Particle X:Y Spawn Coords Based on Rotation

[Slyth2727]

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:

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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;      }    }

[jonjava]

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.

[relminator]

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

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[Slyth2727]

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.

[jonjava]

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

[jonjava]

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.

[Slyth2727]

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.

[Slyth2727]

Aaannd I slept on it and figured it out the next day (with the help you guys gave me ). 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:
 

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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);