Particle X:Y Spawn Coords Based on Rotation

Game Mechanics
#1

Alright, I got a good one for you guys here. I have worked on this for a while to no avail so I thought some more math savvy people on here could point me in the right direction. So I’ve got a missile for my TD game that, when shot, rotates and follows it’s target like a real missile. Now I want to use my particle system to spawn particles in at the base of the rocket so it looks like a nice smoke trail. I already have all the particle and color fade coded, but I have no clue how to get the X and Y to pass it to spawn based on the rotation of the missile. Here is the code for it:


	public void move(Graphics g) {
		if (inGame) {
			if ((xBullet >= targetX - 4 && xBullet <= targetX + 4) && (yBullet <= targetY + 4 && yBullet >= targetY - 4)) {
				inGame = false;
				Screen.currLevel.mobs[shotMob].looseHealth(damage);
				new Explosion((int) xBullet - 26, (int) yBullet - 12);
				xBullet = 900;
				yBullet = 900;
			}
			targetX = Screen.currLevel.mobs[shotMob].x + Screen.currLevel.mobs[shotMob].width / 4;
			targetY = Screen.currLevel.mobs[shotMob].y + Screen.currLevel.mobs[shotMob].width / 4;
			distanceX = (int) ((double) targetX - xBullet);
			distanceY = (int) ((double) targetY - yBullet);
			sep = Math.sqrt((distanceX * distanceX) + (distanceY * distanceY));
			scale = 3;
			xBullet += (distanceX / sep) * scale;
			yBullet += (distanceY / sep) * scale;
			slope = (targetY - yBullet) / (targetX - xBullet);
			damage = tower.damage;

			Graphics2D g2d = (Graphics2D) g.create();
			double correction = 270;
			float rotation;
			rotation = (float) (Math.atan2(yBullet - targetY, xBullet - targetX) * (180 / Math.PI));
			Screen.room.block[y][x].drawImage = false;
			g2d.setTransform(AffineTransform.getRotateInstance(Math.toRadians(rotation + correction), xBullet + Screen.room.blockSize / 2, yBullet + Screen.room.blockSize / 2));
			g2d.drawImage(Screen.tileset_air[Value.missile], (int) xBullet, (int) yBullet, Screen.room.blockSize, Screen.room.blockSize, null);
			Screen.addParticle(XCOORDINATE, YCOORDINATE, .2, 4, 80, 100, false, Color.orange, Color.BLACK, 20);
			g2d.dispose();
		}
	}

	public void draw(Graphics g) {
		if (!Screen.currLevel.mobs[shotMob].isDead() && Screen.inGame) {
			if (timeF >= timeS) {
				move(g);
				timeF = 0;
			} else {
				timeF++;
			}
		} else {
			// inGame = false;
		}
	}
#2

Basically we need a bit of trigonometry

Where the distance should be the distance from the missiles rotation point to the point you want the missile to appear.

Something like that.

#3

You could use atan2(diry,dirx) to get the angle.

#4

what would be dirx and diry? I mean I know it is short for direction but I am not sure which variables you mean by it.

#5 
rotation = (float) (Math.atan2(yBullet - targetY, xBullet - targetX) * (180 / Math.PI));
#6

dirx would actually be delta x and diry you delta y or dx and dry. Whatever you call them it should be the the distance between point A and point B in the X lane and the distance between point A and point B in the Y lane. Because we can imagine that there is a 90 degree triangle between the two points.

Basic trigonometry for 90 degree angled triangles.

#7

Ah ok I understand… I have already done this to get the angle, look up in the code I posted. For your suggestion on the spawn position for the particles, I have tried and failed to use it many different ways. For some reason none of them seem to work.

#8

Aaannd I slept on it and figured it out the next day (with the help you guys gave me :wink: ). So for anyone viewing this thread in the future, what I realized I had to do was plug in half the width and height of the image where missile_x and missile_y was so the final equations were these:


double particleX = (xBullet + Screen.room.blockSize / 2) - Math.toRadians(Math.cos((rotation) + correction)) * (Screen.room.blockSize / 2);
double particleY = (yBullet + Screen.room.blockSize / 2) - Math.toRadians(Math.sin((rotation) + correction)) * (Screen.room.blockSize / 2);