circle-circle collision problem - algorithm

I have a problem with circle-circle collision detection.I used the following algorithm
func collision(id,other.id)
{
var vaP1,vaP2,dis,va1,vb1,va2,vb2,vp1,vp2,dx,dy,dt;
if (id!=other.id)
{
dx=other.x-x;
dy=other.y-y;
dis=sqrt(sqr(dx)+sqr(dy));
if dis<=radius+other.radius
{
//normalize
dx/=dis;
dy/=dis;
//calculate the component of velocity in the direction
vp1=hspeed*dx+vspeed*dy;
vp2=other.hspeed*dx+other.vspeed*dy;
if (vp1-vp2)!=0
{
dt=(radius+other.radius-dis)/(vp1-vp2);
//move the balls back so they just touch
x-=hspeed*dt;
y-=vspeed*dt;
other.x-=other.hspeed*dt;
other.y-=other.vspeed*dt;
//projection of the velocities in these axes
va1=(hspeed*dx+vspeed*dy);
vb1=(vspeed*dx-hspeed*dy);
va2=(other.hspeed*dx+other.vspeed*dy);
vb2=(other.vspeed*dx-other.hspeed*dy);
//new velocities in these axes. take into account the mass of each ball.
vaP1=(va1+bounce*(va2-va1))/(1+mass/other.mass);
vaP2=(va2+other.bounce*(va1-va2))/(1+other.mass/mass);
hspeed=vaP1*dx-vb1*dy;
vspeed=vaP1*dy+vb1*dx;
other.hspeed=vaP2*dx-vb2*dy;
other.vspeed=vaP2*dy+vb2*dx;
//we moved the balls back in time, so we need to move them forward
x+=hspeed*dt;
y+=vspeed*dt;
other.x+=other.hspeed*dt;
other.y+=other.vspeed*dt;
}
}
}
x=ball 1 x-position
y=ball 1 y-position
other.x= ball 2 x position
other.y=ball 2 y position
this algorithm works well when i have a ball image of 40 x 40 pixel and ball center is (20,20) means image consists only ball.But the problem arises when image size is 80 x 80.and ball center position is (60,60),means ball is lower right corner with radius 20.
in this case there are multiple collision occur,means the portion
x+=hspeed*dt;
y+=vspeed*dt;
other.x+=other.hspeed*dt;
other.y+=other.vspeed*dt;
unable to seperate the ball /velocity does not change according to collision.
I have changed the value of x which is the center of image 40,40 to 60,60 center of ball adding 20.but the result is same .Can any one tell me what is the problem.I think algorithm is correct because it works nicely in all other case and lots of people used this algorithm.problem is changing position from image center to ball center.what correction should i do for this??? or any idea.if someone want to help plz give me e-mail address so that i can send my full project.

I didnt have the mental power to digest your entire question, but here is my 2 cents on how to solve your problem
1) The simplest way to detect a circle collision with another is to check if their distance is less than the radius of the combined circles. (i might be wrong with the math, so correct me if i am wrong)
Circle c1,c2;
float distance = DISTANCE(c1.center,c2.center);
if(distance < c1.radius + c2.radius)
{
// collision .. BOOOOOOM
}
2) Try to use accurate data types. Try not to convert floats to integers without checking overflow, underflow and decimal points. Better still, just use floats .
3) Write a log and trace through your values. See if there are any obvious maths errors .
4) Break down your code to its simplest portion. Try to remove all that velocity computation to get the simplest movements to help you debug.

I will not give you the answer that you are looking for and I am not sure someone else will. The amount of code that must be decyphered to get you the answer may not warrant the reward. What I would recommend is to losen the coupling in your algorithm. The function above is doing way too much work.
Ideally you would have a collision detection that concentrated only on the collision and not on advancing the balls. Something like function shown below and that would allow other developers to help you more easily if you still had a problem.
function(firstCircleCenterX, firstCircleCenterY, secondCircleCenterX, secondCircleCenterY, firstCircleRadius, secondCircleRadius)
{
...this code should concentrate on logic to determine collision
...use pythagoran theory to find distance between the two centers
...if the distance between the two centers is less than ((2*firstCircleRadius)+(2*secondCircleRadius) then you have a collision
...return true or false depending on collision
}

Related

How do I synchronize scale and position of map and point layers in d3.js?

