I am reading an android tutorial for game development that explains how to convert in-game coordinates to actual pixels. Simple enough. This is done via function worldToScreen() as follows:
public Rect worldToScreen(float objectX, float objectY, float objectWidth, float objectHeight){
int left = (int) (screenCentreX - ((currentViewportWorldCentre.x - objectX) * pixelsPerMetreX));
int top = (int) (screenCentreY - ((currentViewportWorldCentre.y - objectY) * pixelsPerMetreY));
int right = (int) (left + (objectWidth * pixelsPerMetreX));
int bottom = (int) (top + (objectHeight * pixelsPerMetreY));
convertedRect.set(left, top, right, bottom);
return convertedRect;
}
It seems to return a rectangle object containing the four points that a square object would occupy.
Why does it use a square?
Why is it substracting top/left and adding bottom/right?
A thorough explanation will be much appreciated.
Answer to question 1
He's using a rectangle probably because it's a simple geometry object that is already implemented in Java and in most gaming libraries, like the one you are using (i can see he's using the Rect class).
Rectangle is also a common solution in 2D games when you want to implement simple collision detection for example.
Answer to question 2
You ask why he's adding bottom and right... But i can only see that he's adding top and left.
He's doing that because the y axis goes from up to down, and the x axis goes from left to right.
So to get the bottom point you have to add the y coordinate of the top point to the height of the rectangle.
Same for the right point, you have to add the x coordinate of the left point to the width of the rectangle.
In the hope that my Paint skills can come useful i made a drawing that probably will help you understand:
To make the drawing and my answer even more clear:
top + height = bottom
left + width = right
PS: "he" is the guy who made the tutorial that you're following.
Related
I'm having trouble setting the coordinate of the star are there any better solution for this. I cannot get the the correct shape. Can someone help me on this?
public void star(Graphics shapes)
{
shapes.setColor(color);
int[] x = {42,52,72,52,60,40,15,28,9,32,42};
int [] y = {38,62,68,80,105,85,102,75,58,20,38};
shapes.fillPolygon(x, y, 5);
}
Sun's implementation provides some custom Java 2D shapes like Rectangle, Oval, Polygon etc. but it's not enough. There are GUIs which require more custom shapes like Regular Polygon, Star and Regular polygon with rounded corners. The project provides some more shapes often used. All the classes implements Shape interface which allows user to use all the usual methods of Graphics2D like fill(), draw(), and create own shapes by combining them.
Regular Polygon Star
Edit:
Link
Honestly, I'd use the 2D Graphics shapes API, they allow you to "draw" a shape, which is simpler (IMHO) then using polygon. The advantage is, they are easy to paint and transform
Having said that, the problem you're actually having is the fact that you're not passing the right information to the fillPolygon method.
If you take a look at the JavaDocs for Graphics#fillPolygon, you'll note that the last parameter is the number of points:
nPoints - a the total number of points.
But you're passing 5, where there are actually 11 points in your array
Something like...
shapes.setColor(color);
int[] x = {42,52,72,52,60,40,15,28,9,32,42};
int [] y = {38,62,68,80,105,85,102,75,58,20,38};
shapes.fillPolygon(x, y, 11);
should now draw all the points, but some of your coordinates are slightly off, so you might want to check that
The second to last number of your Y should be 60 not 20
g2.setColor(color);
int[] x = {42,52,72,52,60,40,15,28,9,32,42};
int[] y = {38,62,68,80,105,85,102,75,58,60,38};
g2.fillPolygon(x , y, 11);
I'm having trouble setting the coordinate of the star are there any better solution for this
Check out Playing With Shapes. You should be able to use the ShapeUtils class to generate your shape.
This class will generate the points for you so you don't need to manage every pixel.
a star has 10 points ppl mind that not 11
setBackground(Color.black);
int[]x={250,150,0,150,100,250,400,350,500,350};
int[]y={100,200,200,300,400,300,400,300,200,200};
g.fillPolygon( (x),(y),10);
setForeground(Color.cyan);
this will help to draw a star with black bg and cyan foreground
I have been trying to solve this for a few hours, and the internet is pretty unfruitful on the subject.
I need help detecting and solving collisions between rectangles, and not just detecting, but note I mentioned solving as well.
These are two boxes, with x/y width/heights. I simply need to detect when they are overlapping, and push one of the boxes out of the other one smoothly.
Also, note that one box is stationary - and the other is moving.
Does anyone have anything on this (or can give me an example?) I'd really appreciate it.
I need the boxes to be able to rest on top of each other as well.
Thank you!
I'm not sure what the context here is (Are these boxes moving or stationary? Are you looking for a physically accurate resolution, or simply a geometrically correct one?), but it seems like you could accomplish this in the following way:
1) Determine if there is a box collision
2) Determine the intersection of the two boxes, which would produce a third box. The width and height of the box is your penetration depth.
