Bring It! – Bullet Time

One of the most important parts of the entire game is the ability to shoot at the bad guys. For this you’ll definitely be needing some bullets (and I’ll get to the bad guys soon).

I thought adding bullet sprites would be quite complicated because it would involve finding the angle to be fired at, the start position based on the position and rotation of the character sprite, but that turned out to be quite simple in the end.

Rather than use any complicated mathematics I determined that the centre of the character sprite is something that won’t change because the character is on top of a non-moving tower. I modified the Sprite.java file in the Rokon files to include centreX and centreY values. If I eventually let the character sprite move then I will need to make sure that value gets updated, but that’s a very minor issue.

I create the bullet sprite when the user presses the trackball button, or the centre of the directional pad if that’s what the device has. The bullet sprite appears at the centre of the character sprite and has X and Y velocity applied to it like so:

bulletSprite.setVelocityX((float)Math.sin(
  Math.toRadians(arrowSprite.getRotation()))*100);

bulletSprite.setVelocityY((float)-Math.cos(
  Math.toRadians(arrowSprite.getRotation()))*100);

This gets the bullet moving in the direction the character sprite is pointing as is demonstrated in the video.

The next thing on my list is creating some bad guys that can get hit by the bullets. I imagine that’ll be more complicated than I think at the moment, but I hope to be proved wrong. 🙂

Bring It! – Getting started

To begin developing my game I downloaded the latest Android SDK and set up Eclipse as explained in great (and hopefully up-to-date) detail on Google’s instructions page.

Once this was working I got the latest version of the Rokon game engine and extracted it into my workspace folder. I imported the existing project into Eclipse and added my own class to the list. I copied one of the examples into my own class and then removed the com.rokon.examples class and files. I changed all references of the com.rokon.examples class to my own class in files like the AndroidManifest.xml file. I missed a few places at first, but Eclipse is pretty good at telling you what’s wrong. I found it helpful to clean the project using the Project->Clean menu item to make sure no errors were left over from before certain changes.

Once I had eradicated all errors from Eclipse I ran the source in an Android Virtual Device. To my immense surprise it was working!

I went into the code and started to strip out all the bits I didn’t want. I kept one sprite around, but changed the sprite texture to my own arrow, and made it considerably smaller.

That was all pretty easy, and I thought things were going to well until I realised that I now needed to get down to the harder stuff. The user would need to aim in different directions from one point, so I needed a way for the person to change the angle. My first thought was to use the scroll ball that appears on most HTC devices these days. I thought that might be a bit of a problem though, because I was planning on using the scroll ball press as the trigger, and that might lead to people accidentally pressing it when they didn’t want to.

I decided that it would be cool to have the character point towards where the user pressed on the screen. The problem with that is that code is required to work out what angle to rotate the character by depending on where the character is, and where the player has pressed on the screen. I managed to find various little bits and pieces online that helped me come up with a decent solution.

That looks very complicated, but it’s not that bad, really. The angle is worked out using the position of the centre of the sprite and the position of the press on the screen. I’d explain it in more detail, but that might take a while. You may notice the +90 near the end of the rotation line. Unfortunately that’s a bit of a hack to add 90 degrees of rotation to whatever angle is given to me. It works, so I’m fine with that for now.

Now when I run the “game” I see an arrow on the screen. If I press on the screen then the arrow points at where I pressed. If I move my finger while it’s pressed then the arrow follows my finger around. A good start to coding the actual character.

Now to climb the rest of the mountain!