# Create a flash draw game like Line Rider or others – A different approach

April 21st update: part 2 released
June 15th update: part 3 released

Some days ago I received a very very interesting email from Kevin W. (for Kevin: I can give you more credits if you send me some information about you).

It refers to the Line Rider game creation, with a different approach.

——

I was reviewing parts three and four of your line rider tutorials today because I had never tried out your collision and reaction techniques for myself, when I found some things I didn’t like.

Mostly, the practice of combining and extracting angles constantly using trig, and therefore the need to compensate for the differences between degrees and radians. I didn’t want to do this because I’ve had a lot of bad luck with my own experiments.

I was reminded of a book on Flash MX game design I saw a few months ago. I remembered the author talking about using vectors exactly for this purpose which eliminates the need for taxing trigonometry functions.

The idea is to keep the x and y speeds separate, and use normalization and scalars to determine the resultant speed.

I think the result is somewhat more efficient and basically keeps track of the ball’s momentum which would lead well into the concepts of sliding required for a sled-based game.

I haven’t yet incorporated your fixes for the ball sticking in the terrain and losing energy, but I would like to know what you think of my approach to this problem.

I hope you take this as gratitude on my part for your tutorials. I started in January of this year using just your blog.

By the way, I am Izy who commented on your posts, the one who made the rocket game with the missile.

—–

Kevin sent a fully commented actionscript too, here it is:

```//lineRider_ideas//
//////////////////////////
//Initialize all variables to be used//
//and attach all movieclips.////////////////
//M is the ball, T is the terrain, F is a textfield.
//////////////////////////////////////////////////////////
var m:MovieClip = _root.attachMovie("ball", "ball", 1, {_x:125});
var t:MovieClip = _root.attachMovie("terrain", "terrain", 2, {_x:0});
var f:TextField = _root.createTextField("textfield", 20, 100, 25, 300, 25);
var precision:Number = 1;
var sum_x:Number = 0;
var sum_y:Number = 0;
var collisions:Number = 0;
var normal_x:Number = 0;
var normal_y:Number = 0;
var vector_length = 0;
var go:Boolean = true;
//////////////////////////////////////////////////////////
//Create an object to hold speed information and gravity//
//////////////////////////////////////////////////////////
var speed:Object = {x:0, y:0, g:0.25};
/////////////////////////////
//Position certain elements//
/////////////////////////////
m._y = m._height;
t._y = 400;
f.text = "Click and drag to reposition the ball.";
///////////////////////////////
//Allow the ball to be positioned//
///////////////////////////////
_root.onMouseDown = function() {
go = false;
speed.x = 0;
speed.y = 0;
m._x = _root._xmouse;
m._y = _root._ymouse;
m.startDrag();
};
_root.onMouseUp = function() {
m.stopDrag();
go = true;
};
////////////////////////////////
//The main part of the code.////
////////////////////////////////
_root.onEnterFrame = function() {
//Reset variables each frame.
sum_x = 0;
sum_y = 0;
collisions = 0;
if (go) {
//Check for collisions by generating points in a slightly different//
for (var i:Number = 1; i<360; i += precision) {
var angle = (i*precision)*(Math.PI/180);
if (t.hitTest(spot_x, spot_y, true)) {
collisions++;
sum_x += spot_x;
sum_y += spot_y;
}
}
if (collisions>0) {
//Here's the real key to my approach.//////////////
//First, I find the magnitude of the speed vector//
//to use as a scalar.//////////////////////////////
var total_speed = Math.sqrt(Math.pow(speed.x, 2)+Math.pow(speed.y, 2));
spot_x = (sum_x/collisions)-m._x;
spot_y = (sum_y/collisions)-m._y;
//Next, I find the length of a the vector that is the line//
//between the point of collision and the center of the object.//
//This is equal to the radius of the circle and so could be replaced
vector_length = Math.sqrt(spot_x*spot_x+spot_y*spot_y);
//Here I normalize the component of the vector by dividing their magnitude
//by the total length of the two vectors, thereby creating another vector
//with a magnitude of exactly one.///////////////////////////////////////
normal_x = spot_x/vector_length;
normal_y = spot_y/vector_length;
//Finally, I multiply the normalized vector by the scalar to proportionally
//achieve the final speed of the ball./////////////////////////////////////
speed.x += -(normal_x*total_speed);
speed.y += -(normal_y*total_speed);
}
//Add gravity to the speed object and then add the speed to the ball's position.//
speed.y += speed.g;
m._x += speed.x;
m._y += speed.y;
}
};```

Then, Kevin fixed another problem… read:

—–

I noticed that if you change the code

```speed.x += -(normal_x*total_speed); speed.y += -(normal_y*total_speed);```

to

```speed.x = -(normal_x*total_speed)*friction; speed.y = -(normal_y*total_speed)*friction;```

it will behave almost identically to your part four code without the sticking bug.

—–

What to say? First, I want to thank Kevin for this precious feedback… then, I am going to study this code and eventually use it in future updates of this tutorial.

Here it is his working final movie

And this is the source code.

Any comment?

214 GAME PROTOTYPES EXPLAINED WITH SOURCE CODE
// 1+2=3
// 10000000
// 2 Cars
// 2048
// Avoider
// Ballz
// Block it
// Blockage
// Bloons
// Boids
// Bombuzal
// Breakout
// Bricks
// Columns
// CubesOut
// Dots
// DROP'd
// Dudeski
// Eskiv
// Filler
// Fling
// Globe
// HookPod
// Hundreds
// InkTd
// Iromeku
// Lumines
// Magick
// MagOrMin
// Maze
// Memdot
// Nano War
// Nodes
// o:anquan
// Ononmin
// Pacco
// Phyballs
// Platform
// Poker
// Pool
// Poux
// Pudi
// qomp
// Racing
// Renju
// SameGame
// Security
// Sling
// Slingy
// Sokoban
// Splitter
// Sproing
// Stack
// Stringy
// Sudoku
// Tetris
// Threes
// Toony
// Turn
// TwinSpin
// vvvvvv
// Wordle
// Worms
// Yanga
// Zhed
// zNumbers