3) move the center of one of the boxes by the penetration depth, (x - width, y - height).
This should cause the boxes to become disjoint.
FYI: Intersection of two boxes can be computed by taking the max of the mins and the mins of the maxes from both boxes.
Here is some code from my engine for box intersection:
bool Bounds::IntersectsBounds(const Bounds &other) const
{
return !(min.x > other.max.x || max.x < other.min.x
|| min.y > other.max.y || max.y < other.min.y);
}
bool Bounds::Intersection(const Bounds &other, Bounds &outBounds) const
{
if (!this->IntersectsBounds(other)) {
return false;
}
outBounds.min.x = std::max(min.x, other.min.x);
outBounds.min.y = std::max(min.y, other.min.y);
outBounds.max.x = std::min(max.x, other.max.x);
outBounds.max.y = std::min(max.y, other.max.y);
return true;
}
In this case, the "outBounds" variable is the intersection of the two boxes (which in this case is your penetration depth). You can use the width/height of this box to perform your collision resolution.
Yeah! This is a pretty common problem! You may want to check out the gamedev portion of the stack exchange network!
Detection
bool collide(float x1,float y1,float sx1,float sy1, float x2, float y2, float sx2, float sy2){
if (x1+sx1 <= x2)
return false;
if (x2+sx2 <= x1)
return false;
if (y1+sy1 <= y2)
return false;
if (y2+sy2 <= y1)
return false;
return true;
}
Resolution
As far as an answer, this depends on the type of application you are going for. Is it a sidescroller, top-down, tile based? The answer depends on the response to this question. I'll assume something dynamic like a sidescroller or top-down action game.
The code is not difficult, but the implementation can be. If you have few objects moving on the screen you can use a similar system to mine, which goes something like the following:
Get a list of objects you are currently colliding with, in order of distance from the current object.
Iterate through the objects, and resolve collisions using the following method
Check if the object has some special collision type (teleporter, etc) by sending that object a message, and checking on the return value (a teleporter will take care of the collision resolution)
check if the previous bottom position of our current object (A) was above the top side of the object in question(B), if so that means you have had a bottom collision. Resolve by setting the y position of A to the y position of B minus the height of A
(IF THE PREVIOUS FAILED) check if the previous right side of A was to the left of the left side of B, if so that means you have had a right side collision. Resolve by setting the x position of A to B's position minus A's width
(IF THE PREVIOUS FAILED) check if the previous left side of A was to the right of the right side of B, if so that means you have had a left side collision. Resolve by setting the x position of A to B's x position plus B's width
(IF THE PREVIOUS FAILED) check if the previous top side of A was below the bottom side of B, if so you have had a top side collision. Resolve by setting the y position of A to the y position of B plus B's height
Whew. It is important that you have the objects sorted according to distance, it will catch on edges if you check collisions with an object that is farther away!
I hope that makes sense!
Edit: Apparently doesn't work in Android.
https://stackoverflow.com/a/15515114/3492994
Using the available classes from the 2D Graphics API.
Rectangle r1 = new Rectangle(100, 100, 100, 100);
Line2D l1 = new Line2D.Float(0, 200, 200, 0);
System.out.println("l1.intsects(r1) = " + l1.intersects(r1));
What this doesn't tell you, is where...
This problem is hard to explain, so I'll use an image to aid me:
I'm trying to find an angle between the middle of the tank, to the mouse. The orange dot denotes the mouse position, the red line separates the two instances, and the green/lime line shows the tank's turret angle. I've been looking through stack overflow multiple times, and yet to no avail have I found a solution to my problem. I've even googled. In both, I have found many groups of code to 'find an angle'. I'm sure that these work, so I doubt my problem lies in the hands of bad code. I'm GUESSING that the error is found within the MouseMotionListener.
The two points I'm using to create the psuedo-line (NOT the green or red lines) to are the tank's middle point new Point(Tank.getX() + 16, Tank.getY() + 16) (the tanks size is 32x32) and the mouse point (set when there is a new mouse moved event).
Details about my program:
A frame is created and has a MouseMotionListener attached to it.
A JPanel is created and added to the frame.
Everything is drawn onto the JPanel.
In short, my getAngle() code is wrong, my MouseMotionListener is wrong, or I'm giving the wrong parameters. What is the problem?...
