acceleration

[BenLi]

I thought this simple piece of code may help anyone starting to do some games or graphics in turing. This program accelerates a circle around the screen. Controls are arrow button and the spacebar.

Features:
-bounce off the walls
-spacebar to stop all velocity
-friction

Note: no attempt at actual physics has been made

code:

setscreen ("graphics:800;600,offscreenonly") var x, y : int var vx, vy : real := 0 var key : array char of boolean x := 400 y := 300 loop     Input.KeyDown (key)     if key (KEY_UP_ARROW) then         vy += .3     elsif key (KEY_DOWN_ARROW) then         vy -= .3     end if     if key (KEY_RIGHT_ARROW) then         vx += .3     elsif key (KEY_LEFT_ARROW) then         vx -= .3     end if     if key (' ') then         vx := 0         vy := 0     end if     x += round (vx)     y += round (vy)     if x > maxx then         vx := -vx     elsif x < 0 then         vx := -vx     end if     if y > maxy then         vy := -vy     elsif y < 0 then         vy := -vy     end if     if abs (vx) = vx then         vx -= .1     elsif (vx) not= vx then         vx += .1     end if     if abs (vy) = vy then         vy -= .1     elsif (vy) not= vy then         vy += .1     end if     drawfilloval (x, y, 25, 25, blue)     View.Update     cls end loop

[TokenHerbz]

ok nothing works for me, i press a direction key the balls gets stuck on the left side.

Super glitch each time i run this.

[BenLi]

can someone check this out? It runs perfectly for me.

Are you on an uber fast computer? Perhaps decrease the acceleration values?

[ZeroPaladn]

the ball glitches out every time. gets stuck in the walls and whatnot. other than that, i dont understand the code, which in turn means i cant help you fix this.

[War_Caymore]

pretty cool little game, hold the down htton after letting it jump around and it will glitch into the end of the screen. mabey thickening the colsion at the ends of the screen might prevent the ball from sticking but god only knows what more harm that will do.

A better idea.

code:

if x > maxx then         vx := -vx     elsif x < 0 then         vx := -vx     end if     if y > maxy then         vy := -vy     elsif y < 0 then         vy := -vy     end if

instead of setting the boundries to maxx and maxy, set it to th radius of the ball

code:

if x > maxx - 25 then         vx := -vx     elsif x < 0 + 25 then         vx := -vx     end if     if y > maxy - 25 then         vy := -vy     elsif y < 0 + 25 then         vy := -vy     end if

that might keep the ball inside the screen and it will not go half out, and glithc.

Nevermind.... this only lengthens the time it takes to glith, so it is now mroe preventable. i did what i could with what little skills i have.

[Cervantes]

Cervantes makes his first large post in too long!

War_Caymore wrote:

instead of setting the boundries to maxx and maxy, set it to th radius of the ball

code:

if x > maxx - 25 then         vx := -vx     elsif x < 0 + 25 then         vx := -vx     end if     if y > maxy - 25 then         vy := -vy     elsif y < 0 + 25 then         vy := -vy     end if

that might keep the ball inside the screen and it will not go half out, and glithc.

Nevermind.... this only lengthens the time it takes to glith, so it is now mroe preventable. i did what i could with what little skills i have.

That does nothing but change the place at which the ball gets stuck. If you want to prevent the ball from sticking, we first need to understand why it sticks.

It sticks because somtimes when you bounce against the wall, the numbers work out in the following way: frame 1, you're just outside the wall; frame 2, you're as far inside the wall as can be possible; frame 3, you're still inside the wall because you didn't bounce back far enough in one frame. Why didn't you bounce back far enough? Because a) the retarding code slowed you down (that code that moves the velocity of the ball closer to zero) and b) you are probably still holding the arrow key to move you towards the edge.

What's the solution? The easiest one would be to just step through an extra frame of movement when you hit the wall, by adding a x += vx and y += vy in the respective if statements. Or maybe you should add two steps? Who knows. It would all depend on many constants such as acceleration rate from the arrow keys and deceleration rate, as well as max speed.

The other way to solve the problem would be to do some math to predict when a collision with the wall will occur, before it actually occurs. You can then do some more math to determine where exactly you should be after the collision. This approach won't be limited by how many frames per second you are getting. If you like math, I suggest you try this out.

Now, to clean the code in a few places.

code:

var x, y : int var vx, vy : real := 0

Make x and y reals, and round when drawing. Rounding during your calculations allows for rounding errors to grow from a microscopic scale to potentially a macroscopic scale.

code:

if x > maxx then         vx := -vx     elsif x < 0 then         vx := -vx     end if

First, let's insert the extra step:

code:

if x > maxx then         vx := -vx         x += vx     elsif x < 0 then         vx := -vx         x += vx     end if

Now, we're doing the same action if either condition is true. Let's just use or:

code:

if x > maxx or x < 0 then         vx := -vx         x += vx     end if

Next, the retarding of motion could be done differently. Instead of

code:

if abs (vx) = vx then         vx -= .1     elsif (vx) not= vx then         vx += .1     end if

we could use

code:

vx *= 0.99

This means that the ball no longer slows down linearly. It mimicks things like wind resistance, which increases with the square of the velocity, much better.

Finally, stick a delay in there. It's very much needed for anyone with a decently fast computer.

Here's the complete code with all the changes I suggested:

code:

setscreen ("graphics:800;600,offscreenonly") var x, y : real var vx, vy : real := 0 var key : array char of boolean x := 400 y := 300 loop     Input.KeyDown (key)     if key (KEY_UP_ARROW) then         vy += .3     elsif key (KEY_DOWN_ARROW) then         vy -= .3     end if     if key (KEY_RIGHT_ARROW) then         vx += .3     elsif key (KEY_LEFT_ARROW) then         vx -= .3     end if     if key (' ') then         vx := 0         vy := 0     end if     x += vx     y += vy     if x > maxx or x < 0 then         vx := -vx         x += vx     end if     if y > maxy or y < 0 then         vy := -vy         y += vy     end if     vx *= 0.99     vy *= 0.99     drawfilloval (round (x), round (y), 25, 25, blue)     View.Update     delay (10)     cls end loop