Moving a sprite in a specified angle

[motorollin]

Ok this works:

    if ( x <= 20 )
    {
        x=21;
        angle=(180-angle);
    }
    if ( y <= 20 )
    {
        y=21;
        angle=(180-angle)-180;
    }
    if ( x >= 620 )
    {
        x=619;
        angle=(180-angle);
    }
    if ( y >= 440 )
    {
        y=439;
        angle=(180-angle)-180;
    }

    x+=pinwheelspeed*cos(angle);
    y+=pinwheelspeed*sin(angle);




But every so often the sprite hits the wall at an angle which means it can't escape for some reason. It's probably due to the fact that certain angle can't be represented precisely at such a low resolution. I think it might be better to have tables of x and y velocities to give a variety of "angles" rather than trying to calculate actual angles.

--
moto

[Karlos]

As sin / cos use doubles and return doubles, that's unlikely. It's more likely that the conversion to whatever type x and y are discards fractional data for results that would otherwise be less than one, giving a zero x or y velocity.

Strange that you are using degrees as an angular measure for sin and cos. I thought it only worked in radians?

Anyway, if you are going for strict x / y velocities and only ever inverting their signs on collision with a wall, you might as well do away with angles except when firing them off initially.

That is to say, each sprite is initialised with

xVel = speed*cos(angle);
yVel = speed*sin(angle);

and then basically the values dont change for the sprite except for sign flipping on hitting a wall.

Avoid narrow angles by only allowing say a 5 degree granularity for the starting angle.

[motorollin]

Karlos wrote:
Strange that you are using degrees as an angular measure for sin and cos. I thought it only worked in radians?

Ohhhhh, that could be my mistake - I was treating the result as degrees :roll: However, this seems to work better anyway:

Pinwheel::Pinwheel()
{
...
xVel=4;
yVel-2;
...
}

Pinwheel::move()
{
    if ( x <= 0 )
    {
        x=1;
        xVel = -xVel;
    }
    if ( y <= 0 )
    {
        y=1;
        yVel = -yVel;
    }
    if ( x >= LEVEL_WIDTH-PINWHEEL_WIDTH )
    {
        x=(LEVEL_WIDTH-PINWHEEL_WIDTH)-1;
        xVel = -xVel;
    }
    if ( y >= LEVEL_HEIGHT-PINWHEEL_HEIGHT )
    {
        y=(LEVEL_HEIGHT-PINWHEEL_HEIGHT)-1;
        yVel = -yVel;
    }
   
    x+=xVel;
    y+=yVel;
}



This works perfectly even with 1000 pinwheels on screen. Only problem is, when I tried changing the values of xVel and yVel to "rand() % 10 + 2" to make them a random number, the pinwheels all moved in the same direction every time. Did I get the rand() syntax wrong?

--
moto

[whabang]

My mind hurts...

[Karlos]

C(++) stdlib rand() returns a pseudorandom number between 0 and RAND_MAX, which is implementation defined (from as low as 32767 or as high as (2^32)-1, depending on the implementation).

To convert the output to the range 0 ... N use ((N*rand())/RAND_MAX)

[Karlos]

whabang wrote:
My mind hurts...

Wrong forum, really :lol:

[motorollin]

Hmmm, it doesn't work...

int xv=(3*(rand()/RAND_MAX))+1;
int yv=(3*(rand()/RAND_MAX))+1;
int r=1*(rand()/RAND_MAX);
if ( r == 1 ) xv = -xv;
r=1*(rand()/RAND_MAX);
if ( r == 1 ) yv = -yv;
xVel=xv;
yVel=yv;

Lines 1 and 2 should (I think) generate two random numbers between 1 and 4 inclusive. Lines 3-6 then give each velocity a 50/50 chance of becoming negative (IOW the sprite has a 50/50 chance of having its direction reversed in each axis). Lines 7 and 8 obviously then assign the result to the velocities.

When I run this over and over again, the sprite always moves in the same direction. I then added "if ( xv == 1 && yv == 1 ) quit=true;", and my game quits every single time. That suggests to me that the result of the rand() functions are always returning 0 :-?

--
moto

[Karlos]

As I said, you need

(N*rand())/RAND_MAX

What you are doing is this:

N*(rand()/RAND_MAX)

Since you are dealing with integer arithmetic here, you will always get zero for that except in the rare (about 1 in RAND_MAX ;-) ) chance that rand() returns RAND_MAX.

Order of evaluation is very important here. You want to multiply rand() by your scaling factor *before* dividing it, not the other way around.

[motorollin]

Ok I changed my code to this:

        int xv=((3*rand())/RAND_MAX)+1;
        int yv=((3*rand())/RAND_MAX)+1;
        int r1=((1*rand())/RAND_MAX)+1;
        if ( r1 == 1 ) xv = -xv;
        int r2=((1*rand())/RAND_MAX)+1;
        if ( r2 == 1 ) yv = -yv;
        pinwheels.xVel=xv;
        pinwheels.yVel=yv;

The enemy still moves in the same direction each time the game is launched (though a different one to the one it went in before I changed the rand() lines)

[Karlos]

Well, you didn't expect rand() to be actually random did you? ;-)

Unsurprisingly, rand() returns the next number in a series of pseudorandom numbers that are in fact entirely predictable if you know the algorithm in use and the seed that it started with.

Try initialising the random number generator at the start with something reasonably likely to be different each time your program runs. The current system time is a favourite random seed of old.

[motorollin]

Well actually I do have "srand( time(NULL) );" at the top of my main function, before the rand() commands :-?

--
moto