Laboratory: Transforming Images
In this laboratory, you will be creating a few images and manipulating
others. We will also be working with some colors.
a. Create a new 4x3 image, call it canvas. Do not
show it, since you'll want to see the pixel transformations.
b. Load an existing PNG or JPEG image of your choice, call it
picture, and show it. Please choose an image that is
not too large (say, not much more than 250x250). If you'd like, you
can find the image from the reading at
/home/rebelsky/MediaScheme/Images/kitten.png.
Exercise 1: Image Variants
a. Set a few pixels in canvas to colors of your choice. Leave
others black or white.
b. What do you expect to happen when you use
image-variant to complement each pixel in
canvas, using the following instruction?
> (image-show (image-variant canvas irgb-complement))
c. Check your answer experimentally.
d. What do you expect to have happen if you use
image-variant to complement each pixel in
picture? (You would use nearly the same instruction,
substituting picture for canvas.)
e. Check your answer experimentally.
Exercise 2: Transforming “In Place”
Although we emphasized image-variant in the reading,
the MediaScheme library also provides a procedure,
(image-transform! image
colortrans), that changes the image, rather
than creating a new image.
a. Create, but do not show, a new copy of canvas.
If you have not done so already, set a few pixels in
canvas to colors of your choice. Leave others black
or white.
b. What do you expect to happen when you use image-transform!
to complement each pixel in canvas, using the following
instruction?
> (image-transform! canvas irgb-complement)
c. Check your answer experimentally.
d. Create, but do not show, a new version of picture What
do you expect to have happen if you use image-transform!
to complement each pixel in picture? (You would use
nearly the same instruction, substituting picture for
canvas.)
e. Check your answer experimentally. You should show the image
after complementing.
f. What do you expect to have happen if you once again complement each
pixel in picture?
g. Check your answer experimentally.
Exercise 3: Undoing Transformations
a. What do you expect to have happen if you use image-transform!
to redden each pixel in canvas?
b. Check your answer experimentally.
c. You may have noticed that in exercise 2, we were able to undo the
complement transformation by complementing again. Is there an easy
way to undo the redden operation? (You do not have to write code;
just explain how to do it.)
d. Are there transformations or sequences of transformations that would
be impossible to undo? (That is, can you do something to an image such
that there is nothing that you can do to the revised image that will
bring back the original image?)
Exercise 4: Pure Transformations, Revisited
As you may have just observed, there are times that transforming an
image can be dangerous, because we cannot easily undo some transformations
(at least not without the magic “Undo” menu item). In spite
of this problem, many programmers still prefer side-effecting operations
like image-transform!.
With your partner, come up with some reasons that it may be advantageous
to use image-transform! rather than
image-variant. Be prepared to share those reasons
with your classmates.
Consider the following definitions.
(define much-darker (compose irgb-darker irgb-darker))
(define red (iirgb 255 0 0))
a. What color do you expect (much-darker red) to compute?
(Answer the question in terms of red, green, and blue values.)
b. Check your answer experimentally.
c. Set the top-left pixel of canvas to red.
d. What effect do you expect the following instruction to have?
(image-transform-pixel! canvas 0 0 (compose irgb-lighter irgb-lighter))
e. Check your answer experimentally.
f. What effect do you expect the following instruction to have?
(image-transform-pixel! canvas 0 0
(compose irgb-lighter (compose irgb-lighter irgb-lighter)))
g. Check your answer experimentally.
Exercise 6: Composition and Order of Operations
Consider the composition (compose irgb-darker irgb-phaseshift).
a. Does this darken the image first or phase-shift the image first?
b. Does it matter? That is, do you get the same result either way?
c. Check your answer experimentally, using
image-variant to compute the new images.
Exercise 7: Undoing Transformations
Earlier in this lab, we saw that some transformations had natural inverses
and some did not. For example, if you complement a color twice, you get
the original color. For many colors, if you lighten and then darken the
color, you get the original color. (If any of the components is very large,
then we may not be able to restore the original color.)
a. Consider the irgb-redder operation. How would
you write an inverse to that operation using only the basic transformations
along with composition? (That is, you do not get to write a
lambda expression.)
b. Test your answer using image-variant.
For example, if you decided that the answer was to make the image
greener and bluer, you might write something like the following.
(define redder-canvas (image-variant canvas irgb-redder))
(define not-redder-canvas (image-variant redder-canvas (compose irgb-greener irgb-bluer)))
(image-show redder-canvas)
(image-show not-redder-canvas)
In case you were wondering, making the image greener and bluer does not
invert the redder operation.
Exercise 8: Anonymous Transformations
a. Consider the following instruction. What effect do you expect it to have
on the red component of each pixel? What effect do you expect it to have on
the overall image?
> (image-show (image-variant picture
(lambda (color)
(irgb (modulo (+ (irgb-red color) 128) 255)
(irgb-green color)
(irgb-blue color)))))
b. Check your answers experimentally.
c. Write an instruction to build a variant of picture in which
the green component of each pixel is set to 255.
d. Write an instruction to build a variant of picture in which
the red component is restricted to the range [64..192], with numbers above
192 dropped to 192 and numbers below 64 raised to 64. In writing this,
you may want to rely on the bound procedure you used
in a previous lab.
e. Write an instruction to build a variant of picture in which
the red component is set to 255 minus the red component.
Explorations are intended for students interested in further
exploring the design aspects of these techniques. They also provide
students who finish early with extra activities that may challenge
them in different ways. You may do them in any order.
Exploration 1: Combining Transformations
While we only have a few basic transformations, there are, in some sense,
an infinite number of ways to combine them. Try to find an interesting
composition of basic transformations that someone might want to use as a
filter.
Exploration 2: Arithmetic Transformations, Revised
As you have undoubtedly noticed, RGB colors are represented as integers.
That means that we can transform colors with arithmetic operations as
well as with component based operations. What do you think the following
operations will do to your image? Try some of them to find out.
Then, try a few of your own devising.
(define t1 (lambda (c) (* 2 c)))
(define t2 (lambda (c) (* 3 c)))
(define t2 (lambda (c) (* -1 c)))
(define t3 (lambda (c) (* 256 c)))
(define t4 (lambda (c) (quotient (+ color-red c) 2)))
(define t5 (lambda (c) (- c color-blue)))
Make sure you save your work regularly. Some of these
procedures have the potential to crash the GIMP or MediaScript.
Extra 3: Your Own
Transformations
Try developing your own interesting RGB transformation procedures and
applying them to picture.