I've seen many example maps in d3 where points added to a map automatically align as expected, but in code I've adapted from http://bl.ocks.org/bycoffe/3230965 the points I've added do not line up with the map below.
Example here: https://naltmann.github.io/d3-geo-collision/
(the points should match up with some major US cities)
I'm pretty sure the difference is due to the code around scale/range, but I don't know how to unify them between the map and points.
Aligning geographic features geographically with your example will be challenging - first you are projecting points and then scaling x,y:
node.cx = xScale(projection(node.coordinates)[0]);
node.cy = yScale(projection(node.coordinates)[1]);
The ranges for the scales is interesting in that both limits of both ranges are negatives, this might be an attempt to rectify the positioning of points due to the cumulative nature of forces on the points:
.on('tick', function(e) {
k = 10 * e.alpha;
for (i=0; i < nodes.length; i++) {
nodes[i].x += k * nodes[i].cx
nodes[i].y += k * nodes[i].cy
This is challenging as if we remove the scales, the points move farther and farther right and down. This cumulative nature means that with each tick the points drift further and further from recognizable geographic coordinates. This is fine when dealing with a set of geographic data that undergoes the same transformation, but when dealing with a background that doesn't undergo the same transformation, it's a bit hard.
I'll note that if you want a map width of 1800 and a height of 900, you should set the mercator projection's translate to [1800/2,900/2] and the scale to something like 1800/Math.PI/2
The disconnection between geographic coordinates and force coordinates appears to be very difficult to rectify. Any solution for this particular layout and dimensions is likely to fail on different layouts and dimensions.
Instead I'd suggest attempting to use only a projection to place coordinates and not cumulatively adding force changes to each point. This is the short answer to your question.
For a longer answer, my first thought was to get rid of the collision function and use an anchor point linked to a floating point for each city, only drawing the floating point (using link distance to keep them close). This is likely a cleaner solution, but one that is unfortunately completely different than what you've attempted.
However, my second thoughts were more towards keeping your example, but removing the scales (and the cumulative forces) and reducing the forces to zero so that the collision function can work without interference. Based on those thoughts, here's a demonstration of a possible solution.

Game Maker - Touch Event

I´m making my first game in Game Maker.
In the game i need to the user to draw a figure, for example a rectangle, and the game has to recognize the figure. How can i do this?
Thanks!
Well, that is a pretty complex task. To simplify it, you could ask him to place a succession of points, using the mouse coordinates in the click event, and automatically connect them with lines. If you store every point in the same ds_list structure, you will be able to check conditions of angle, distance, etc. This way, you can determine the shape. May I ask why you want to do this ?
The way I would solve this problem is pretty simple. I would create a few variables for each point when someone clicked on one of the points it would equal true. and wait for the player to click on the next point. If the player clicked on the next point i would call in a sprite as a line using image_angle to line both points up and wait for the player to click the next point.
Next I would have a step event waiting to see if all points were clicked and when they were then to either draw a triangle at those coordinates or place an sprite at the correct coordinates to fill in the triangle.
Another way you could do it would be to decide what those points would be and check against mouse_x, and mouse_y to see if that was a point and if it was then do as above. There are many ways to solve this problem. Just keep trying you will find one that works for your skill level and what you want to do.
You need to use draw_rectangle(x1, y1, x2, y2, outline) function. As for recognition of the figure, use point_in_rectangle(px, py, x1, y1, x2, y2).
I'm just wondering around with ideas cause i can't code right now. But listen to this, i think this could work.
We suppose that the user must keep his finger on touchscreen or an event is triggered and all data from the touch event is cleaned.
I assume that in future you could need to recognize other simple geometrical figures too.
1 : Set a fixed amount of pixels of movement defined dependent on the viewport dimension (i'll call this constant MOV from now on), for every MOV you store in a buffer (pointsBuf) the coordinates of the point where the finger is.
2 : Everytime a point is stored you calculate the average of either X and Y coordinates for every point. (Hold the previous average and a counter to reduce time complexity). Comparing them we now can know the direction and versus of the line. Store them in a 2D buffer (dirVerBuf).
3 : If a point is "drastically" different from the most plain average between the X and Y coordinates we can assume that the finger changed direction. This is where the test part of MOV comes critical, we must assure to calculate an angle now. Since only a Parkinsoned user would make really distorted lines we can assume at 95% that we're safe to take the 2nd point that didn't changed the average of the coordinate as vertex and let's say the last and the 2nd point before vertex to calculate the angle. You have now one angle. Test the best error margin of the user to find if the angle is about to be a 90, 60, 45, ecc.. degrees angle. Store in a new buffer (angBuf)
4 : Delete the values from pointsBuf and repeat step 2 and 3 until the user's finger leaves the screen.
5 : if four of the angles are of 90 degrees, the 4 versus and two of the directions are different, the last point is somewhat near (depending from MOV) the first angle stored and the two X lines and the Y lines are somewhat equal, but of different length between them, then you can connect the 4 angles using the four best values next to the 4 coordinates to make perfect rectangular shape.
It's late and i could have forgotten something, but with this method i think you could even figure out a triangle, a circle, ecc..
With just some edit and confronting.
EDIT: If you are really lazy you could instead use a much more space complexity heavy strategy. Just create a grid of rectangles or even triangles of a fixed dimension and check which one the finger has touched, connect their centers after you'have figured out the shape, obviously ignoring the "touched for mistake" ones. This would be extremely easy to draw even circles using the native functions. Gg.

