This laboratory is also available in PDF.
Summary: We explore techniques for algorithmically
modifying existing images.
Contents:
a. Create a new file for your procedures for this lab (say,
image-manip.scm). Open that file in DrScheme.
b. Start the GIMP and open a Script-Fu console.
c. Load your new file and
/home/rebelsky/Web/Courses/CS151/2007S/Examples/hog.scm.
d. If you're working on a computer outside the MathLAN, you may want to
download a new copy of gimp.scm.
e. For this laboratory, you will need a moderately small image (say
128x128), since the number of points we process is approximately
width*height. (A 128x128 image has more than sixteen thousand points.)
You can make a copy of the sample
image or you can select a portion of some image you find interesting
(such as your StalkerNet photo). I'd suggest that you call your image
sample.jpg.
a. Load your sample image with
(define sample (load-image "/home/username/Desktop/sample.jpg"))
b. Display it with
c. Determine the color of your image at the position (10,10) with
(color->list (get-color-at sample 10 10))
d. Set the foreground color to that color with
(set-fgcolor (get-color-at sample 10 10))
e. Try getting a few other colors.
The reading suggests that you can set colors with the
set-color-at! procedure, but that you can't necessarily
see those changes.
a. Try setting the color at (10,10) to red with
(set-color-at! sample 10 10 RED)
b. Verify that you've set the color with
(color->list (get-color-at sample 10 10))
c. Zoom in to 800% or so (using ) and see
if you can see the change. Don't worry if you can't; it's one of the
strange design features of GIMP.
d. Open a new copy of your sample image, set the color at (10,10), and
then show it.
(define sample (load-image "/home/username/Desktop/sample.jpg"))
(set-color-at! sample 10 10 RED)
(show-image sample)
e. Verify that you've set the color with
(color->list (get-color-at sample 10 10))
f. Zoom in to 800% or so and see if you can see the change. This time, you
should be able to see the change. If you can't, get help.
a. Convert your sample image to greyscale with
(define sample (load-image "/home/username/Desktop/sample.jpg"))
(modify-image! greyscale-better sample)
(show-image sample)
Warning! It takes about a minute for modify-image! to do
its job.
b. Rotate the color components of the original image with
(define sample (load-image "/home/username/Desktop/sample.jpg"))
(modify-image! rotate-components sample)
(show-image sample)
c. Increment the red component of the original image with
(define sample (load-image "/home/username/Desktop/sample.jpg"))
(modify-image! redder-64 sample)
(show-image sample)
d. Decrement the blue component of the original image with
(define sample (load-image "/home/username/Desktop/sample.jpg"))
(modify-image! lessblue-64 sample)
(show-image sample)
As you may recall from the reading, you can increment the red component
of a color by 64 in a variety of ways.
-
(redder-64 color)
-
(change-red 64 color)
-
((redder 64) color)
-
((l-s change-red 64) color)
a. Create the color light-grey with
(define light-grey (rgb 64 64 64))
b. Determine the components of that color with
c. Verify that each of the procedures above changes the red component by
64. For example,
(color->list (redder-64 light-grey))
d. Figure out a way to increment the red component of light-grey
by 128.
e. What do you expect to happen if you decrement the red component of
light-grey by 128? Verify your answer experimentally.
f. Create a new copy of your sample image with the red component of each
point incremented by 128.
g. Create a new copy of your sample image with the red component of each
point set to 0.
We've seen that there are two ways to sequence image modifications:
- We can simply type the two
modify-image! commands in
sequence.
- We can use one call to
modify-image! and compose the
color transformations.
For example, if we want to both increment the blue component by 96 and
decrement the red component by 64, we can write
(modify-image! (bluer 96) sample)
(modify-image! (redder -64) sample)
or we can write
(modify-image! (compose (redder -64) (bluer 96)) sample)
a. One way to compare the efficacy of the two approaches is to time
them. Try both modifications (on separate copies of the image, preferably),
timing each. Which is faster? Are the results the same?
c. At this point, you know a few basic modifications: (1) setting the
image to greyscale with greyscale-better; (2) changing
one of the color components with (redder amt) (or
bluer or greener); rotating the color components with
rotate-component. Try composing some of these.
At times, we only want to change color values if certain conditions hold.
For example, we might only want to increment the red value if it is below
or above a certain amount.
a. In your code file, add the following procedure
(define sometimes-change-red
(lambda (cutoff amt)
(lambda (color)
(if (> (red color) cutoff)
(change-red amt color)
color))))
b. What would you write to increment by 128 the red component of all
of the pixels with a red component of at least 64?
c. One hopes that you wrote something like
(modify-image! (sometimes-change-red 64 128) sample)
Try that code.
As you may recall from a previous laboratory, when experimenting with
transformations, it may be useful to try some anonymous ones before deciding
on a final one.
For example, suppose we want to increment the red component by 25%, the green
component by 50%, and the blue component by 75%. We can write
(modify-image! (lambda (color) (rgb (* 1.25 (red color)) (* 1.5 (green color)) (* 1.75 (blue color)))) sample)
Similarly, if we want to try setting the blue component
to 128 and swap the red and green components, we can write:
(modify-image! (lambda (color) (rgb (green color) (red color) 128)) sample)
a. Verify that these techniques work.
b. Consider the following procedure
(define extreme (lambda (val) (if (< val 128) 0 255)))
What do you expect to happen with the following?
(modify-image! (lambda (color) (rgb (extreme (green color)) (extreme (red color)) (extreme (blue color)))) sample)
c. Try it.
d. Try a few interesting modifications of your own.
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