Two sprites collide - which one rear ended the other? - javascript

I am writing a simple game (in Javascript) where circular sprites can collide with each other. I store an angle (in degrees) and an x,y for each object. I have the collision detection working, but I need to work out which sprite hit the other. So for two objects colliding which one drove into the other (i.e. the front of which sprite made contact with the 'non front' of the other). Or to put it another way, which one rear ended the other. In the case of very close head on accidents e.g. where they make an absolute direct head on collision (180 degree collision) or are within say 5 degrees of doing so (175-185 degree collision), then I need to spot this and attribute no 'blame' to either sprite. What would the maths, or better still the code, for this be?
var angleA = Math.abs(collidedWith.angle - 180);
var angleB = Math.abs(this.angle - 180);
var angleConsideredHeadOn = 5;
var anglesSubtracted = Math.abs(angleA - angleB);
if (anglesSubtracted < angleConsideredHeadOn) {
var headOn = true;
}
if (anglesSubtracted >= angleConsideredHeadOn) {
// There was a non-head on crash
var xA = collidedWith.velocityX;
var yA = collidedWith.velocityY;
var xB = this.velocityX;
var yB = this.velocityY;
// Need to determine which sprites 'front' collided with the other's side or read
}

To detect the head-on case, take the dot product of their two (pre-impact) velocity vectors which will give you the cosine between their headings. If that value is less than -0.9 Pi then the collision is head on.
Otherwise, the sprite to blame is the one with the highest absolute velocity (since a slow sprite cannot possibly rear-end a fast sprite).

This seems to work...
// Javascript
// Two objects 'this' and 'collidedWith' have collided
// this code determines who head butted the other
// Based on having an angle and an X and Y velocity (xDir and yDir) where volicity could be negative (going left or up) or positive (going right or down)
// 'degrees' is a function which converts radians to degrees
var angleConsideredHeadOn = 15;
var theta_a = degrees(Math.atan2(this.xDir, this.yDir));
var theta_b = degrees(Math.atan2(collidedWith.xDir, collidedWith.yDir));
var a_to_b = degrees(Math.atan2(this.posY - collidedWith.posY, this.posX - collidedWith.posX)); // Normal angle from 'this's perspective
var b_to_a = -a_to_b % 360; // ditto from collidedWith's perspective
var collision_angle_a = Math.abs(theta_a - a_to_b) % 360
var collision_angle_b = Math.abs(theta_b - b_to_a) % 360
// Collision is close to head on
if (Math.abs(collision_angle_a - collision_angle_b) < angleConsideredHeadOn) return;
//
if (collision_angle_a < collision_angle_b) {
// 'collidedWith' head butted the other object
} else {
// 'this' head butted the other object
}

