Data definition: 

;; {(idx 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. 

;; {(idx 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:
  • {(idx image-color?)} : anything -> boolean
    to determine if the input is a valid image color
    • The first group of functions creates basic shapes (Image):
  • {(idx rectangle)} : Int Int Mode Color -> Image
    to create a rectangle using the given width, height, mode, and color
  • {(idx circle)} : Int Mode Color -> Image
    to create a circle using the given radius, mode, and color
  • {(idx ellipse)} : Int Int Mode Color -> Image
    to create an ellipse using the given width, height, and color
  • {(idx triangle)} : Int Mode Color -> Image
    to create an upward pointing equilateral triangle using the given edge size and color
  • {(idx star)} : Int[>=2] Int[>=1] Int[>=1] Mode Color -> Image
    to create a multi-pointed star; the first number specifies the number of points, the second specifies the radius where the points begin and the third specifies the radius where they end.
  • {(idx line)} : Int Int Color -> Image
    to create an image with a colored line from (0,0) to the point with the given coordinates
  • {(idx add-line)} : Image Int Int Int Int Color -> Image
    to add a line to an existing image, drawn between the two given points
  • {(idx 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:
  • {(idx pinhole-x)} : Image -> Int
    to determine the x coordinate of the pinhole, measuring from the left of the image
  • {(idx pinhole-y)} : Image -> Int
    to determine the y coordinate of the pinhole, measuring down from the top of the image
  • {(idx 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.
  • {(idx 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:
  • {(idx 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.
  • {(idx 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:
  • {(idx 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.

  • {(idx 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:
  • {(idx image-width)} : Image -> Int
    to obtain an Image's width in pixels
  • {(idx 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:
  • {(idx image->color-list)} : Image -> List-of-color
    to convert an image to a list of colors
  • {(idx 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.
  • {(idx 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.
  • {(idx 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.
  • {(idx 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.
  • {(idx 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.
  • {(idx 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.
  • {(idx image->alpha-color-list)} : image -> list-of-alpha-color
    to convert an image to a list of alpha colors
  • {(idx 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).
  • {(idx make-alpha-color)} : int int int int -> color
    to construct an alpha color
  • {(idx alpha-color?)} : anything -> boolean
    to determine if its input is a color
  • {(idx alpha-color-alpha)} : color -> int
    to extract the alpha value of a color
  • {(idx alpha-color-red)} : color -> int
    to extract the red component of a color
  • {(idx alpha-color-green)} : color -> int
    to extract the green component of a color
  • {(idx alpha-color-blue)} : color -> int
    to extract the blue component of a color"