Develop Reference

Shooting a bullet from the direction that a rectangle is facing

I have a Rectangle object that is placed on the screen and rendered using paintComponent.
I also have a rotation variable that determines the rotation of the object (using right and left keys to rotate) and repaints the object on the screen using Affine Transform.
In the keyPressed method, I have this which allows me to shoot bullets:
else if(key == KeyEvent.VK_SPACE) {
Bullet b = new Bullet(handler, player.x + 10, player.y - 10, ID.Bullet);
handler.addObject(b);
b.setDY(-3*Math.cos(player.rotation));
b.setDX(3*Math.sin(player.rotation));
}
If you see where I create a new bullet, in the second line, the second and third arguments in the new Bullet() are what determines where the bullets are created. Currently they just shoot from the same position on the rectangle regardless of the rotation.
I have failed at allowing the player to shoot a bullet from the direction they are facing, so if anyone has any suggestions that would be very helpful.

So basically you are trying to find the offset Vector from the player's position.
Let's assume, that you have the offset Vector, when you haven't rotated the object.
Then your task would be to rotate the given Vector by the rotation variable.
To do this we first need to look into geometry a bit. Let's think of a 2d-Vector as a triangle, that we can split into it's x and y components. We know, that the rotated Vector should have the same hypothenuse length as the original offset or in other terms the same magnitude.
This means that:
x₀² + y₀² = x₁² + y₁²
Since:
x² + y² = Vector-Magnitude
Your rotation variable keeps track of the inner angle of that "triangle vector" and hence we can describe the lengths as:
x = sin(rotation) * Vector Magnitude
y = cos(rotation) * Vector Magnitude
Now the only thing left to do is evalute those values. To do that, let us jump into the code, shall we?
float magnitude = offset.x*offset.x + offset.y*offset.y;
float x = Math.sin(rotation) * magnitude;
float y = Math.cos(rotation) * magnitude;
Bullet b = new Bullet(handler, player.x + x, player.y + y, ID.Bullet);
possible Errors and fixes
Make sure that you have the correct angle-format. The Java-Math class uses radiens for trigonometric methods.
There could be a constant rotation deviation, depending on what exactly you want your result to look like. This is a rather easy fix, as you just have to add this constant to the rotation variable. In unlikely instances you maybe also need to negate the rotation variable simply by using -rotation in the places I used rotation in my code.

If you know the rotation angle and the width of the rectangle then use this information to rotate the start position of the bullet too. For example to let the start position at the right side:
x = rectMiddleX + width/2 * cos(angle)
y = rectMiddleY + width/2 * sin(angle)
If angle is 0 it will start at the right side
else if(key == KeyEvent.VK_SPACE) {
Bullet b = new Bullet(handler, player.x + width/2 * cos(player.rotation), player.y - width/2 * sin(player.rotation), ID.Bullet);
//if player.x/y is in corner, add width/2 / height/2
handler.addObject(b);
b.setDY(-3*Math.cos(player.rotation));
b.setDX(3*Math.sin(player.rotation));
}

Rotating Coordinates (Java and Geometry)