Related

Rotation facing mouse not correct with p5 libraries

i am making a javascript shooter game.i want the the player to rotate towards the mouse.it is working, but the rotation was not correct.
I tried this with an image, and it works, but with the sprite itself(player1), is not.
i have asked this once before but received no answer
I am a beginner in javascript, so help would be appreciated.
I am using the p5.js libraries
Here is my code snippet:
//variables
var player1;
var gun1;
var gun2;
function preload(){
img = loadImage('rect1.png');
}
function setup(){
//creating sprites
player1 = createSprite(200,200,30,30)
gun = createSprite(player1.x,player1.y-20,5,30)
gun.shapeColor = "black"
player1.addImage("player",img)
player1.scale = 0.2
}
function draw(){
canvas = createCanvas(displayWidth-20, displayHeight-120);
background("#32CD32");
push()
gun.x = player1.x;
gun.y = player1.y-15;
// functions to move
//up
if(keyDown("up")){
player1.y = player1.y - 5;
}
//down
if(keyDown("down")){
player1.y = player1.y + 5;
}
//right
if(keyDown("right")){
player1.x = player1.x + 5;
}
//left
if(keyDown("left")){
player1.x = player1.x - 5;
}
angleMode(DEGREES)
imageMode(CENTER)
let a = atan2(mouseY - height / 2, mouseX - width / 2);
translate(width/2, height/2);
//rotate(a)
player1.rotation = a
//image(img,0,0,40,40)
pop()
drawSprites();
}
I think I'm using a dated version of p5.play, so there's not much in your code that works for me, but here's what I think is going on based on what you're saying.
If you want to understand what the deal is with atan2(), you first have to understand atan(). Basically, you have the ordinary trig functions sin, cos, and tan. Then you have the inverse trig functions arcsin, arccos, and arctan (abbreviated asin, acos, and atan). The arctan function is useful because you can input a slope and it will give you the angle of that slope. There's a catch, though; atan will only give values between -pi/2 and pi/2. This covers all non-vertical lines, but what if you wanted to use it for a vector or something that has direction? atan2() solves that problem. Instead of taking one input (a ratio: rise/run), it takes two inputs, a rise and a run. This prevents dividing by zero (for vertical lines) and signs of rise and run cancelling. The first input is the rise, and the second is the run. The output is the angle between the vector with those coordinates and the x-axis. So atan2() will give you some angle between -pi and pi.
Now let's look at what you have put into the atan2() function:
atan2(mouseY - height / 2, mouseX - width / 2);
So the vector you're considering is the vector from the middle of the canvas to the mouse. If this is what you want, great. If it's not, maybe consider
atan2(mouseY - player1.y, mouseX - player1.y);
which yields the "heading" (not really the heading) of the vector from the player's position to the mouse.
There are a couple of other potential problems (I can't figure out which one it is because p5.play isn't behaving, or I'm doing something else wrong):
radians/degrees: sometimes this stuff goes wrong. Try printing a and seeing if it's what you're looking for. If it's in degrees, consider saying player1.rotation = radians(a) instead. I know that p5.Vector.fromAngle() doesn't care about angleMode, maybe sprite.rotation doesn't either?
I don't know how drawSprites works, but you might consider putting it inside of the push()/pop() section. When drawing other shapes, this is how you get a rotation about the point (x, y):
let x = 100;
let y = 200;
let a = atan2(mouseY - y, mouseX - x);
push();
translate(x, y);
rotate(a);
square(-10, -10, 20);
pop();
The square is centered at (x,y), and is rotated about (x,y) toward the cursor. It's necessary that you do it in this order: push, translate, rotate, shape, pop. If you mix up that order, it doesn't do it right. But what you have is push, translate, pop, shape. I don't know how sprite.rotate works, so maybe it's supposed to do what you want. But here is another way to do a rotation. (If you're going to do it this way, I think you'd have to draw player1 "at (0,0)" after the translation and rotation, before pop)
I hope this helped!

Turning objects so they reset their y-rotation in Three.js

Using three.js, I'm creating a game with cars that move in a specific direction, depending on their y-rotation. An example would be 90 degrees. I use object.translateZ() to move them forward but I've run into a problem.
I'm using physijs to simulate the cars. Cars that run into each other may change their y-rotation (because of the crash) and I want to find a way for the cars to slowly change their rotation back to the original rotation like they are turning to get back on the road. Without this my city is very chaotic.
Here's the code that I'm already using (this is just part of it):
var targetRotation = 90
var rotation = car.mesh.rotation.y * 180 / Math.PI //to convert to degrees
I want to find a way to slowly change the car's rotation so it's the same as the target rotation.
Any help is appreciated! (but some sort of function would be perfect)
I've done stuff like this before in other systems (2D, not Three.js), and essentially all you want to do is gradually increment the angle until you reach something close enough to the target angle. Usually this means the float is less than the turning speed you're incrementing by (so you don't "overshoot" the value).
The amount of the increment depends on how quickly you want them to turn.
You also want to check if it's better to increase the angle or decrease (do you turn clockwise or counterclockwise) depending on if you're closer to 360 or 0. This prevents the car from turning the "long way" around. You can find this out by seeing if the difference is greater/less than 180.
We can use the modulus operator to get the "real" angle between -360/360.
var currentAngle = car.mesh.rotation.y * 180 / Math.PI; //car's angle in degrees
var targetAngle = 90; //preferred angle *this time* ;)
var turningSpeed = 1; //one degree per update (wayyy to high for real stuff, usually)
currentAngle = currentAngle % 360; //get the 0-360 remainder
if ( Math.abs(targetAngle - currentAngle) >= turningSpeed) {
var addto = 0
if (targetAngle - currentAngle < 0) {
addto = 360
}
if ( targetAngle - currentAngle + addto <= 180 ) {
currentAngle += turningSpeed;
}
else {
currentAngle -= turningSpeed;
}
}
else { currentAngle = targetAngle; } //we're close enough to just set it
car.mesh.rotation.y = ( currentAngle * Math.PI ) / 180; // back to radians!

