I think you're the one that's supposed to make it create two balls on the screen.

In the notes it says you should be able to use 

(my-animate (list ball1 ball2) <your draw function> <your update function> <your termination function>)

whereas the method you have uses a list of y positions.

dbravoru wrote:

I think you're the one that's supposed to make it create two balls on the screen.

In the notes it says you should be able to use 

(my-animate (list ball1 ball2) <your draw function> <your update function> <your termination function>)

whereas the method you have uses a list of y positions.

isn't that what this

(define (create-scene y-pos-pair)
          ;; create the background image
  (let* ([image0 (empty-scene WIDTH HEIGHT)] 
         [x-pos1 (/ WIDTH 3)]
         [y-pos1 (min (first y-pos-pair) LOWEST-Y-POS)]
          ;; put the object on the background image to create image1
         [image1 (place-image FALLING-OBJECT x-pos1 y-pos1 image0)]
         [x-pos2 (* 2 (/ WIDTH 3))]
         [y-pos2 (max (second y-pos-pair) HIGHEST-Y-POS)]
          ;; put the second object on image1 to create image2.
         [image2 (place-image FALLING-OBJECT x-pos2 y-pos2 image0)]
         )
     ;; return image2 as the result.
    image2))

function does?

I had previously thought so as well. 

This basically sets up the animation.

In an animation, you have still images that appear to be moving, but what is really happening is the same image is being placed in a slightly different position than before.

Think of a slow-motion movie; if you pause and unpause, it looks like each frame can be individual images.

That is what is happening here.

Amedrano wrote:

dbravoru wrote:

I think you're the one that's supposed to make it create two balls on the screen.

In the notes it says you should be able to use 

(my-animate (list ball1 ball2) <your draw function> <your update function> <your termination function>)

whereas the method you have uses a list of y positions.

 

isn't that what this

(define (create-scene y-pos-pair)
          ;; create the background image
  (let* ([image0 (empty-scene WIDTH HEIGHT)] 
         [x-pos1 (/ WIDTH 3)]
         [y-pos1 (min (first y-pos-pair) LOWEST-Y-POS)]
          ;; put the object on the background image to create image1
         [image1 (place-image FALLING-OBJECT x-pos1 y-pos1 image0)]
         [x-pos2 (* 2 (/ WIDTH 3))]
         [y-pos2 (max (second y-pos-pair) HIGHEST-Y-POS)]
          ;; put the second object on image1 to create image2.
         [image2 (place-image FALLING-OBJECT x-pos2 y-pos2 image0)]
         )
     ;; return image2 as the result.
    image2))

function does?

By the way, the code from the wiki

[image2 (place-image FALLING-OBJECT x-pos2 y-pos2 image0)]

should be.

[image2 (place-image FALLING-OBJECT x-pos2 y-pos2 image1)]

Just a typo I think.

You're right. The line should be

[image2 (place-image FALLING-OBJECT x-pos2 y-pos2 image1)]

And you're right that this is the function that puts two balls on the screen.