EDIT: As asked for in the comments here is my code and the output:
Code:
public static float getAngle(Point source, Point destination) {
System.out.println(source + "\n" + destination);
double xDiff = source.x - destination.x;
double yDiff = source.y - destination.y;
System.out.println((float) Math.toDegrees(Math.atan2(yDiff, xDiff)));
return (float) Math.toDegrees(Math.atan2(yDiff, xDiff));
}
Output:
java.awt.Point[x=116,y=116] // Source point
java.awt.Point[x=134,y=123] // Destination point
-158.7495
Well, after plenty of Googling, I found that there really was no error. All the code was correct, all.. pretty much everything was correct. The only thing that was wrong was that it was off by 90 degrees. This thought had crossed my mind multiple times, but every time I looked at the dot and the line, it just didn't seem right... The fixed code is below:
public static float getAngle(Point source, Point destination) {
System.out.println(source + "\n" + destination);
double xDiff = source.x - destination.x;
double yDiff = source.y - destination.y;
System.out.println((float) Math.toDegrees(Math.atan2(yDiff, xDiff)));
return (float) Math.toDegrees(Math.atan2(yDiff, xDiff)) + 90.0F;
}
For some reason, I feel like I've just dropped a couple IQ points for overlooking this multiple time.
So I'm programming a recursive program that is supposed to draw Koch's snowflake using OpenGL, and I've got the program basically working except one tiny issue. The deeper the recursion, the weirder 2 particular vertices get. Pictures at the bottom.
EDIT: I don't really care about the OpenGL aspect, I've got that part down. If you don't know OpenGL, all that the glVertex does is draw a line between the two vertices specified in the 2 method calls. Pretend its drawLine(v1,v2). Same difference.
I suspect that my method for finding points is to blame, but I can't find anything that looks incorrect.
I'm following the basically standard drawing method, here are the relevant code snips
(V is for vertex V1 is the bottom left corner, v2 is the bottom right corner, v3 is the top corner):
double dir = Math.PI;
recurse(V2,V1,n);
dir=Math.PI/3;
recurse(V1,V3,n);
dir= (5./3.)* Math.PI ;
recurse(V3,V2,n);
Recursive method:
public void recurse(Point2D v1, Point2D v2, int n){
double newLength = v1.distance(v2)/3.;
if(n == 0){
gl.glVertex2d(v1.getX(),v1.getY());
gl.glVertex2d(v2.getX(),v2.getY());
}else{
Point2D p1 = getPointViaRotation(v1, dir, newLength);
recurse(v1,p1,n-1);
dir+=(Math.PI/3.);
Point2D p2 = getPointViaRotation(p1,dir,newLength);
recurse(p1,p2,n-1);
dir-=(Math.PI*(2./3.));
Point2D p3 = getPointViaRotation(p2, dir, newLength);
recurse(p2,p3,n-1);
dir+=(Math.PI/3.);
recurse(p3,v2,n-1);
}
}
I really suspect my math is the problem, but this looks correct to me:
public static Point2D getPointViaRotation(Point2D p1, double rotation, double length){
double xLength = length * Math.cos(rotation);
double yLength = length * Math.sin(rotation);
return new Point2D.Double(xLength + p1.getX(), yLength + p1.getY());
}
N = 0 (All is well):
N = 1 (Perhaps a little bendy, maybe)
N = 5 (WAT)
I can't see any obvious problem code-wise. I do however have a theory about what happens.
It seems like all points in the graph are based on the locations of the points that came before it. As such, any rounding errors that occurs during this process eventually start accumulating, eventually ending with it going haywire and being way off.
What I would do for starters is calculating the start and end points of each segment before recursing, as to limit the impact of the rounding errors of the inner calls.
One thing about Koch's snowflake is, that the algorithm will lead to a rounding issue one time (it is recursive and all rounding errors add up). The trick is, to keep it going as long as possible. There're three things you can do:
If you want to get more detailed, the only way is to expand the possibilities of Double. You will need to use your own range of coordinates and transform them, every time you actually paint on the screen, to screen coordinates. Your own coordinates should zoom and show the last recursion step (the last triangle) in a coordination system of e.g. 100x100. Then calculate the three new triangles on top of that, transform into screen coordinates and paint.
The line dir=Math.PI/3; divides by 3 instead of (double) 3. Add the . after the 3
Make sure you use Point2D.Double anywhere. Your code should do so, but I would explicitely write it everywhere.
You won the game, when you still have a nice snowflake but get a Stackoverflow.
So, it turns out I am the dumbest man alive.
Thanks everyone for trying, I appreciate the help.
This code is meant to handle an equilateral triangle, its very specific about that (You can tell by the angles).
I put in a triangle with the height equal to the base (not equilateral). When I fixed the input triangle, everything works great.
I want to move a label in a GUI in a circular path.I hope to maintain the speed using threads.
Can anyone tell me how to do circular movement? What is the logic we should use for it?
You can use the circumference formula : x^2 + y^2 = radius^2
Knowing that formula, if you know the radius you want (constant), you can do something like this (pseudocode, I hope you get the idea):
for (x from initial_x to end_x) {
y = Math.sqrt(radius^2 - x^2)
draw the label at position (x, y)
}