Canvas water/blob physics on a circular path with texture?

this is my first question after having relied on this site for years!
Anyway, I'd like to accomplish something similar to this effect:
http://www.flashmonkey.co.uk/html5/wave-physics/
But on a circular path, instead of a horizon. Essentially, a floating circle/blob in the center of the screen that would react to mouse interaction. What I'm not looking for is gravity, or for the circle to bounce around the screen - only surface ripples.
If at all possible I'd like to apply a static texture to the shape, is this a possibility? I'm completely new to Canvas!
I've already tried replacing some code from the above example with circular code from the following link, to very limited success:
http://www.html5canvastutorials.com/tutorials/html5-canvas-circles/
If only it were that easy :)
Any ideas?
Thanks in advance!
I tried to figure out how wave simulation works using View Source and JavaScript console. It's working fine but threw some JS errors. Also, it seems physics update is entangled with rendering in the render() method.
Here is what I found about the code:
The mouseMove() method creates disturbances on the wave based on mouse position, creating a peak around the mouse. The target variable is the index of the particle that needs to be updated, it's calculated from mouse pos.
if (particle && mouseY > particle.y) {
var speed = mouseY - storeY;
particles[target - 2].vy = speed / 6;
particles[target - 1].vy = speed / 5;
particles[target].vy = speed / 3;
particles[target + 1].vy = speed / 5;
particles[target + 2].vy = speed / 6;
storeY = mouseY;
}
Then, the particles around target are updated. The problem I found is that it does no bounds checking, i.e. it can potentially particles[-1] when target == 0. If that happens, an exception is thrown, the method call ends, but the code does not stop.
The render() method first updates the particle positions, then renders the wave.
Here is its physics code:
for (var u = particles.length - 1; u >= 0; --u) {
var fExtensionY = 0;
var fForceY = 0;
if (u > 0) {
fExtensionY = particles[u - 1].y - particles[u].y - springs[u - 1].iLengthY;
fForceY += -fK * fExtensionY;
}
if (u < particles.length - 1) {
fExtensionY = particles[u].y - particles[u + 1].y - springs[u].iLengthY;
fForceY += fK * fExtensionY;
}
fExtensionY = particles[u].y - particles[u].origY;
fForceY += fK / 15 * fExtensionY;
particles[u].ay = -fForceY / particles[u].mass;
particles[u].vy += particles[u].ay;
particles[u].ypos += particles[u].vy;
particles[u].vy /= 1.04;
}
Basically, it's Hooke's Law for a chain of particles linked by springs between them. For each particle u, it adds the attraction to the previous and next particles (the if statements check if they are available), to the variable fForceY. I don't fully understand the purpose of the springs array.
In the last four lines, it calculates the acceleration (force / mass), updates the velocity (add acceleration), then position (add velocity), and finally, reduce velocity by 1.04 (friction).
After the physics update, the code renders the wave:
context.clearRect(0, 0, stageWidth, stageHeight);
context.fillStyle = color;
context.beginPath();
for (u = 0; u < particles.length; u++) {
...
}
...
context.closePath();
context.fill();
I'm not explaining that, you need to read a canvas tutorial to understand it.
Here are some ideas to get started, note that I didn't test these code.
To modify the code to draw a circular wave, we need introduce a polar coordinate system, where the particle's x-position is the angle in the circle and y-position the distance from center. We should use theta and r here but it requires a large amount of refactoring. We will talk about transforming later.
mouseMove(): Compute particle index from mouse position on screen to polar coordinates, and make sure the disturbance wrap around:
Define the function (outside mouseMove(), we need this again later)
function wrapAround(i, a) { return (i + a.length) % a.length; }
Then change
particles[target - 2] --> particles[wrapAround(target - 2, particles)]
particles[target - 1] --> particles[wrapAround(target - 1, particles)]
...
The modulo operator does the job but I added particles.length so I don't modulo a negative number.
render(): Make sure the force calculation wrap around, so we need to wrapAround function again. We can strip away the two if statements:
fExtensionY = particles[wrapAround(u - 1, particles)].y - particles[u].y - springs[wrapAround(u - 1, springs)].iLengthY;
fForceY += -fK * fExtensionY;
fExtensionY = particles[u].y - particles[wrapAround(u + 1, particles)].y - springs[warpAround(u, springs)].iLengthY;
fForceY += fK * fExtensionY;
Here is the result so far in jsfiddle: Notice the wave propagate from the other side. http://jsfiddle.net/DM68M/
After that's done, the hardest part is rendering them on a circle. To do that, we need coordinate transform functions that treat particle's (x, y) as (angle in the circle, distance from center), and we also need inverse transforms for mouse interaction in mouseMove().
function particleCoordsToScreenCoords(particleX, particleY) {
return [ radiusFactor * particleY * Math.cos(particleX / angleFactor),
radiusFactor * particleY * Math.sin(particleX / angleFactor) ];
}
function screenCoordsToParticleCoords(screenX, screenY) {
// something involving Math.atan2 and Math.sqrt
}
Where the ...Factor variables needed to be determined separately. The angleFactor is two pi over the highest x-position found among particles array
Then, in the coordinates supplied to the context.lineTo, context.arc, use the particleCoordsToScreenCoords to transform the coordinates.