Path generation for non-intersecting disc movement on a plane

What I'm looking for
I have 300 or fewer discs of equal radius on a plane. At time 0 each disc is at a position. At time 1 each disc is at a potentially different position. I'm looking to generate a 2D path for each disc for times between 0 and 1 such that the discs do not intersect and the paths are relatively efficient (short) and of low curvature if possible. (for example, straight lines are preferable to squiggly lines)
Lower computation time is generally more important than exactness of solution. (for example, a little intersection is okay, and I don't necessarily need an optimal result)
However, discs shouldn't teleport through each other, stop or slow abruptly, or change direction abruptly -- the "smoother" the better. Only exception is time 0 and 1.
Paths can be expressed in a sampled form or piecewise linear nature (or better) -- I'm not worried about having truly smooth paths via splines. (I can approximate that if I so need.)
What I've tried
You can see a demo of my best attempt (via Javascript + WebGL). Be warned, it will load slowly on older computers due to the computations involved. It appears to work in Firefox/Chrome/IE11 under Windows.
In this demo I've represented each disc as an "elastic band" in 3D (that is, each disc has a position at each time) and ran a simple game-style physics engine that resolves constraints and treats each point in time like a mass with springs to the previous/next time. ('Time' in this case is just the third dimension.)
This actually works pretty well for small N (<20), but in common test cases (for example, start with discs arranged in circle, move each disc to the opposite point on the circle) this fails to generate convincing paths since the constraints and elasticity propagate slowly throughout the springs. (for example, if I slice time into 100 discrete levels, tension in the elastic bands only propagates one level per each simulation cycle) This makes good solutions require many (>10000) iterations, and that is tediously slow for my application. It also fails to reasonably resolve many N>40 cases, but this may be simply because I can't feasibly run enough iterations.
What else I've tried
My initial attempt was a hill-climber that started with straight-line paths which were gradually mutated. Solutions which measured better than the currently best solution replaced the currently best solution. Better measurements resulted from the amount of intersection (that is, completely overlapping measured worse than just grazing) and the length of the paths (shorter paths were better).
This produced some surprisingly good results, but unreliably, likely getting stuck in local minima very often. It was extremely slow for N>20. I tried applying a few techniques (simulated annealing, a genetic algorithms approach, etc) in an attempt to get around the local minima issue, but I never had much success.
What I'm trying
I'm optimizing the "elastic band" model so that tension and constraints propagate much more quickly in the time dimension. This would save a good deal of needed iterations in many cases, however in highly-constrained scenarios (for example, many discs trying to cross the same location) an untenable amount of iterations would still be required. I'm no expert on how to solve constraints or propagate springs more quickly (I've tried reading a few papers on non-stretchable cloth simulation, but I haven't been able to figure out if they apply), so I'd be interested in if there's a good way to go about this.
Ideas on the table
Spektre has implemented a very fast RTS-style unit movement algorithm that works admirably well. It's fast and elegant, however it suffers from RTS-movement style problems: sudden direction changes, units can stop abruptly to resolve collisions. Additionally, units do not all arrive at their destination at the same time, which is essentially an abrupt stop. This may be a good heuristic to make viable non-smooth paths after which the paths could be resampled in time and a "smoothing" algorithm could be run (much like the one used in my demo.)