I am working on a 2D java game engine using AWT canvas as a basis. Part of this game engine is that it needs to have hitboxes with collision. Not just the built in rectangles (tried that system already) but I need my own Hitbox class because I need more functionality. So I made one, supports circular and 4-sided polygon shaped hitboxes. The way the hitbox class is setup is that it uses four coordinate points to serve as the 4 corner vertices that connect to form a rectangle. Lines are draw connecting the points and these are the lines that are used to detect intersections with other hitboxes. But I now have a problem: rotation.
There are two possibilities for a box hitbox, it can just be four coordinate points, or it can be 4 coordinate points attached to a gameobject. The difference is that the former is just 4 coordinates based on 0,0 as the ordin while the attached to gameobject stores offsets in the coordinates rather than raw location data, so (-100,-100) for example represents the location of the host gameobject but 100 pixels to the left, and 100 pixels up.
Online I found a formula for rotating points about the origin. Since Gameobject based hitboxes were centered around a particular point, I figured that would be the best option to try it on. This code runs each tick to update a player character's hitbox
//creates a rectangle hitbox around this gameobject
int width = getWidth();
int height = getHeight();
Coordinate[] verts = new Coordinate[4]; //corners of hitbox. topLeft, topRight, bottomLeft, bottomRight
verts[0] = new Coordinate(-width / 2, -height / 2);
verts[1] = new Coordinate(width / 2, -height / 2);
verts[2] = new Coordinate(-width / 2, height / 2);
verts[3] = new Coordinate(width / 2, height / 2);
//now go through each coordinate and adjust it for rotation
for(Coordinate c : verts){
if(!name.startsWith("Player"))return; //this is here so only the player character is tested
double theta = Math.toRadians(rotation);
c.x = (int)(c.x*Math.cos(theta)-c.y*Math.sin(theta));
c.y = (int)(c.x*Math.sin(theta)+c.y*Math.cos(theta));
}
getHitbox().vertices = verts;
I appologize for poor video quality but this is what the results of the above are: https://www.youtube.com/watch?v=dF5k-Yb4hvE
All related classes are found here: https://github.com/joey101937/2DTemplate/tree/master/src/Framework
edit: The desired effect is for the box outline to follow the character in a circle while maintaining aspect ratio as seen here: https://www.youtube.com/watch?v=HlvXQrfazhA . The current system uses the code above, the effect of which can be seen above in the previous video link. How should I modify the four 2D coordinates to maintain relative aspect ratio throughout a rotation about a point?
current rotation system is the following:
x = x*Cos(theta) - y *Sin(theta)
y = x*Sin(theta) + y *Cos(theta)
where theta is degree of rotation in raidians

You made classic mistake:
c.x = (int)(c.x*Math.cos(theta)-c.y*Math.sin(theta));
c.y = (int)(c.x*Math.sin(theta)+c.y*Math.cos(theta));
In the second line you use modified value of c.x. Just remember tempx = c.x
before calculations and use it.
tempx = c.x;
c.x = (int)(tempx*Math.cos(theta)-c.y*Math.sin(theta));
c.y = (int)(tempx*Math.sin(theta)+c.y*Math.cos(theta));
Another issue: rounding coordinates after each rotation causes distortions and shrinking after some rotations. It would be wise to store coordinates in floats and round them only for output, or remember starting values and apply rotation by accumulated angle to them.

Android Java - postRotate not rotating around center?

I believe this is more of a logic question than a java question, sorry.
My intent is rather straightforward, i want the ship to move and rotate with a matrix, with the bitmap ship1 being the center pivot of the rotation. The code works great except the pivot is off by a strange offset. (picture of conundrum linked at bottom)
The default value rotation at 0 works but all the other values seem to slide away from the center, with 180 being the furthest from the center.
centerX = playerValues[Matrix.MTRANS_X] + ship1.getWidth()/2;
centerY = playerValues[Matrix.MTRANS_Y] + ship1.getHeight()/2;
newRotation = ((float) Math.toDegrees(Math.atan2(fingery1 - centerY, fingerx1 - centerX)));
matrix.postRotate((newRotation - prevRotation), centerX, centerY);
prevRotation = newRotation;
if (fingerx1 > playerX) {
xspeed = 1;
} else
if (fingerx1 < playerX) {
xspeed = 0;
} else
if (fingery1 > playerY) {
yspeed = 1;
} else
if (fingery1 < playerY) {
yspeed = 0;
}
matrix.postTranslate(xspeed, yspeed);
matrix.getValues(playerValues);
I tried to draw how the relation of the bitmap looks at different angles. (the blue dot is where I intend to rotate the bitmap around, the arrow pointing right is the only correct one).
http://i.stack.imgur.com/2Yw76.png
Please let me know if you see any errors or any feedback helps! I just need a second pair of eyes on this because mine are going to explode soon.

Consider studying a good computer graphics text re matrix math. Foley and Van Dam is always a safe bet.
The matrix A is applied to point x with multiplication Ax. You have A = RT a rotation with translation post multiplied. The result is RTx which is R (T x) meaning the point is translated then rotated, when you probably meant the opposite.
Additionally it appears you are concatenating incremental changes repeatedly. Floating point errors will accumulate, visible as worsening distortions. Instead maintain orientation parameters x, y, theta for each ship. These are controlled by the UI. Set the matrix from these in each rendering. The transform will be rotation about the point (w/2, h/2) followed by translation to (x, y). But the matrix to effect this is the translation post multiplied by the rotation! Also you must reset the matrix for each ship.

