racket encryption video 2/4 — dissection

February 9, 2020

startup Doctor Racket make sure you’re using the racket
language itself and also under the language menu check that
you’re indeed using the language specified at source, so there’s no
surprises. You’ll need some image tools, so
require 2htdp/image, standard image library
and then put my dsheap/color-utils from planet. These may take awhile to load at first, but, subsequently, not so long. Click F5, and then Control-D, and you’re working in
the interactions pane. Now you’ll need an elephant to work with…
there’s one! cut the elephant so that you can paste it
later on in to your program. We’ll paste it into this definition, so
that jumbo becomes a shortcut for our elephant. Jumbo is a riot of colors. We’d like to make a list of those colors, by turning an image into a color-list. It’s a pretty lengthy list though, and
not practical to work with all at once, so instead of all fifty thousand colors,
let’s just look at the first five. They have a predictable pattern: first the red intensity, then the green,
then the blue, and then the opacity, and these are all numbers from 0 through
255. Let’s look at just one color. So the
seventeenth color in jumbo looks like that: intensity of 174 for red. We can pull out just the red part of that, with color-red, or we can pull out just the blue intensity
with color-blue We can base a new color on an existing color by saying: I’m going to specify the red and then
use the existing color for the rest of it. So the red will be 25, everything else
will be the same as color 17. Or i can specified the blue part and
make everything else the same as the previous color. so now the blue part will be 26,
and everything looks will be as in the 17th color. Here’s how we go about changing all the
colors in an image at once. We bind the name old-col to each color in turn and
then I can do something like: create a new green, which will be the average of 255 or the
greenest possible and the existing green in old-col. And then the template will be the old
color in jumbo itself. So I’m making it all a bit greener…

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