Assignment #2

For our second assignment, we were given instructions to make adjustments to a code which produced an image between two pictures, that could "dissolve" into either picture.

The most challenging aspect of this assignment was writing the check-expect for a function called interpolate-image. interpolate-image takes two images and a number and produces an image that is an exact intermediate between the two images' colours and opacities. The definition of interpolate-image is as follows:

; interpolate-image : image image number -> image
; Produce image interpolated n/DENOM between
; i1 and i2
; !!! write two check-expect expressions for interpolate-image, one
;     for when n = DENOM, the other for whem n = NUMER

(define (interpolate-image i1 i2 n)
  (local ; f is defined only inside interpolate-image
    [(define (f c1 c2) (interpolate-color (/ n DENOM) c1 c2))]
    (map-images f i1 i2)))


I was initially unfamiliar with the "local" clause and so this created a bit of confusion for me. Luckily, Professor Heap was available during office hours and helped me understand what the function was trying to do:
You should know that interpolate-color takes in a fraction and two colours to produce a colour that is an interpolate of the colours, by whatever fraction was given. For example, (interpolate-color 2/3 (make-color 0 0 0 255) (make-color 6 9 12 255)) will make a colour (make-color 4 6 8 255).
By taking in a number (n), and dividing it by DENOM (20), you have the fraction that the image will be interpolated at. The check-expect tests ask to use n=NUMER=0 and n=DENOM=20. When n=NUMER, the interpolate is at a minimum and will just produce the leftmost image (p1). However, when n=DENOM, the interpolate is at a maximum and will produce the rightmost image (p2).

Therefore, the check-expects can be listed as follows:

(check-expect (interpolate-image p1 p2 DENOM) p2)
(check-expect (interpolate-image p1 p2 NUMER) p1)