6-crash-test-dummies.md

6. Crash test dummies

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So when are we gonna pick up the cherries?

We want our player token pick up all the cherry tokens it reaches. For this we need to implement a collision check. Unfortunately PICO-8 API doesn't provide one, so you'll need to make one yourself.

The easiest way to do a collision check is to check if both the player token and any cherry is close enough. We can check the distance between points, such as the center of player and cherry and see if it is less than 4 pixels or so.

A distance function, if you don't remember math, is given from pythagorean function. You may create a function that takes two tables (that presumably have X and Y) and calculate the distance. We need to define some variables, and to not pollute the global scope you can use LOCAL to define variables to be bound by the function scope it is in:

FUNCTION DIST(A,B)
 LOCAL DX,DY=A.X-B.X,A.Y-B.Y
 RETURN SQRT(DX*DX+DY*DY)
END

Now lets use that in our _UPDATE function and check the distances with with our DIST function between all cherries and the player. IF they are close enough THEN remove the cherry with PICO-8's DEL function.

FUNCTION _UPDATE()
 PLAYER:UPDATE()
 FOR CHERRY IN ALL(CHERRIES) DO
  IF DIST(PLAYER, CHERRY)<4 THEN
   DEL(CHERRIES, CHERRY)
  END
 END
END

Do note that as with FOR loops, IF-statements in LUA doesn't need parenthesis for it's arguments.

Running CTRL+R you'll be able to pick up the cherries in the stage now.

Points to review

• IF-THEN-END statements for decisions, like in other languages No need for parenthesis to enclose the expression
• DEL removes/deletes the given object from the given table

Next Chapter - Keeping the Score

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