Ashkan Kzme has suggested that the problem may be related to network flows. It would appear that the minimum cost flow problem could work, as long as space and time could be discritized in a reasonable manner, and the running times could be kept down. The advantage here is that it's a well studied set of problems, but sudden velocity changes would still be an issue and some sort of "smoothing" post-steps may be desirable. The stumbling block I'm currently having is deciding on a network representation of space-time that wouldn't result in discs teleporting through each other.
Jay Kominek posted an answer that uses a nonlinear optimizer to optimize quadratic Bezier curves with some promising results.
Have played with this for fun a bit and here the result:
Algorithm:
process each disc
set speed as constant*destination_vector
multiplicative constant a
and limit the speed to constant v afterwards
test if new iterated position does not conflict any other disc
if it does rotate the speed in one direction by some angle step ang
loop until free direction found or full circle covered
if no free direction found mark disc as stuck
This is how it looks like for circle to inverse circle path:
This is how it looks like for random to random path:
stuck disc are yellow (none in these cases) and not moving discs are at destination already. This can also get stuck if there is no path like if disc already in destination circles another ones destination. To avoid that you need also change the colliding disc also ... You can play with the ang,a,v constants to make different appearance and also you could try random direction of angle rotation to avoid that swirling/twister movement
Here the source code I used (C++):
//---------------------------------------------------------------------------
const int discs =23; // number of discs
const double disc_r=5; // disc radius
const double disc_dd=4.0*disc_r*disc_r;
struct _disc
{
double x,y,vx,vy; // actual position
double x1,y1; // destination
bool _stuck; // is currently stuck?
};
_disc disc[discs]; // discs array
//---------------------------------------------------------------------------
void disc_generate0(double x,double y,double r) // circle position to inverse circle destination
{
int i;
_disc *p;
double a,da;
for (p=disc,a=0,da=2.0*M_PI/double(discs),i=0;i<discs;a+=da,i++,p++)
{
p->x =x+(r*cos(a));
p->y =y+(r*sin(a));
p->x1=x-(r*cos(a));
p->y1=y-(r*sin(a));
p->vx=0.0;
p->vy=0.0;
p->_stuck=false;
}
}
//---------------------------------------------------------------------------
void disc_generate1(double x,double y,double r) // random position to random destination
{
int i,j;
_disc *p,*q;
double a,da;
Randomize();
for (p=disc,a=0,da=2.0*M_PI/double(discs),i=0;i<discs;a+=da,i++,p++)
{
for (j=-1;j<0;)
{
p->x=x+(2.0*Random(r))-r;
p->y=y+(2.0*Random(r))-r;
for (q=disc,j=0;j<discs;j++,q++)
if (i!=j)
if (((q->x-p->x)*(q->x-p->x))+((q->y-p->y)*(q->y-p->y))<disc_dd)
{ j=-1; break; }
}
for (j=-1;j<0;)
{
p->x1=x+(2.0*Random(r))-r;
p->y1=y+(2.0*Random(r))-r;
for (q=disc,j=0;j<discs;j++,q++)
if (i!=j)
if (((q->x1-p->x1)*(q->x1-p->x1))+((q->y1-p->y1)*(q->y1-p->y1))<disc_dd)
{ j=-1; break; }
}
p->vx=0.0;
p->vy=0.0;
p->_stuck=false;
}
}
//---------------------------------------------------------------------------
void disc_iterate(double dt) // iterate positions
{
int i,j,k;
_disc *p,*q;
double v=25.0,a=10.0,x,y;
const double ang=10.0*M_PI/180.0,ca=cos(ang),sa=sin(ang);
const int n=double(2.0*M_PI/ang);
for (p=disc,i=0;i<discs;i++,p++)
{
p->vx=a*(p->x1-p->x); if (p->vx>+v) p->vx=+v; if (p->vx<-v) p->vx=-v;
p->vy=a*(p->y1-p->y); if (p->vy>+v) p->vy=+v; if (p->vy<-v) p->vy=-v;
x=p->x; p->x+=(p->vx*dt);
y=p->y; p->y+=(p->vy*dt);
p->_stuck=false;
for (k=0,q=disc,j=0;j<discs;j++,q++)
if (i!=j)
if (((q->x-p->x)*(q->x-p->x))+((q->y-p->y)*(q->y-p->y))<disc_dd)
{
k++; if (k>=n) { p->x=x; p->y=y; p->_stuck=true; break; }
p->x=+(p->vx*ca)+(p->vy*sa); p->vx=p->x;
p->y=-(p->vx*sa)+(p->vy*ca); p->vy=p->y;
p->x=x+(p->vx*dt);
p->y=y+(p->vy*dt);
j=-1; q=disc-1;
}
}
}
//---------------------------------------------------------------------------
Usage is simple:
call generate0/1 with center and radius of your plane where discs will be placed
call iterate (dt is time elapsed in seconds)
draw the scene
if you want to change this to use t=<0,1>
loop iterate until all disc at destination or timeout
