you can store velocity to some variable, and make it increase when getKey is true, and decrease to 0 when getKey is false.

If you're referring to acceleration, deceleration, and gravity, those three are actually fairly easy to implement.

Say you have a sprite for your character in Pic1, and the characters position in two variables, X and Y. Every frame, to update the screen, you go like this:

.Initialize
0->X->Y

.Main Game Loop
Repeat getkey(15)

ClrDraw
Pt-On(X/256,Y/256,Pic1)

Dispgraph

.Just so it's not too fast...
Pause 75

End

Because you'll probably want to use 256x inflation, to insure smoothness and acuracy. Some say 256x is too much, but dividing by 256 is a highly optimized procedure (read: it's way fast)

So to do gravity, you need to implement constant acceleration along the Y-Axis. To do that, you need a variable to store the current velocity on the Y-Axis, and add to it every frame. I'm going to use the variable E.

Adding to our previous code:


0->X->Y->E

Repeat getkey(15)

.Add Y_Velocity to Y_Position
Y+E->Y

.Add a constant acceleration to Y_Velocity
E+100->E

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End

If you run that code, you'll notice a problem - the character falls right off the bottom of the screen! You need to add some code to prevent that.


0->X->Y->E

Repeat getkey(15)

Y+E->Y
E+100->E

.Make sure he doesn't fall off the bottom of the screen
If Y>>14336
.^^make sure it's >>, not >.  >> works on negative numers, > doesn't.
.14336 = 56 (the lowest we want our character to be able to go) * 256 (256x inflation)

14336->Y
0->E
.there are other ways of stopping the character, like in Builderboy's Zedd physics library, objects bounce when they hit the ground and stuff like that.

End

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End


Now, we want to add the capability to fly. I'm going to add some code that adds the functionality to fly. If, when we want to fall, we just make E a bigger number, to fly, we just need to make E a negative number, thus subtracting from and making smaller the Y position variable.


0->X->Y->E

Repeat getkey(15)

Y+E->Y
E+100->E

If Y>>14336
14336->Y
0->E
End

.Press [2nd] to fly
If getkey(54)

E-150->E
.This number needs to be bigger than 100, to overcome gravity

End

.Make it so the character doesn't fly off the top of the screen
If Y<<0
0->Y->E
End

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End


Great! Now, we need to add acceleration on the X axis. We need a variable for X axis velocity. I'll use D. To do this, when the user presses right, we want to add to the X velocity, and when the user presses left, we want to subtract from it. Piece of cake!


0->X->Y->E

Repeat getkey(15)

.Every frame, add X_Velocity to X_position
X+D->X

Y+E->Y
E+100->E

If Y>>14336
14336->Y
0->E
End

If getkey(54)
E-150->E
End

If Y<<0
0->Y->E
End

.When the user presses right, add to X_Velocity
If getkey(3)
D+50->D
End

.When the user presses left, subtract from it
If getkey(2)
D-50->D
End

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End


You can already see the problem here - the character will go off the edges of the screen. We need to add boundary checking.


0->X->Y->E

Repeat getkey(15)

X+D->X
Y+E->Y
E+100->E

If Y>>14336
14336->Y
0->E
End

If getkey(54)
E-150->E
End

If Y<<0
0->Y->E
End

If getkey(3)
D+50->D
End

If getkey(2)
D-50->D
End

.If the character is off the right edge...
If X>>22528
.bring him back
22528->X
0->D
End

.If the character is off the left edge...
If X<<0
.we can take care of this
0->X->D
End

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End


Now, we need one final feature - automatic deceleration. Right now the character never loses his energy, and that's not very practical for most games. (note that it's not practical for MOST games - you could still design some kind of a game around that concept)

The way we're going to do this is by checking to see if the user DIDN'T press right or left, decelerate. There are very efficient ways to do this, but for clarity purposes, I'm going to use a very simple method.


0->X->Y->E

Repeat getkey(15)

X+D->X
Y+E->Y
E+100->E

If Y>>14336
14336->Y
0->E
End

If getkey(54)
E-150->E
End

If Y<<0
0->Y->E
End

If getkey(3)
D+50->D
End

If getkey(2)
D-50->D
End

If X>>22528
22528->X
0->D
End

If X<<0
0->X->D
End

.if the user isn't pressing right and the user isn't pressing left
If getkey(2)=0 and (getkey(3)=0)
.divide the acceleration value by 2
D//2->D
End

ClrDraw
Pt-On(X/256,Y/256,Pic1)
Dispgraph
Pause 75

End


That should be everything! You now have a simple but working physics movement engine that supports boundary checking, gravity, and acceleration/deceleration.

Keep in mind, take all this code with a grain of salt. It's not perfect, it's not the best way, it doesn't have every little detail you want to add to make games great, but it works!

Great tutorial, lol, I could have used this a while ago! I like all the explaining

BUTTSFREDKIN I LOVE YOU NOW...
(in a manly way... )

Now I feel stupid.
How would I get this to go backwards (i guess)
Im trying to get my sprite to jump fast up and slow down, then fall.

When you hit the jump key, set the y-velocity to a negative value (this can vary depending on how high you want the jump to be, and make sure the y-velocity is stored in a signed variable). Then, every frame add your gravity constant, add the y-velocity to the y-position, and you should be set.

leafy you just made me go "ah hah!"
who knew it was so simple?

Meeeee!

Yeah, I thought it might be a bit tricky for gravity, bt I started and I noticed how really simple it was

yeah lol

how would I implement a generic "stop  falling if there is a pixel below you" with this? (It works when I fall just a few pixels to it, but when falling from great heights it accelerates through it).
I don't understand bboy's virtual pixel testing.