Due: 11:59 p.m. Tuesday, March 5
Summary: You will explore some novel color transforms.
Purposes: To practice conditionals and have some fun with representations of colors and their transforms.
Expected Time: Two to three hours.
Collaboration: We encourage you to work in groups of size two. You may, however, work alone or work in a group of size three. You may discuss this assignment with anyone, provided you credit such discussions when you submit the assignment.
Email your answer to
<email@example.com>. The subject of your email
should have the form
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. Write a predicate,
(, that returns
value is either an RGB color or a color
name recognized by MediaScript. In all other cases, it should return
You can use the predicates
color-name?, which are already defined, to help
you. You should not use the predicate
want you to write your own version.
b. Write your own version of
which is a very useful procedure.
Your procedure should have the following behavior:
If given an RGB color, it returns that color unchanged.
If given a color name, it returns the result of calling
color-name->rgb on that color name.
If given any other type of value, it returns
You should not use the built-in
procedure; we want you to write your own version.
The previous transform tries to enhance the most pronounced color in the RGB representation. It may be more useful for us to explore alternate representations of color that are somewhat more intuitive. The HSV (for hue, saturation, and value) representation is one possibility. Hue represents the pure color (e.g., red, blue, yellow, green, or a combination of one of these). Saturation represents the "colorfulness" of the hue in the color. For instance, a completely saturated color would be a pure hue (like red), while a lesser saturated color might appear just as bright but somewhat faded (perhaps rose or pink). Value, then represents the brightness relative to a similarly bright white.
Hue is represented as an angle, or a point on a circle. Thus, the values 0-360 sweep through colors red (0 degrees), yellow (60 degrees), green (120 degrees), cyan (180 degrees), blue (240 degrees), magenta (300 degrees), and back to red (at 360 or 0 degrees).
There are a variety of transformations that can take an RGB color and give an HSV representation. In this problem, we'll focus on just extracting the hue.
Before we describe how to calculate hue, we need some basic values
to refer to. Let
refer to the red, green, and blue components of an RGB color,
chroma of a color is the
largest of the RGB components minus the smallest of the RGB
components. For example, the chroma of (128,64,50) is 128-50, or 78;
the chroma of (0,255,0) is 255-0, or 255. The chroma of
(255,255,255) is 255-255, or 0.
The raw hue can then be calculated as follows:
redis a largest component
chroma) + 2 if
greenis a largest component
chroma) + 4 if
blueis a largest component
chroma=0 the hue is undefined, because all the components are the same and we would have a gray. In this case, one convention is to set the hue to 0.
The raw hue as given above produces a value between -1 and 6 (corresponding to the 6 cardinal colors described above). If it is negative, we should add 6 to get us back to a positive representation. The final result is converted to the range 0-360 by multiplying by 60 degrees (which is 360/6).
What happens if
chroma is not 0, and two of
the components are both the largest? If the formula is
well-designed, it shouldn't matter. Hence, you can try
the “largest component” tests in order.
Using the algorithm given above, write a
that takes an RGB color and produces its hue. For example,
(round (rgb->hue-angle RGB-PINK))
Note: You may wish to use a
statement or helper procedures to decompose your implementation into
managable, meaningful units. Please be sure to give your variables
and procedures meaningful names.
Being able to manipulate the hue in a color can actually be quite
useful. MediaScheme can convert an HSV color into an RGB with the
hsv-list is a three-element list
containing the hue, saturation, and value components of a HSV
MediaScheme can also extract the saturation and value from an RGB
color with the procedures
Write a procedure
( that takes an RGB color and a hue value (in the range 0-360) and creates a new RGB color using the given hue with the saturation and value of
works with exact numbers. Thus you may need to
It turns out that making color transforms based on hue be can visually interesting.
|Original Image||Hue rotated 30 deg||Hue rotated 90 deg|
a. Using the procedures you have written so far, write a procedure
that takes a color and produces a new rgb color where the HSV equivalent
has a hue rotated by
angle degrees, a number
Hint: If the rotated angle is greater than 360,
be sure to wrap around properly (e.g.,
modulo) to get the correct hue angle.
b. Write an expression as concisely as possible that will rotate the
hues in an entire image (say, called
picture) by 30 degrees.
Warning: When you
rgb-rotate-hue thousands of times (as you
will in an image of non-trivial size), it is likely to take some
time. Do your testing on small images.
Create your own RGB color transform that transforms the hue of its input color conditionally using a manner of your own choosing. For instance, you may choose to shift colors close to blue more toward green, leaving the rest unchanged.
Include a description of the effect of your transform in English. We
should be able to apply your transform using
For extra credit, design, document, and implement a Color Scheme Generation Toolkit. Your toolkit should consist of a suite of related procedures. Each procedure will take an RGB color as a parameter and produce a list of colors forming a coherent color scheme. Some procedures may need to take additional parameters: You decide what those parameters will be. You will find the procedures related to hue, saturation, and value very useful.
You should at least write procedures to do the following:
Janet Davis (firstname.lastname@example.org)