remember any change in speed for each disc in a list
need the position or speed vector and time it occur
after loop rescale the discs list all to the range of <0,1>
render/animate the rescaled lists
[Notes]
My test is running in real time but I did not apply the <0,1> range and have not too many discs. So you need to test if this is fast enough for your setup.
To speed up you can:
enlarge the angle step
test the collision after rotation against last collided disc and only when free test the rest...
segmentate the disc into (overlapping by radius) regions handle each region separately
also I think some field approach here could speed up things like create field map once in a while for better determine the obstacle avoidance direction
[edit1] some tweaks to avoid infinite oscillations around obstacle
For more discs some of them get stuck bouncing around already stopped disc. To avoid that just change the ang step direction once in a while this is the result:
you can see the oscillating bouncing before finish
this is the changed source:
void disc_iterate(double dt) // iterate positions
{
int i,j,k;
static int cnt=0;
_disc *p,*q;
double v=25.0,a=10.0,x,y;
const double ang=10.0*M_PI/180.0,ca=cos(ang),sa=sin(ang);
const int n=double(2.0*M_PI/ang);
// process discs
for (p=disc,i=0;i<discs;i++,p++)
{
// compute and limit speed
p->vx=a*(p->x1-p->x); if (p->vx>+v) p->vx=+v; if (p->vx<-v) p->vx=-v;
p->vy=a*(p->y1-p->y); if (p->vy>+v) p->vy=+v; if (p->vy<-v) p->vy=-v;
// stroe old and compute new position
x=p->x; p->x+=(p->vx*dt);
y=p->y; p->y+=(p->vy*dt);
p->_stuck=false;
// test if coliding
for (k=0,q=disc,j=0;j<discs;j++,q++)
if (i!=j)
if (((q->x-p->x)*(q->x-p->x))+((q->y-p->y)*(q->y-p->y))<disc_dd)
{
k++; if (k>=n) { p->x=x; p->y=y; p->_stuck=true; break; } // if full circle covered? stop
if (int(cnt&128)) // change the rotation direction every 128 iterations
{
// rotate +ang
p->x=+(p->vx*ca)+(p->vy*sa); p->vx=p->x;
p->y=-(p->vx*sa)+(p->vy*ca); p->vy=p->y;
}
else{
//rotate -ang
p->x=+(p->vx*ca)-(p->vy*sa); p->vx=p->x;
p->y=+(p->vx*sa)+(p->vy*ca); p->vy=p->y;
}
// update new position and test from the start again
p->x=x+(p->vx*dt);
p->y=y+(p->vy*dt);
j=-1; q=disc-1;
}
}
cnt++;
}
It isn't perfect, but my best idea has been to move the discs along quadratic Bezier curves. That means you've got just 2 free variables per disc that you're trying to find values for.
At that point, you can "plug" an error function into a nonlinear optimizer. Longer you're willing to wait, the better your solution will be, in terms of discs avoiding each other.
Only one actual hit:
Doesn't bother displaying hits, the discs actually start overlapped:
I've produced a full example, but the key is the error function to be minimized, which I reproduce here:
double errorf(unsigned n, const double *pts, double *grad,
void *data)
{
problem_t *setup = (problem_t *)data;
double error = 0.0;
for(int step=0; step<setup->steps; step++) {
double t = (1.0+step) / (1.0+setup->steps);
for(int i=0; i<setup->N; i++)
quadbezier(&setup->starts[2*i],
&pts[2*i],
&setup->stops[2*i],
t,
&setup->scratch[2*i]);
for(int i=0; i<setup->N; i++)
for(int j=i+1; j<setup->N; j++) {
double d = distance(&setup->scratch[2*i],
&setup->scratch[2*j]);
d /= RADIUS;
error += (1.0/d) * (1.0/d);
}
}
return error / setup->steps;
}
Ignore n, grad and data. setup describes the specific problem being optimized, number of discs, and where they start and stop. quadbezier does the Bezier curve interpolation, placing its answer into ->scratch. We check ->steps points part way along the path, and measure how close the discs are to one another at each step. To make the optimization problem smoother, it doesn't have a hard switch when the discs start touching, it just tries to keep them all as far apart from one another as possible.
Completely compilable code, Makefile and some Python for turning a bunch of quadratic bezier curves into a series of images is available at https://github.com/jkominek/discs
Performance is a bit sluggish on huge numbers of points, but there are a number of options for improvement.
