Fundamentals of Computer Science I: Media Computing (CS151.01 2008S)
Due: 9:00 a.m., Wednesday, 20 February 2008
Summary: In this assignment, you will use the color manipulation operations you learned in the lab on RGB colors to blend colors together.
Purposes: To get you comfortable working with the basic color operations in the GIMP. To help you think more about colors.
Expected Time: Two to three hours.
Collaboration: You must work in a group of size two or size three. (If you need help identifying a group, please let me know.) You may discuss this assignment and possible solutions with anyone you wish. If you discuss this assignment with people other than group members, make sure to include a citation (e.g., “I consulted this person, who helped me do this”).
Email your answer to
<firstname.lastname@example.org>. The title of your email
should have the form
CSC151.01 2008S Assignment 4: Blending Colors and
should contain your answers to all parts of the assignment. Scheme code
should be in the body of the message.
Warning: So that this assignment is a learning experience for everyone, we may spend class time publicly critiquing your work.
A common effect in digital graphics is a color blend, in which colors range more or less smoothly from one color to another. For example, the following image shows a blend from blue to red. Isn't it beautiful?
What tools does one need to construct blends like the above? You must
know how to manipulate the red, green, and blue components of colors
used in digital images. And you know how to do that. In particular,
you can extract the red, green, and blue components of a color using
rgb-blue, and you can construct an RGB color
Along with basic mathematical operations and the
image-set-pixel! operation, this is enough to let you
construct small color blends.
a. Write and document a procedure,
takes two RGB colors as parameters
and computes the average of those two colors.
That is, the procedure should produce a new RGB color where
the red component is
the average of the red components of the two colors,
the green component is the average of the
green components of the two colors, and the blue component is
the average of the blue components of the two colors.
You should document the
rgb-average procedure using
the 6 P's.
b. Consider the following procedures that build an image for a
five-pixel blend. You should test the procedure with a few pairs of
colors, such as blue and red, green and yellow, and Pacific salmon
and Oregon salmon. (You will need to use
cname->rgb to convert color names into the
corresponding RGB colors.)
;;; Procedure: ;;; image-five-pixel-blend ;;; Parameters: ;;; startcolor, an RGB color ;;; endcolor, an RGB color ;;; Purpose: ;;; Creates a 5x1 pixel image that contains a blend from startcolor to ;;; endcolor. ;;; Produces: ;;; fiveblend, an image identifier (define image-five-pixel-blend (lambda (startcolor endcolor) (image-five-pixel-blend-helper (image-new 5 1) startcolor endcolor))) (define image-five-pixel-blend-helper (lambda (fivebyone startcolor endcolor) (image-set-pixel! fivebyone 0 0 startcolor) (image-set-pixel! fivebyone 4 0 endcolor) (image-set-pixel! fivebyone 2 0 (rgb-average startcolor endcolor)) (image-set-pixel! fivebyone 1 0 (rgb-average startcolor (image-get-pixel fivebyone 2 0))) (image-set-pixel! fivebyone 3 0 (rgb-average endcolor (image-get-pixel fivebyone 2 0))) fivebyone))
i. Explain why you think
uses a helper procedure.
ii. Explain why you think
image-five-pixel-blend-helper ends with
c. Write and document a procedure,
that takes a weight between 0 and 1 and two RGB colors, and computes
the RGB color made up of
weight) parts of
It is easiest to think about this procedure in terms of particular
components. Suppose the red component of
is 0 and the red component of
color2 is 120.
weight is 0.2, then the red component of
the weighted average will be
96 (that is, 0.2*0 + 0.8*120). If
weight is 0.75, then the red component
of the weighted average will be 30 (that is, 0.75*0 + 0.25*120).
For another example,
suppose the green component of
is 200 and the green component of
weight is 0.2, then the green component
of the weighted average will be 40 (that is 0.2*200 + 0.8*0).
weight is 0.75, then the
green component of the weighted average will be
150 (that is 0.75*200 + 0.25*0).
What if the components are both non-zero? Suppose the blue
color1 is 120 and the
blue component of
color2 is 180.
weight is 0.2, then the green component
of the weighted average will be 168 (0.2*120+0.8*180 =
0.75, then the green component of the weighted average will
be 135 (you can do the math).
d. Write a
that creates an image 11 pixels wide and 1 pixel high that is a blend
should be RGB colors.
To compute the color for each pixel, you should call the
rgb-weighted-average procedure that you wrote in
image-five-pixel-blend and its helper as a
model for your procedure(s).
image-11-pixel-blend to compute color blends from
three colors to their pseudo-complements. What do you notice about
the colors in the middle of these blends? Why do you get those
Write a procedure,
(, that creates a 5x5
image with each parameter RGB color in the specified position and a blend
of those colors in between.
You should be able to blend the colors along the edges, using a technique much like the one we just used for one-dimensional blends. How should you blend the colors in the middle? It's up to you.
We intend to evaluate your assignment on the correctness and elegance of your solution. That is, is what you've done something that could be reasonably considered a blend, and have you chosen a technique that is clear and easy to understand.
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.
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