Fundamentals of Computer Science I: Media Computing (CS151.01 2008S)

Laboratory: Building Images by Iterating Over Positions

Summary: In this laboratory, you will explore techniques for building and modifying images by iterating over the positions in an image.

(image-compute-pixels! image first-col first-row last-col last-row function)
DrFu procedure. Create a portion of an image by setting each pixel in the specified region to the result of applying function to the position of the pixel. function must be a function from position to rgb-color. Warning! The current version is buggy, and may affect other pixels.
DrFu Constant. A special color value, used only by image-compute-pixels!. If the color function returns color-transparent for a particular position, the color at that position is left unchanged.
(position-new col row)
DrFu procedure. Build a new position that represents the point at (col,row).
(position-col pos)
DrFu procedure. Extract the column from position pos.
(position-row pos)
DrFu procedure. Extract the row from position pos.


Add the following instructions to your definitions pane and click Run.

  ((or (not (defined? 'canvas)) (not (image? canvas)))
   (define canvas (image-new 200 200))
   (image-show canvas)))


Exercise 1: Drawing Rectangles

As you learned in the reading, one of the simplest ways to use image-compute-pixels! is to create a rectangle of a uniform color. For example, we might add a small square near the middle of canvas with

(define rect-color (cname->rgb "hot pink"))
(image-compute-pixels! canvas 90 90 110 110 (lambda (pos) rect-color))

a. Confirm that these instructions work as advertised.

b. What do you expect the following instructions to do? What do you expect to have happen to the pixels that were in the first rectangle?

(define rect-color (cname->rgb "firebrick"))
(image-compute-pixels! canvas 95 100 120 120 (lambda (pos) rect-color))

c. Check your answer experimentally.

d. Consider the following instructions. What do you expect to have happen when you execute them?

(define rect-color (rgb-new 255 255 255))
(image-compute-pixels! canvas 
                        0 0 
                        (image-width canvas) (image-height canvas) 
                        (lambda (pos) rect-color))

e. Check your answer experimentally and then read the notes on this problem.

f. Using a similar instruction, make the entire canvas black.

g. Write instructions to draw a blue rectangle in the upper-left corner of canvas, a green rectangle in the upper-right corner, a yellow rectangle in the lower-right corner, and a red rectangle in the lower-left corner. You can choose the size of the rectangles.

Exercise 2: Ranges

a. The reading claims that the following instructions will provide a range of intensities of blue. Check to confirm that the claim is correct.

(define blues (image-new 17 1))
(image-show blues)
(image-compute-pixels! blues 0 0 16 0 
                        (lambda (pos) (rgb-new 0 0 (* 16 (position-col pos)))))

b. Use similar code to create a range of intensities of green in a 17x17 image, keeping each column the same intensity of green.

c. Use similar code to create a range of intensities of red from top to bottom (instead of from left to right) in a 17x17 image. In this case, make the intensities vary from top to bottom, instead of from left to right. That is, each row should have the same intensity of red.

d. Sometimes we don't want our color ranges to fill the entire image. Write instructions to put the previous red blend in the square that starts at 60,60 and ends at 76,76 in canvas.

Exercise 3: Two-Dimensional Color Ranges

In the previous problem, you made images that showed color intensities ranging over a single dimension, either horizontally or vertically. However, we can certainly vary colors in both dimensions.

Write instructions to create a 17x17 image that shows a range of intensities of blue that vary over all the pixels. In particular, make the blue component proportional to the sum of the row and column number.

Optional: Fill in the image again, making the blue component proportional to the product of the row and column number.

Exercise 4: Generalizing Color Ranges

One thing that a good programmer does is generalize her code, so that it works in a variety of situations. The color range code we've written so far is fairly specific to the image size (and even the image placement).

For example, let's consider the computation of the blue pixels in a 17x17 image. The core part of that code is likely to read as follows:

(define blues (image-new 17 17))
(image-compute-pixels! blues 0 0 16 16
                        (lambda (pos) (rgb-new 0 0 (* 16 (position-col pos)))))

The number 16 appears three times in this code. The first time it represents the final column of the image. The second time, it represents the final row of the image. The third time, it represents 256/16.

a. Write a procedure, (image-blue-range image) that fills image with a similar range of blues, and that works with any width and height. For example, for a shorter, wider, image, one might write

(image-blue-range (image-new 65 12))

b. Sometimes we want to draw color ranges in the middle of an image, rather than filling it. Write a procedure, (image-blue-area image left top width height), that fills a blue range within the specified area of the image.

c. Test your code by drawing a variety of blue color ranges. Here are a few to get you started.

