Teachpacks for How to Design Programs

# Images

### image.ss

This teachpack provides primitives for constructing and manipulating images. Basic images are created as outlines or solid shapes. Additional primitives allow for the composition of images.

Data definition:

    ;; Mode is one of the following two symbols or strings:
    ;; -- 'solid
    ;; -- 'outline
    ;; -- "solid"
    ;; -- "outline"

    ;; Interpretation: 'solid is used for creating solid basic
    ;; shapes; 'outline is used for creating outlines of basic
    ;; shapes. Strings are used in an analogous manner.

Data definition:

    (define-struct color (red blue green))
    ;; A CS is a structure: (make-color N N N)
    ;; where N is between 0 and 255.

    ;; Color is one of:
    ;; -- a color symbol, e.g., 'blue
    ;; -- a color string, e.g., "blue"
    ;; -- a CS, e.g., (make-color 0 0 255), which also denotes blue.

    ;; Interpretation: Color arguments are used to paint the shapes
    ;; or their outlines. See below for more information about color structs.

The following predicate precisely specifies what a valid image color is:
- `image-color? : anything -> boolean`
  to determine if the input is a valid image color
The first group of functions creates basic shapes (`Image`):
- `rectangle : Int Int Mode Color -> Image`
  to create a rectangle using the given width, height, mode, and color
- `circle : Int Mode Color -> Image`
  to create a circle using the given radius, mode, and color
- `ellipse : Int Int Mode Color -> Image`
  to create an ellipse using the given width, height, and color
- `triangle : Int Mode Color -> Image`
  to create an upward pointing equilateral triangle using the given edge size and color
- `line : Int Int Color -> Image`
  to create an image with a colored line from (0,0) to the point with the given coordinates
- `add-line : Image Int Int Int Int Color -> Image`
  to add a line to an existing image, drawn between the two given points
- `text : String Size Color -> Image`
  to create an image of the text in the given string, with the point size, and color specified by the last two arguments
Images have many properties. To understand how functions manipulate and create images, we need to understand one of these properties immediately: pinholes. Each image, including primitive shapes, come with a pinhole. Usually the pinhole is in the center of the shape except for those created from `line` and `text`, which have pinholes at the top left. When in doubt you can always find out where the pinhole is and even place it somewhere else:
- `pinhole-x` : Image -> Int
  to determine the x coordinate of the pinhole, measuring from the left of the image
- `pinhole-y : Image -> Int`
  to determine the y coordinate of the pinhole, measuring down from the top of the image
- `put-pinhole : Image Int Int -> Image`
  to put the pinhole in the location specified by the arguments, counting from the left and down from the top, respectively.
- `move-pinhole : Image Int Int -> Image`
  to move the pinhole down and to the right (by the specified amounts) of its current location. Use negative numbers to move it up or to the left.
The next group of functions build images from images:
- `overlay : Image Image Image ... -> Image`
  to add the pixels of the second Image onto the first image. The operation lines up the images via their pinholes.
- `overlay/xy : Image Int Int Image -> Image`
  to add the pixels of the second image onto the first image. Instead of lining up on the pinhole, the second image's pinhole is lined up with an offset from the first image's pinhole. The two coordinates specify how far down and to the right the offset should be. The pinhole of the resulting image is the same place as the pinhole in the first image.
For composite images, it is always possible to determine whether one occurs in the other and where:
- `image-inside? : Image Image -> Boolean`
  to determine whether the pixels of the second image appear in the first.

  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?` will not recognize the cropped image, due to standard compression applied to JPEG images.

  Use PNG images instead whenever possible.

- `find-image` : Image Image -> Posn
  to determine where the pixels of the second image appear in the first, with respect to the pinhole of the first image.
Two more properties of images are useful for image manipulations: their width and height. The two functions for extracting these properties are:
- `image-width : Image -> Int`
  to obtain an Image's width in pixels
- `image-height : Image -> Int`
  to obtain an image's height in pixels
Data definition:

    ;; List-of-color is one of:
    ;; -- empty
    ;; -- (cons Color List-of-color)

Interpretation: represents a sequence of colors It is possible to extract an image's colors and pixels and to create images from a list of colors:
- `image->color-list : Image -> List-of-color`
  to convert an image to a list of colors
- `color-list->image : List-of-color Nat Nat Nat Nat -> Image`
  to convert a list of colors to an image with the given width and height, and pinhole coordinates (the pinhole coordinates are with respect to the top-left of the image).
The shrink functions trim an image by eliminating extraneous pixels.
- `shrink-tl : Image Int Int -> Image`
  to shrink the image, starting from the top-left corner. The two numbers indicate how many pixels to save. The pinhole of the resulting image is in the middle of the image.
- `shrink-tr : Image Int Int -> Image`
  to shrink the image, starting from the top-right corner. The two numbers indicate how many pixels to save. The pinhole of the resulting image is in the middle of the image.
- `shrink-bl : Image Int Int -> Image`
  to shrink the image, starting from the bottom-left corner. The two numbers indicate how many pixels to save. The pinhole of the resulting image is in the middle of the image.
- `shrink-br : Image Int Int -> Image`
  to shrink the image, starting from the bottom-right corner. The two numbers indicate how many pixels to save. The pinhole of the resulting image is in the middle of the image.
- `shrink : Image Int Int Int Int -> Image`
  to shrink an image around its pinhole. The numbers are the pixels to save to left, above, to the right, and below the pinhole, respectively. The pixel directly on the pinhole is always saved.
The last group of functions extracts the consitiuent colors from an image and combine colors into an image, but the functions provide alpha-channel information as well. Alpha channels are a measure of transparency; 0 indicates fully opaque and 255 indicates fully transparent.
- `image->alpha-color-list : image -> list-of-alpha-color`
  to convert an image to a list of alpha colors
- `alpha-color-list->image : list-of-alpha-color int int int int -> image`
  to convert a list of alpha colors to an image with the given width and height, and pinhole coordinates (the pinhole coordinates are with respect to the top-left of the image).
- `make-alpha-color : int int int int -> color`
  to construct an alpha color
- `alpha-color? : anything -> boolean`
  to determine if its input is a color
- `alpha-color-alpha : color -> int`
  to extract the alpha value of a color
- `alpha-color-red : color -> int`
  to extract the red component of a color
- `alpha-color-green : color -> int`
  to extract the green component of a color
- `alpha-color-blue : color -> int`
  to extract the blue component of a color"