Java: Rotate a Point around an other using Google Maps Coordinates

From Google Earth I got a Box with coordinates for a picture, like following:
<LatLonBox>
<north>53.10685</north>
<south>53.10637222222223</south>
<east>8.853144444444444</east>
<west>8.851858333333333</west>
<rotation>-26.3448</rotation>
</LatLonBox>
Now I want to test weather a point intersect with this LatLonBox.
My base idea to check, whether a point intersect with the LatLonBox was, to rotate the point back by the given angle, and then to test whether the point intersect with a regular (not rotated) rectangle.
I tried to calculate the rotation manually:
public static MyGeoPoint rotatePoint(MyGeoPoint point, MyGeoPoint origion, double degree)
{
double x = origion.getLatitude() + (Math.cos(Math.toRadians(degree)) * (point.getLatitude() - origion.getLatitude()) - Math.sin(Math.toRadians(degree)) * (point.getLongitude() - origion.getLongitude()));
double y = origion.getLongitude() + (Math.sin(Math.toRadians(degree)) * (point.getLatitude() - origion.getLatitude()) + Math.cos(Math.toRadians(degree)) * (point.getLongitude() - origion.getLongitude()));
return new MyGeoPoint(x, y);
}
public boolean intersect(MyGeoPoint geoPoint)
{
geoPoint = MyGeoPoint.rotatePoint(geoPoint, this.getCenter(), - this.getRotation());
return (geoPoint.getLatitude() < getTopLeftLatitude()
&& geoPoint.getLatitude() > getBottomRightLatitude()
&& geoPoint.getLongitude() > getTopLeftLongitude()
&& geoPoint.getLongitude() < getBottomRightLongitude());
}
And it seems that the results are wrong.
LatLonBox box = new LatLonBox(53.10685, 8.851858333333333, 53.10637222222223, 8.853144444444444, -26.3448);
MyGeoPoint point1 = new MyGeoPoint(53.106872, 8.852311);
MyGeoPoint point2 = new MyGeoPoint(53.10670378322918, 8.852967186822669);
MyGeoPoint point3 = new MyGeoPoint(53.10652664993972, 8.851994565566875);
MyGeoPoint point4 = new MyGeoPoint(53.10631650700605, 8.85270995172055);
System.out.println(box.intersect(point1));
System.out.println(box.intersect(point2));
System.out.println(box.intersect(point3));
System.out.println(box.intersect(point4));
The result is true, false, false, true. But it should be 4x true.
Probably I´, making some kind of error in reasoning.
Maybe because the latitude values are getting bigger upwards. But I don´t knwo how to change the formular.
I need some help ...
EDIT:
I think my basic idea and formular is right. Also I found similar solutions eg. link and couldn´t find any difference.
So I think the only possible error source is, that the axis are not proportional. So the problem is how to take account of this.
I hope someone has got an idea.

The problem was indeed that the axis were not proportional.
The following method takes care of it.
public static MyGeoPoint rotatePoint(MyGeoPoint point, MyGeoPoint origion, double degree)
{
double x = origion.longitude + (Math.cos(Math.toRadians(degree)) * (point.longitude - origion.longitude) - Math.sin(Math.toRadians(degree)) * (point.latitude - origion.latitude) / Math.abs(Math.cos(Math.toRadians(origion.latitude)));
double y = origion.latitude + (Math.sin(Math.toRadians(degree)) * (point.longitude - origion.longitude) * Math.abs(Math.cos(Math.toRadians(origion.latitude))) + Math.cos(Math.toRadians(degree)) * (point.latitude - origion.latitude));
return new MyGeoPoint(x, y);
}

if I understand correctly you want to check if these four points are in rotated rectangle.
I would recommend checking not by corner points because your rectangle is rotated but:
if you have rotated rectangle ABCD then calculate lines |AB|, |BC|,|CD| and |DA|. If you have two points then use y=ax+b (you will calculate a,b by by giving [x,y] of both coordinates that gives you two easy equatations).
Finally function intersect will check
if point <= line |CD|
AND point >= line |AB|
AND point <= line |BC|
AND point >= |DA|
then it is inside rect.
This can be done when your point P[x,y] you put in ax+y+b (a>0 or -ax-y-b). If it is zero it is lying on the line, if it is < than it is under line or "on the left side". Hope I helped..
BTW why are you using -degree value, which you multiply by -1 , is it necessary?