If the user is making minor tweaks to the starting and finishing positions, then after every tweak, rerun the optimization in the background, using the previous solution as the new starting point. Fixing up a close solution should be faster than recreating it from scratch every time.
Parallelize the n^2 loop over all points.
Check to see if other optimization algorithms will do better on this data. Right now it starts with a global optimization pass, and then does a local optimization pass. There are algorithms which already "know" how to do that sort of thing, and are probably smarter about it.
If you can figure out how to compute the gradient function for free or close to, I'm sure it would be worth it to do so, and switch to algorithms that can make use of the gradient information. It might be worth it even if the gradient isn't cheap.
Replace the whole steps thing with a suboptimization that finds the t at which the two discs are closest, and then uses that distance for the error. Figuring out the gradient for that suboptimization should be much easier.
Better data structures for the intermediate points, so you don't perform a bunch of unnecessary distance calculations for discs that are very far apart.
Probably more?
The usual solution for this kind of problem is to use what is called a "heat map" (or "influence map"). For every point in the field, you compute a "heat" value. The disks move towards high values and away from cold values. Heat maps are good for your type of problem because they are very simple to program, yet can generate sophisticated, AI-like behavior.
For example, imagine just two disks. If your heat map rule is equi-radial, then the disks will just move towards each other, then back away, oscillating back and forth. If your rule randomizes intensity on different radials, then the behavior will be chaotic. You can also make the rule depend on velocity in which case disks will accelerate and decelerate as they move around.
Generally, speaking the heat map rule should make areas "hotter" at they approach some optimal distance from a disk. Places that are too near a disk, or too far away get "colder". By changing this optimal distance you can determine how close the disks congregate together.
Here are a couple of articles with example code showing how to use heat maps:
http://haufler.org/2012/05/26/beating-the-scribd-ai-challenge-implementing-traits-through-heuristics-part-1/
http://www.gamedev.net/page/resources/_/technical/artificial-intelligence/the-core-mechanics-of-influence-mapping-r2799
Game AI Pro, Volume 2, chapter on Heat Maps
I don't have enough rep to comment yet, so sorry for the non-answer.
But to the RTS angle, RTS's generally use the A* algorithm for path finding. Is there a reason you're insisting on using a physics-based model?
Secondly, your attempt you linked that operates rather smoothly, but with the acceleration in the middle, behaves how I initially thought. Since your model treats it as a rubber band, it basically is looking for which way to rotate for the shortest path to the desired location.
If you arent worried about a physical approach, I would attempt as follows:
Try to move directly toward the target. if it collides, it should attempt to roll clockwise around its most recent collision until it is in a position on the vector at 90 degrees to the vector from current location to the target location.
If we assume a test case of 5 in a row at the top of a box and five in a row at the bottom, they will move directly toward each other until they collide. The entire top row will slide to the right until they fall over the edge of the bottom row as it moves to the left and floats over the edge of the top row. (Think of what the whiskey and water shot glass trick looks like when it starts)
Since the motion is not determined by a potential energy stored in the spring which will accelerate the object during a rotation, you have complete control over how the speed changes during the simulation.
In a circular test like you have above, if all disks are initialized with the same speed, the entire clump will go to the middle, collide and twist as a unit for approximately a quarter turn at which point they will break away and head for their goal.
If the timing is lightly randomized, I think you'll get the behavior you're looking for.
I hope this helps.

