3-smooth-runnings.md

3. Smooth Runnings

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Can we make the sprite not move so sharply?

Is it possible for the character to go around in a smoother fashion? Sure you can! You need the program to keep track of a new state, the velocity. So we need to add the following global state variables at the top of our program:

X,Y=60,60
DX,DY=0,0

The DX and DY variables keep track of the velocity (or the distance taken per frame). The player will now instead control these directly instead of the sprite's X and Y values. So lets change the _UPDATE function to do this:

FUNCTION _UPDATE()
 IF BTN(0) THEN
  DX-=0.6
 END
 IF BTN(1) THEN
  DX+=0.6
 END
 IF BTN(2) THEN
  DY-=0.6
 END
 IF BTN(3) THEN
  DY+=0.6
 END
 -- MAX VELOCITY 4 PIXELS PER FRAME
 DX=MID(-4,DX,4)
 DY=MID(-4,DY,4)
 -- UPDATE THE X,Y WITH DX,DY
 X+=DX
 Y+=DY
 -- CLAMP THE X,Y
 X=MID(0,X,120)
 Y=MID(0,Y,120)
END

Running the program and controlling the character will look a bit like this:

It is a bit out of control, and this is because we don't slow down when we release the keys. Lets correct this by "applying some breaks" at the end of our _UPDATE function. Easiest is to reduce the DX and DY values for each frame with a factor:

 -- AT THE END OF _UPDATE FUNCTION
 DX*=0.8
 DY*=0.8

When you're not touching the keys the DX and DY values will go towards zero, giving the impression of a slow down. When running the program it'll look like this:

Points to review

• Creating games is an iterative process. Don't be afraid of a little math to make things smoother.
• It's common to use X and Y to track position, DX and DY to track the change per frame (aka. the velocity).

Next Chapter - They Call Him Bobby Tables

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