The problem appears to be that the data structure LatLonBox doesn't make any sense as a description for the boundary of a picture. A box in lat-lon coordinates is not a geometric rectangle. (Think about a box near or including the north pole.) You need to re-think your application to deal in a lat/lon coordinate for the center of the picture and then deal with the rotation as an angle with respect to lines of latitude (parallel to the equator). (Even then, a picture with center on the north or south pole will be a degenerate case that must be handled separately.) So a box should properly be something like:
<geobox>
<center_lat>41</center_lat>
<center_lon>-74</center_lon>
<rotation_degrees_ccw>-23</rotation_degrees_ccw>
<width>1000</width> <!-- in pixels or meters, but not in degrees! -->
<height>600</height> <!-- same as above -->
</geobox>
Having said all that, suppose you have a true geometric box centered at (x0,y0), width w, height h, rotated angle T about its center. Then you can test a point P(x,y) for membership in the box with the following. You need the transformation that takes the box to the origin and aligns it with the axes. This is Translate(-x0,-y0) then Rotate(-T). This transformation as a matrix is
[cos(-T) -sin(-T) 0][1 0 -x0] [ cos(T) sin(T) -x0*cos(T)-y0*sin(T)]
[sin(-T) cos(-T) 0][0 1 -y0] = [-sin(T) cos(T) x0*sin(T)-y0*cos(T)]
[0 0 1][0 0 1] [ 0 0 1 ]
You want to apply this transformation to the point to be tested and then see if it lies in the desired box:
// Transform the point to be tested.
ct = cos(T);
st = sin(T);
xp = ct * x + st * y - x0 * ct - y0 * st;
yp = -st * x + ct * y + x0 * st - y0 * ct;
// Test for membership in the box.
boolean inside = xp >= -w/2 && xp <= w/2 && yp >= -h/2 && yp <= h/2;
It's late and I haven't checked this arithmetic, but it's close. Say if it doesn't work.

AndEngine - Getting Center Coordinates of a Sprite

I have a sprite in my game and I want to get it's center coordinates. The sprite can be rotated though so I can't just get it's x/y coordinates then add half of the sprite's width/height because the getWidth and getHeight methods return the dimensions of the original, unrotated sprite.
I tried getSceneCenterCoordinates() but that for some reason returns the same coordinates for all sprites even if they are nowhere near each other.
Here's a graphic to describe what I want, the red dot is the coordinate I want, and the width/height labels on the right side figure respresent what I WANT the getWidth/Height methods to return (but they don't):

You can use coordinates transformer
final float[] spriteCoordinates = sprite.convertLocalToSceneCoordinates(x,y);
final float canonX = spriteCoordinates[VERTEX_INDEX_X];
final float canonY = spriteCoordinates[VERTEX_INDEX_Y];
Sprite sprite_in_fixedpoint_of_the_first_sprite=new Sprite(canonX,canonY,textureregion)

Get the minimum and maximum value of all the corners (for both - x and y components).
Then take average from max and min to get middle:
middleX = (maxX + minX) / 2
Repeat the same for y component.

Consider something like this
Rectangle test = new Rectangle(100, 100, 50, 100, vboManager);
mainScene.attachChild(test);
System.out.println("Before RotCtrX = " + test.getRotationCenterX());
System.out.println("Before RotCtrY = " + test.getRotationCenterY());
test.setRotation(45);
System.out.println("After RotCtrX = " + test.getRotationCenterX());
System.out.println("After RotCtrY = " + test.getRotationCenterY());
and the result are
System.out(4526): Before RotCtrX = 25.0
System.out(4526): Before RotCtrY = 50.0
System.out(4526): After RotCtrX = 25.0
System.out(4526): After RotCtrY = 50.0
AndEngine Entities rotate around the center (unless you change the RotationCenter values), so applying setRotate() will not affect the "center" point location
those "center" points are relative to the Entity, so if you need the actual screen coordinates, you will need to add those to the getX() and getY() values - which BTW also won't change based solely on applying a setRotate()

I figured out how to use getSceneCenterCoordinates() properly, which is to hand it a float[]. Here is my solution:
float[] objCenterPos = new float[2];
obj.sprite.getSceneCenterCoordinates(objCenterPos);
Log.d(this.toString(), "Coordinates: ("+objCenterPos[0]+","+objCenterPos[1]+")");