XNA 2D Camera loosing precision

I have created a 2D camera (code below) for a top down game. Everything works fine when the players position is close to 0.0x and 0.0y.
Unfortunately as distance increases the transform seems to have problems, at around 0.0x 30e7y (yup that's 30 million y) the camera starts to shudder when the player moves (the camera gets updated with the player position at the end of each update) At really big distances, a billion + the camera wont even track the player, as I'm guessing what ever error is in the matrix is amplified by too much.
My question is: Is there either a problem in the matrix, or is this standard behavior for extreme numbers.
Camera Transform Method:
public Matrix getTransform()
{
Matrix transform;
transform = (Matrix.CreateTranslation(new Vector3(-position.X, -position.Y, 0)) *
Matrix.CreateRotationZ(rotation) * Matrix.CreateScale(new Vector3(zoom, zoom, 1.0f)) *
Matrix.CreateTranslation(new Vector3((viewport.Width / 2.0f), (viewport.Height / 2.0f), 0)));
return transform;
}
Camera Update Method:
This requests the objects position given it's ID, it returns a basic Vector2 which is then set as the cameras position.
if (camera.CameraMode == Camera2D.Mode.Track && cameraTrackObject != Guid.Empty)
{
camera.setFocus(quadTree.getObjectPosition(cameraTrackObject));
}
If any one can see an error or enlighten me as to why the matrix struggles I would be most grateful.
I have actually found the reason for this, it was something I should have thought of.
I'm using single precision floating points, which only have precision to 7 digits. That's fine for smaller numbers (up to around the 2.5 million mark I have found). Anything over this and the multiplication functions in the matrix start to gain precision errors as the floats start to truncate.
The best solution for my particular problem is to introduce some artificial scaling (I need the very large numbers as the simulation is set in space). I have limited my worlds to 5 million units squared (+/- 2.5 million units) and will come up with another way of granulating the world.
I also found a good answer about this here:
Vertices shaking with large camera position values
And a good article that discusses floating points in more detail:
What Every Computer Scientist Should Know About Floating-Point Arithmetic
Thank you for the views and comments!!

Pong: How does the paddle know where the ball will hit?