Finding a location on cosine curve with a specified distance to another location JS

I am working on a "rally" game where a car is drawing on hills made of cosine curves. I know the current xspeed of the car (without hills) but the problem is that I need to know the xspeed of the car on the hills to be able to draw the wheels on right places and keep the speed steady.
At the moment my solution looks like this.
function drawWheelOnBasicHill(hillStart, xLocWheel, wheelNro) {
var cw = 400 //the width of the hill
t_max = 2*Math.PI;
var scale = 80, step = cw, inc = t_max/step;
var t1 = (xLocWheel-hillStart)*inc
var y1 = -scale*0.5 * Math.cos(t1);
if(wheelNro == 1 ){ //backwheel
drawRotatedImage(wheel, car.wheel1x, car.wheel1y-y1-45,sx);
//drawing the wheel on canvas
} else { //frontwheel
drawRotatedImage(wheel, car.wheel2x, car.wheel2y-y1-45,sx);
}
for(var i=1; i<=car.speed; i++){ //finding the next xlocation of the wheel with the
//same distance (on the curve) to the previous location as the speed of the car(=the
//distance to the new point on the flat ground)
var t2 = (xLocWheel + i -hillStart)*inc
var y2 = -scale*0.5 * Math.cos(t2);
if(Math.round(Math.sqrt(i^2+(y2-y1)^2))==car.speed){
sx = sx+i; //the new xcoordinate break;
}
}
}
The for loop is the problem. It might bee too slow (animation with fps 24). I cant understand why the if statement isnt working at the moment. It works sometimes but most of the times the value of the condition newer reaches the actual xspeed.
Are there some more efficient and easier ways to do this? Or does this code contain some errors? I really appreciate your efforts to solve this! Ive been looking at this piece of code the whole day..
So i is the variable and
x2=x1+i
t2=t1+i*inc
y1=-scale*0.5 * Math.cos(t1)
y2=-scale*0.5 * Math.cos(t2)
which somehow is strange. The landscape should be time independent, that is, y should be a function of x only. The time step is external, determined by the speed of the animation loop. So a more logical model would have dx as variable and
dt = t2-t1
x2 = x1 + dx
y1 = f(x1) = -0.5*scale*cos(x1)
y2 = f(x2) = -0.5*scale*cos(x2)
and you would be looking for the intersection of
(x2-x1)^2+(y2-y1)^2 = (speed*dt)^2
which simplifies to
(speed*dt)^2=dx^2+0.25*scale^2*(cos(x1+dx)-cos(x1))^2
For small values of dx, which would be the case if dt or speed*dt is small,
cos(x1+dx)-cos(x1) is approx. -sin(x1)*dx
leading to
dx = (speed*dt) / sqrt( 1+0.25*scale^2*sin(x1)^2 )
To get closer to the intersection of curve and circle, you can then iterate the fixed point equation
dydx = 0.5*scale*(cos(x1+dx)-cos(x1))/dx
dx = (speed*dt) / ( 1+dydx^2 )
a small number of times.

