[Tutorial] How to Jump

Mazer · **Posted:** Sat Dec 22, 2007 10:46 am

If your game involves any kind of movement you should already be keeping track of the following variables:

players x and y position
players x and y velocity

It seems as though alot of you forget about the second one. Go ahead and add it now.
Now then... [copied from another post where I explained jumping]

When you are running right your x velocity should be 10 (or whatever) and when running left, -10.

Your y velocity is somewhat different. You should always be subtracting some value (9.8m/s^2 is acceleration due to gravity, but the units you are using may not correspond so use a value that looks about right) from the y velocity. When the player presses the jump button, first make sure that they are on the ground (y position is 0 or something like that) and then set the y velocity to something relatively high. Don't worry about it looking like the value is too high, you're still decreasing it every time you go through the loop. In this way the y velocity will eventually reach zero and then go below zero, so your character should move through the air in a smooth arc. Don't forget to stop the character when it hits the ground.

Now for some source code.

code:

% some constants, tweak this to your liking
const GROUND_HEIGHT := 120
const RUN_SPEED := 10
const JUMP_SPEED := 30
const GRAVITY := 2

var win := Window.Open ("graphics:640;480,offscreenonly")
var chars : array char of boolean

% player position and velocity
var posx, posy : int
var velx, vely : real

posx := 20
velx := 0
posy := 400
vely := 0

loop
Input.KeyDown (chars)
if chars ('q') then
exit
end if

% to make the player move
if chars (KEY_LEFT_ARROW) then
velx := -RUN_SPEED
elsif chars (KEY_RIGHT_ARROW) then
velx := RUN_SPEED
else
velx := 0
end if

% remember, in order to jump you MUST be on the ground when pressing UP
if chars (KEY_UP_ARROW) and posy = GROUND_HEIGHT then
vely := JUMP_SPEED
end if

% subtract your "gravity" constant from the velocity EACH frame
vely -= GRAVITY
posx += round (velx)
posy += round (vely)

% simple "collision" check. just makes sure the player stays above ground
if posy < GROUND_HEIGHT then
posy := GROUND_HEIGHT
vely := 0
end if

% different colours just to illustrate whether or not you may jump
if posy = GROUND_HEIGHT then
drawfillbox (posx - 10, posy, posx + 10, posy + 20, green)
else
drawfillbox (posx - 10, posy, posx + 10, posy + 20, red)
end if

drawline (0, GROUND_HEIGHT, maxx, GROUND_HEIGHT, blue)
View.Update
delay (25)
cls
end loop

Window.Close (win)

That's a really simple example, but it should be enough to get going. If you can't adapt that to account for things like floating platforms or ceilings or pits (ie, real collision detection) then you've probably picked a project that is too complicated for you right now.

Zampano · **Posted:** Sat Dec 22, 2007 10:37 pm

A tutorial like this is is very helpful, as are all that teach one how to perform tasks without processes.
I do have three questions, though.
First, is there anyway to do the same things you did but without declaring a costly 'real' variable? If the user decides not to jump, vely will continue to decrease and decrease even though it will not be used (unless one jumps). Surely there is a way to avoid something as redundant as that.
Second, what is the purpose of of velx if you could simply increment posx by the value of ground speed?
Third, what is the purpose of declaring those four constants if there values are already constants by themselves?

I think that though a tutorial such as this does not fit into the Turing Walkthrough, tutorials like these should be listed elsewhere; perhaps in some sort of index to solutions to common programming goals.

Tony · **Posted:** Sun Dec 23, 2007 12:31 am