After implementing Pacman and Snake I'm implementing the next very very classic game: Pong.
The implementation is really simple, but I just have one little problem remaining. When one of the paddle (I'm not sure if it is called paddle) is controlled by the computer, I have trouble to position it at the correct position.
The ball has a current position, a speed (which for now is constant) and a direction angle. So I could calculate the position where it will hit the side of the computer controlled paddle. And so Icould position the paddle right there. But however in the real game, there is a probability that the computer's paddle will miss the ball. How can I implement this probability?
If I only use a probability of lets say 0.5 that the computer's paddle will hit the ball, the problem is solved, but I think it isn't that simple.
From the original game I think the probability depends on the distance between the current paddle position and the position the ball will hit the border.
Does anybody have any hints how exactly this is calculated?
Quoting from the very enjoyable book "Racing the Beam" (Google Books: http://books.google.co.uk/books?id=DqePfdz_x6gC&lpg=PP1&dq=racing%20the%20beam&pg=PA40#v=onepage&q&f=false) the original technique was:
To help simulate the human error inherent in precise paddle positioning, The AI paddle skips its adjustment every eight frames. The resulting behaviour is visibly unnoticeable, but it allows the computer player's aim to drift enough that it occasionally misses the ball. It is also technically trivial to implement, requiring only a single mask and the binary AND operation, for which there exists a corresponding 6502 instruction. The programmer can test if the result is zero with another single opcode, branching if needed to skip the instructions that move the paddle.
Even this behaviour must be modified slightly for the game to work at all. If the AI player simply stopped tracking the ball every eight frames, it would be hopelessly out of sync within a few seconds. To prevent this, the AI follows a secondary ball-tracking scheme near the top and bottom of the playfield. If the ball collides with one of these walls while the paddle is also aligned with it, the paddle readjusts, recovering from any drift that had accumulated since the ball last struck the wall. The result is a stochastic misalignment and realignment of computer paddle and ball.
We made a (pseudo-)3D ping-pong game for our high school CS class. What we did was, we made the computer always move the paddle toward the ball, but with a maximum speed -- that way, it could miss the ball if it's too far, but it's still smart. Does this help?
I would think you should have the paddle always move from its current position to a specific point, which would be where the ball is expected to align vertically with the paddle. You could then have a 50% chance that the paddle will move to this exact point and deflect the ball, a 25% chance that it will overshoot, perhaps by some X pixels, and 25% chance that it will undershoot. An even better way to do it might be to have it move to that position on a bell curve so that it misses by different amounts each time.
Other answers discuss how to decide the correct direction to move in in different circumstances. You can also add a lag time before the computer player "reacts" by starting to move in this direction -- that mirrors the typical response of human players.
I'm not sure what the 'official' way to do this is, but I've always just used (pseudocode)
if (ball is above paddle) {
move paddle up
}
if (ball is below paddle) {
move paddle down
}
Then I gave the paddle a slightly varying speed that was slow enough that it couldn't always keep up with the ball. Kind of crude, but it works.
This thread also has some interesting ideas you might want to look at: http://www.gamedev.net/community/forums/topic.asp?topic_id=439576
It just so happens I wrote a pong clone the other day just for fun.
You can play it here and view the source code here.
The AI takes the ball's current speed and multiplies the x-distance away from the wall. It then moves towards that calculated position at a capped speed. It doesn't account for vertical bounces, but that's sort of intended (to make the AI beatable).
Here is the relevant snippet:
/* Update enemy based on simple AI */
enemy.update = function (delta) {
var impactDistance, impactTime, targetY, speed;
speed = .25; // a little slower than the human player
if (ball.vx < 0) {
// Ball is moving away, AI takes a nap ..
return;
}
// Figure out linear trajectory ..
impactDistance = width - ball.width - ball.x;
impactTime = impactDistance / (ball.vx * .25 * 1000);
targetY = ball.y + (ball.vy * .25 * 1000) * impactTime;
if (Math.abs(targetY - (this.y + this.height/2)) < 10) {
// AI doesn't need to move
return;
}
if (targetY < this.y + (this.height / 2)) {
// Move up if ball is going above paddle
speed = -speed;
}
this.y += speed * delta;
this.keepInBounds();
};
after several iterations, if your paddle (that is yours or the AI's depending on your perspective) is too close to the top or bottom of the screen it is simply impossible to cover the distance required. The paddle can easily just follow the y-value of the ball. If you are too far you'll miss.
Alternatively, you can have the AI reset to center after each hit, or travel toward the center as long as the ball is moving away (i.e. toward the opponent)
Or more briefly:
- move toward the center while the ball is moving away
- imitate y-coordinate of ball while it is approaching.
from here you can change the difficulty by making the y-coord imitation mechanism slower than the ball (more consistent, and probably what the original implementation was like; as difficulties are abstracted into simple speed coefficients), or adding random error to each movement or otherwise.

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