1.1 Manipulating Images: "image.rkt"

NOTE: This library is deprecated; use 2htdp/image, instead. For the foreseeable time, we will continue to support the teachpack for your existing programs.

The teachpack provides functions for constructing and manipulating images. Basic, colored images are created as outlines or solid shapes. Additional functions allow for the composition of images.

1.1.1 Images

1.1.2 Modes and Colors

Mode (one-of/c 'solid 'outline "solid" "outline")

A Mode is used to specify whether painting a shape fills or outlines the form.

struct (struct   color (red green blue)

#:extra-constructor-name make-color)

red : (and/c natural-number/c (<=/c 255))

green : (and/c natural-number/c (<=/c 255))

blue : (and/c natural-number/c (<=/c 255))

RGB color?

A RGB describes a color via a shade of red, blue, and green colors (e.g., (make-color 100 200 30)).

Color (or/c symbol? string? color?)

A Color is a color-symbol (e.g., 'blue) or a color-string (e.g., "blue") or an RGB structure.

1.1.3 Creating Basic Shapes

In DrRacket, you can insert images from your file system. Use PNG images whenever possible. In addition, you can create basic shapes with the following functions.

1.1.4 Basic Image Properties

To understand how images are manipulated, you need to understand the basic properties of images.

For the composition of images, you must know about pinholes. Every image come with a pinhole. For images created with the above functions, the pinhole is at the center of the shape except for those created from line and text. The text function puts the pinhole at the upper left corner of the image, and line puts the pinhole at the beginning of the line (meaning that if the first two arguments to line are positive, the pinhole is also in the upper left corner). The pinhole can be moved, of course, and compositions locate pinholes according to their own rules. When in doubt you can always find out where the pinhole is and place it where convenient.

1.1.5 Composing Images

Images can be composed, and images can be found within compositions.

The pinhole of the resulting image is the same place as the pinhole in the first image.

Be careful when using this function with jpeg images. If you use an image-editing program to crop a jpeg image and then save it, image-inside? does not recognize the cropped image, due to standard compression applied to JPEG images.

1.1.6 Manipulating Images

Images can also be shrunk. These “shrink” functions trim an image by eliminating extraneous pixels.

1.1.7 Scenes

A scene is an image, but with the pinhole in the upper-left corner, i.e. an image where pinhole-x and pinhole-y both return 0.

Scenes are particularly useful with the 2htdp/universe and htdp/world teachpacks, since it displays only scenes in its canvas.

1.1.8 Miscellaneous Image Manipulation and Creation

The last group of functions extracts the constituent colors from an image and converts a list of colors into an image.

The remaining functions provide alpha-channel information as well. Alpha channels are a measure of transparency; 0 indicates fully opaque and 255 indicates fully transparent.