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POFollowing a straight line (via Path?)
 

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  1. POFollowing a straight line (via Path?)
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    <p>I'm working on a game which will use projectiles. So I've made a Projectile class and a new instance is created when the user touches the screen:</p> <pre><code>@Override public boolean onTouch(View v, MotionEvent e){ float touch_x = e.getX(); float touch_y = e.getY(); new Projectile(touch_x, touch_y); } </code></pre> <p>And the Projectile class:</p> <pre><code>public class Projectile{ float target_x; float target_y; Path line; public Projectile(float x, float y){ target_x = x; target_y = y; line = new Path(); line.moveTo(MyGame.mPlayerXPos, MyGame.mPlayerYPos); line.lineTo(target_x, target_y); } } </code></pre> <p>So this makes a Path with 2 points, the player's position and and touch coords. My question is - How can you access points on this line? For example, if I wanted to get the x,y coords of the Projectile at the half point of the line, or the point the Projectile would be at after 100 ticks (moving at a speed of X pixels/tick)?</p> <p>I also need the Projectile to continue moving after it reaches the final point.. do I need to use line.addPath(line) to keep extending the Path?</p> <p><strong>EDIT</strong></p> <p>I managed to get the Projectiles moving in a straight line, but they're going in strange directions. I had to fudge some code up:</p> <pre><code>private void moveProjectiles(){ ListIterator&lt;Projectile&gt; it = Registry.proj.listIterator(); while ( it.hasNext() ){ Projectile p = it.next(); p.TimeAlive++; double dist = p.TimeAlive * p.Speed; float dx = (float) (Math.cos(p.Angle) * dist); float dy = (float) (Math.sin(p.Angle) * dist); p.xPos += dx; p.yPos += -dy; } } </code></pre> <p>The <code>Angle</code> must be the problem.. I'm using this method, which works perfectly:</p> <pre><code>private double getDegreesFromTouchEvent(float x, float y){ double delta_x = x - mCanvasWidth/2; double delta_y = mCanvasHeight/2 - y; double radians = Math.atan2(delta_y, delta_x); return Math.toDegrees(radians); } </code></pre> <p>However, it returns 0-180 for touches above the center of the screen, and 0 to -180 for touches below. Is this a problem?</p>
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    1. COI bet you are working in a left handed coordinate system -- (0,0) in the top left of the screen. Most trig functions assume a right handed coordinate system -- (0,0) in the bottom left of the screen. If you use my solution it doesn't matter which coordinate system you choose.
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      1. USJon Snyder
 

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