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POMovement of a sprite according to rotation
 

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  1. POMovement of a sprite according to rotation
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    <p>I hope this is an acceptable question for this forum, as it's more of a maths question than a programming question.</p> <p>I am developing a basic space shooter game using the cocos2d framework, where the user controls a spaceship and shoots asteroids/enemies, and the enemies also shoot back at the spaceship (original eh?).</p> <p>The 'lasers' that are fired by the enemies are a tube like shape, and generally only travel form right to left on the x axis. This is easy to achieve, and the movement is created with this line of code:</p> <pre><code>currentEnemyProjectile.position = ccp(currentEnemyProjectile.position.x - ((screenSize.width/kLaserSpeed)* dtSixty), currentEnemyProjectile.position.y); </code></pre> <p>I also have one particular enemy, which resides in the middle of the screen and rotates to fire its projectiles at the spaceship, so if the space ship is at a different y position than the enemy then the projectile sprite will be rotated and then will move to the spaceship's position, so it will need to also move through the y axis.</p> <p>I'm currently using code to achieve this:</p> <pre><code>float xDiff = ((screenSize.width/kLaserSpeed)* dtSixty); float yDiff = (xDiff / 57) * currentEnemyProjectile.rotation; currentEnemyProjectile.position = ccp(currentEnemyProjectile.position.x - xDiff, currentEnemyProjectile.position.y + yDiff); </code></pre> <p>I just worked this out as a temporary fix, I know it is not the optimal way of doing things. It sort of works, but as I am working out the y offset from the movement along the x axis, the projectiles will move faster if they have further to travel along the y axis. Ideally they should move slower along the x axis if they are moving further upwards or downwards, so that the rate of travel is constant for any trajectory. (I hope I've explained that well enough)</p> <p>It's been roughly 10 years since I last had a maths lesson, so my geometry/trigonometry is rather hazy/non-existent, can anyone shed any light on how this can be done?</p> <p>Also, it's worth noting that I would like to do this without the use of cocos2d actions. I would rather move my sprites each frame as I have been currently doing as I manipulating the flow of time as part of the game, so I need more control than the cocos2d actions can offer.</p> <p>Thanks in advance for any help.</p>
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    1. COI'm a little confused by your variables. kLaserSpeed should be in units of distance/second. Is dtSixty something like delta time for a 60Hz clock rate? Why is xDiff dependent on the screen width? All you have to do is multiply velocity*dt to get the distance moved, call it "d", along the hypotenuse of the triangle. So, dx = d*sin(angle) and dy = d*cos(angle). Make sure you check for the appropriate sign.
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    2. COThe reason I am dividing screen width by kLaserSpeed is that this will give me a constant gameplay experience across different devices (iPhone 4/iPhone 5/ipad), for instance if kLaser speed is 500, then the object will travel 1/500th width of the screen, independent of screen size. dtSixty is just the time since the last frame multiplied by sixty. I had originally started the project without adjusting for the current frame rate, so when I wanted to do that later, as it was running a 60 fps, if I multiplied the time since the last frame by 60, I could keep the same figures for the speed.
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    3. COI'll have a look at the equation you game me and see if I can get it to work, I think I could do with brushing up on my trigonometry a bit! Thanks for the help.
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