUp) {
// Loop throw all points except the last 3
for (int i = 0; i < points.size()-3; i++) {
Line line1 = new Line(points.get(i), points.get(i+1));
// Begin after the line above and check all points after that
for (int j = i + 2; j < points.size()-1; j++) {
Line line2 = new Line(points.get(j), points.get(j+1));
// Call method to check intersection
if(checkIntersection(line1, line2)) {
Log.i("Intersection", "Yes!");
}
}
}
}
And the method:
// Method to check for intersection between lines
private boolean interceptionControl(Line line1, Line line2) {
int x1 = line1.pointX1;
int x2 = line1.pointX2;
int x3 = line2.pointX1;
int x4 = line2.pointX2;
int y1 = line1.pointY1;
int y2 = line1.pointY2;
int y3 = line2.pointY1;
int y4 = line2.pointY2;
// Check if lines are parallel
int denom = (y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1);
if(denom == 0) { // Lines are parallel
// ??
}
double a = ((x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3)) / denom;
double b = ((x2 - x1) * (y1 - y3) - (y2 - y1) * (x1 - x3)) / denom;
// Check for intersection
if( a >= 0.0f && a <= 1.0f && b >= 0.0f && b <= 1.0f) {
return true;
}
return false;
}

You are using int for coordinates and so it does integer division (i.e., 3/2 = 1). This might be the reason when you are dividing by denom. You can fix it by dividing by ((double)denom) instead of simply denom.