(image-blue-area canvas 0 0 17 2)
(image-blue-area canvas 90 0 20 9)

Exercise 5: Color Blends

a. The reading suggested that the following code will provide a blend from red to blue. Check that claim.

(define red-to-blue (image-new 129 16))
(image-show red-to-blue)
(image-compute-pixels! red-to-blue
                        0 0
                        128 15
                        (lambda (pos) (rgb-new (* 2 (- 128 (position-col pos)))
                                               (* 2 (position-col pos)))))

b. Write similar code to provide a blend from yellow to red.

c. Put this blend in canvas, using the rectangle from 10,120 to 138,136.

Exercise 6: More Complex Computations

You may recall a more complex computation of colors from the reading, one that used sin to determine values. Here's a simplified version of that code:

(define what-am-i (image-new 40 50))
(image-compute-pixels! what-am-i
                       0 0 39 49
                       (lambda (pos)
                         (rgb-new 0
                                  (* 128 (+ 1 (sin (* pi 0.025 (position-col pos)))))

a. What range of values does (* pi 0.025 (position-col pos)) compute?

b. What range of values does (sin ...) compute?

c. What range of values does (+ 1 (sin ...)) compute?

d. What range of values does (* 128 (+ 1 (sin ...))) compute?

e. Given that analysis, what kind of image do you expect the preceding code to draw?

f. Check your answer experimentally.

g. The reading had a somewhat more complex set of instructions.

(define what-am-i (image-new 40 50))
(image-compute-pixels! what-am-i
                       0 0 39 49
                       (lambda (pos)
                         (rgb-new 0
                                  (* 128 (+ 1 (sin (* pi 0.025 (position-col pos)))))
                                  (* 128 (+ 1 (sin (* pi 0.020 (position-row pos))))))))

Explain why there might be a .020 rather than a .025 in the computation of the blue component.

h. Predict the appearance of the computed image.

i. Check your answer experimentally.

Exercise 7: Fun with Stripes

Consider the following instructions.

(define stripe-colors
  (list (rgb-new 255 0 0) 
        (rgb-new 255 255 0) 
        (rgb-new 0 255 0) 
        (rgb-new 0 255 255)
        (rgb-new 0 0 255)))
 canvas 0 0 99 99
 (lambda (pos) (list-ref stripe-colors (modulo (position-col pos) 5))))

a. What do you expect these instructions to do?

b. Check your answer experimentally.

c. Extend that code to use seven colors instead of five. That is, you'll need to add two values to the list (presumably, by redefining the list) and use a different modulus.

d. As you should have observed, this makes a sequence of vertical stripes, each of width one pixel. We can make the stripes wider by computing the quotient of the column and the desired width. Try expanding your columns to a width of 5 and then 10. (Note that in order to use the result as a parameter to modulo and then in list-ref, we need to guarantee that it is an integer. Hence, you should compute the quotient with quotient rather than /.

e. Predict the result of the following instructions, and then try executing them.

 canvas 100 0 199 99
 (lambda (pos) 
    (list-ref stripe-colors (modulo (quotient (position-col pos) 10) 5))))
 canvas 0 100 99 199
 (lambda (pos) 
   (list-ref stripe-colors (modulo (quotient (position-row pos) 10) 5))))
 canvas 100 100 199 199
 (lambda (pos) 
   (list-ref stripe-colors 
             (modulo (quotient (+ (position-row pos) (position-col pos)) 10) 5))))

Exercise 8: Circles, Circles, and More Circles

Consider the following code from the reading.

(image-compute-pixels! canvas
                       10 20 70 80
                       (lambda (pos) 
                         (let ((col (position-col pos))
                               (row (position-row pos)))
                           (if (<= (+ (square (- col 40)) (square (- row 50)))
                                  (square 30))

a. Confirm that the code does, in fact, draw a red circle on a black background.

b. What do you expect to have happen if we change the initial 10 to 40, the initial 20 to 40, and color-red to color-yellow?

c. Check your answer experimentally.

d. Suppose we want to draw another red circle of radius 20, centered at (20,20). We might write

(image-compute-pixels! canvas
                       0 0 40 40
                       (lambda (pos) 
                         (let ((col (position-col pos))
                               (row (position-row pos)))
                           (if (<= (+ (square (- col 20)) (square (- row 20)))
                                  (square 20))

What effect do you expect this code to have?

e. Check your answer experimentally.

f. As you should have noticed, in addition to drawing the red circle, we wiped out some of the previous image with black. Erase the image, sketch something new (e.g., the circle centered at (40,50)), then rerun the previous instruction, substituting color-transparent for color-black.

g. As the reading suggested, instead of just using color-red, we can substitute a procedure that computes different colors. Try doing so.