Finding point n% away from the centre of a semicircle in Javascript?

I'm sorry to say that Math really isn't my strong suit. Normally I can get by, but this has got me totally stumped.
I'm trying to code up a quiz results screen in HTML/CSS/Javascript.
On my interface, I have a semicircle (the right hemisphere of a target).
I have a range of 'scores' (integers out of 100 - so 50, 80, 90 etc.).
I need to plot these points on the semicircle to be n% away from the centre, where n is the value of each score - the higher the score, the closer to the centre of the target the point will appear.
I know how wide my semicircle is, and have already handled the conversion of the % values so that the higher ones appear closer to the centre while the lower ones appear further out.
What I can't wrap my head around is plotting these points on a line that travels out from the centre point (x = 0, y = target height/2) of the target at a random angle (so the points don't overlap).
Any suggestions are gratefully received!
Do you have an example of what you want this to look like? It sounds like you want to divide up the circle into N slices where N is the number of points you need to display, then plot the points along each of those radii. So you might have something like:
Edit: code was rotating about the origin, not the circle specified
var scores = [];
//...
//assume scores is an array of distances from the center of the circle
var points = [];
var interval = 2 * Math.PI / N;
var angle;
for (var i = 0; i < N; i++) {
angle = interval * i;
//assume (cx, cy) are the coordinates of the center of your circle
points.push({
x: scores[i] * Math.cos(angle) + cx,
y: scores[i] * Math.sin(angle) + cy
});
}
Then you can plot points however you see fit.
After much headscratching, I managed to arrive at this solution (with the help of a colleague who's much, much better at this kind of thing than me):
(arr_result is an array containing IDs and scores - scores are percentages of 100)
for (var i = 0; i < arr_result.length; i++){
var angle = angleArray[i]; // this is an array of angles (randomised) - points around the edge of the semicircle
var radius = 150; // width of the semicircle
var deadZone = 25 // to make matters complicated, the circle has a 'dead zone' in the centre which we want to discount
var maxScore = 100
var score = parseInt(arr_result[i]['score'], 10)
var alpha = angle * Math.PI
var distance = (maxScore-score)/maxScore*(radius-deadZone) + deadZone
var x = distance * Math.sin(alpha)
var y = radius + distance * Math.cos(alpha)
$('#marker_' + arr_result[i]['id'], templateCode).css({ // target a specific marker and move it using jQuery
'left' : pointX,
'top': pointY
});
}
I've omitted the code for generating the array of angles and randomising that array - that's only needed for presentational purposes so the markers don't overlap.
I also do some weird things with the co-ordinates before I move the markers (again, this has been omitted) as I want the point to be at the bottom-centre of the marker rather than the top-left.

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