For Those With Extra Time

You may find it appropriate to do an extra problem (which you can do in any order), an exploration, or any combination.

Extra 1: Bordering Blends

At the end of some of the previous exercises, you created interestingly colored rectangles in the middle of canvas. Sometimes, to make such rectangles stand out, we would like a fixed-color border around the rectangles.

a. Describe at least two ways to create such borders. After doing so, read the notes on this problem.

b. Pick one of those two ways, and draw the yellow-red blend from an earlier problem in the same location, but with a four-pixel-wide black border.

Extra 2: Expanding Images

Consider the final image from exercise 6. Suppose we wanted to make a similar, but larger, version of the image. To do so, we'd certainly need to change the width and height of the image and the ending column and row.

Do we need to change the .025 and .020? We can, but it depends on how we define “similar”.

If you change .025 and .020, you might scale them similarly to how you scale the image. For example, if we double the width and height of the image, we might write

(define what-am-i (image-new 80 100))
(image-compute-pixels! what-am-i
                        0 0 79 99
                        (lambda (pos)
                        (rgb-new 0
                                  (* 128 (+ 1 (sin (* pi 0.0125 (position-col pos)))))
                                  (* 128 (+ 1 (sin (* pi 0.01 (position-row pos))))))))

a. What do you expect to have happen if you use the preceding code to create a larger, similar, image?

b. Check your answer experimentally.

c. What do you expect to have happen if you restore the .025 and .020 to the preceding code and then rerun it?

d. Check your answer experimentally.

Write a procedure, (image-draw-circle! image col row radius colorfun), that draws a circle of radius radius centered at (col,row), using colorfun to compute the color. For example, we might write

(image-draw-circle! canvas 60 30 20
                    (lambda (pos) 
                      (rgb-new (modulo (* 3 (position-row pos)) 256)
                               (modulo (square (position-col pos)) 256)
                               (modulo (square (position-row pos)) 256))))


Choose either, both, or neither of these explorations.

Exploration 1: Simulating Depth

As Professor Kluber suggested when he visited, one way we can simulate depth in an image is by overlaying one thing over another.

a. Create an image you find aesthetically pleasing by overlaying fixed-color rectangles on top of each other.

b. Create an image you find aesthetically pleasing by overlaying computed-color rectangles on top of each other.

Exploration 2: Complex Color Computations Continued

In the final exercises in this lab, you used trigonometric functions to compute color values. Clearly, we can use these functions, and others, in a variety of ways. For example, we need not linearly scale the row and column, or can scale them in a way to get different ranges. Similarly, we can choose functions other than sin to compute results. In the end, all that we care is that we end up with each component value in the range 0 .. 255.

Experiment with a variety of functions and multipliers to find a combination that you find pleasing.

For example, consider the original computation of the green component

(* 128 (+ 1 (sin (* pi 0.025 (position-col pos)))))

We might replace the 0.25 with another number. We might replace the sin with cos. We might cube the result of that multiplication with

(expt (* 0.025 (position-col pos)) 3)

We might even square the result of the sine computation.


Notes on Exercise 1: Drawing Rectangles

While the obvious answer to part d is “it makes the image white”, there is a subtle bug in the code, which should cause it to report an error, rather than fill the image with the color white.

We include this intentional bug, as we have in the past, to remind you that you need not just consider what code is supposed to do; you should also make sure that it is logically correct.

Notes on Extra 1: Bordering Blends

One technique for putting a border around a rectangular part of the image is to draw a slightly larger rectangle before you draw that part of the image. For example, if our blend is from 10,20 to 25,35, we might first draw a rectangle from 8,18 to 27,37.

Another technique is to draw four thin rectangles, one above, one to the left, one to the right, and one to the bottom. For example, if our blend is from 10,20 to 25,35, we might draw a two-pixel border with

(define bgcolor (rgb-new 0 0 0))
(image-compute-pixels! canvas  8 18 27 19 (lambda (pos) bgcolor))
(image-compute-pixels! canvas  8 18  9 37 (lambda (pos) bgcolor))
(image-compute-pixels! canvas 26 18 27 37 (lambda (pos) bgcolor))
(image-compute-pixels! canvas  8 36 27 37 (lambda (pos) bgcolor))

The second technique is a bit more complicated to write, but has the advantage that we can add the border after we've drawn the image.

Creative Commons License

Samuel A. Rebelsky,

Copyright (c) 2007-8 Janet Davis, Matthew Kluber, and Samuel A. Rebelsky. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)

This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.