► |  | Teachpacks

1 | Ht DP Teachpacks
2 | Ht DP/ 2e Teachpacks

▼ | 2 | Ht DP/ 2e Teachpacks

2.1 | Batch Input/ Output: "batch-io.ss"
2.2 | Images: "image.ss"
2.3 | Worlds and the Universe: "universe.ss"
2.4 | Porting World Programs to Universe

► | 2.2 | Images: "image.ss"

2.2.1 | Basic Images
2.2.2 | Polygons
2.2.3 | Overlaying Images
2.2.4 | Placing Images & Scenes
2.2.5 | Rotating, Scaling, Flipping, Cropping, and Framing Images
2.2.6 | Bitmaps
2.2.7 | Image Properties
2.2.8 | Image Predicates
2.2.9 | Equality Testing of Images
2.2.10 | Pinholes
2.2.11 | The nitty gritty of pixels, pens, and lines
2.2.12 | Exporting Images to Disk

On this page:

2.2.1`` Basic Images
circle
ellipse
line
add-line
add-curve
text
text/ font
2.2.2`` Polygons
triangle
right-triangle
isosceles-triangle
triangle/ sss
triangle/ ass
triangle/ sas
triangle/ ssa
triangle/ aas
triangle/ asa
triangle/ saa
square
rectangle
rhombus
star
star-polygon
radial-star
regular-polygon
polygon
2.2.3`` Overlaying Images
overlay
overlay/ align
overlay/ xy
underlay
underlay/ align
underlay/ xy
beside
beside/ align
above
above/ align
2.2.4`` Placing Images & Scenes
empty-scene
place-image
place-image/ align
scene+ line
scene+ curve
2.2.5`` Rotating, Scaling, Flipping, Cropping, and Framing Images
rotate
scale
scale/ xy
flip-horizontal
flip-vertical
crop
frame
2.2.6`` Bitmaps
bitmap
image->color-list
color-list->bitmap
2.2.7`` Image Properties
image-width
image-height
image-baseline
2.2.8`` Image Predicates
image?
mode?
image-color?
color
y-place?
x-place?
angle?
side-count?
step-count?
pen
pen-style?
pen-cap?
pen-join?
2.2.9`` Equality Testing of Images
2.2.10`` Pinholes
center-pinhole
put-pinhole
pinhole-x
pinhole-y
clear-pinhole
overlay/ pinhole
underlay/ pinhole
2.2.11`` The nitty gritty of pixels, pens, and lines
2.2.12`` Exporting Images to Disk
save-image

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#### 2.2`` Images: "image.ss"

( require 2htdp/image )

The image teachpack provides a number of basic image construction functions, along with combinators for building more complex images out of existing images. Basic images include various polygons, ellipses and circles, and text, as well as bitmaps (typically bitmaps come about via the Insert Image... menu item in DrRacket). Existing images can be rotated, scaled, flipped, and overlaid on top of each other.

##### 2.2.1`` Basic Images

( circle radius mode color) → image?
radius : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?
( circle radius outline-mode pen-or-color) → image?
radius : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a circle with the given radius, height, mode, and color.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( circle 30 "outline" "red")
> ( circle 20 "solid" "blue")

( ellipse width height mode color) → image?
width : ( and/c real? ( not/c negative? ))
height : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?
( ellipse width height mode pen-or-color) → image?
width : ( and/c real? ( not/c negative? ))
height : ( and/c real? ( not/c negative? ))
mode : ( or/c 'outline "outline")
pen-or-color : ( or/c image-color? pen? )

Constructs an ellipsis with the given width, height, mode, and color.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( ellipse 40 20 "outline" "black")
> ( ellipse 20 40 "solid" "blue")

( line x1 y1 pen-or-color) → image?
x1 : real?
y1 : real?
pen-or-color : ( or/c pen? image-color? )

Constructs an image representing a line segment that connects the points (0,0) to (x1,y1).

Examples:
> ( line 30 30 "black")
> ( line -30 20 "red")
> ( line 30 -20 "red")

( add-line image x1 y1 x2 y2 pen-or-color) → image?
image : image?
x1 : real?
y1 : real?
x2 : real?
y2 : real?
pen-or-color : ( or/c pen? image-color? )

Adds a line to the image image, starting from the point (x1,y1) and going to the point (x2,y2). Unlike scene+line, if the line passes outside of image, the image gets larger to accomodate the line.

Examples:

> ( add-line ( ellipse 40 40 "outline" "maroon")
0 40 40 0 "maroon")

> ( add-line ( rectangle 40 40 "solid" "gray")
-10 50 50 -10 "maroon")

> ( add-line
( rectangle 100 100 "solid" "darkolivegreen")
25 25 75 75
( make-pen "goldenrod" 30 "solid" "round" "round"))

( add-curve |  | image |  |  |  |
|  | x1 |  |  |  |
|  | y1 |  |  |  |
|  | angle1 |  |  |  |
|  | pull1 |  |  |  |
|  | x2 |  |  |  |
|  | y2 |  |  |  |
|  | angle2 |  |  |  |
|  | pull2 |  |  |  |
|  | pen-or-color) |  | → |  | image?

image : image?
x1 : real?
y1 : real?
angle1 : angle?
pull1 : real?
x2 : real?
y2 : real?
angle2 : angle?
pull2 : real?
pen-or-color : ( or/c pen? image-color? )

Adds a curve to image, starting at the point (x1,y1), and ending at the point (x2,y2).

The angle1 and angle2 arguments specify the angle that the curve has as it leaves the initial point and as it reaches the final point, respectively.

The pull1 and pull2 arguments control how long the curve tries to stay with that angle. Larger numbers mean that the curve stays with the angle longer.

Unlike scene+curve, if the line passes outside of image, the image gets larger to accomodate the curve.

Examples:

> ( add-curve ( rectangle 100 100 "solid" "black")
20 20 0 1/3
80 80 0 1/3
"white")

> ( add-curve ( rectangle 100 100 "solid" "black")
20 20 0 1
80 80 0 1
"white")

> ( add-curve
( add-curve
( rectangle 40 100 "solid" "black")
20 10 180 1/2
20 90 180 1/2
( make-pen "white" 4 "solid" "round" "round"))
20 10 0 1/2
20 90 0 1/2
( make-pen "white" 4 "solid" "round" "round"))

> ( add-curve ( rectangle 100 100 "solid" "black")
-20 -20 0 1
120 120 0 1
"red")

( text string font-size color) → image?
string : string?
font-size : ( and/c integer? (<=/c 1 255))
color : image-color?

Constructs an image that draws the given string, using the font size and color.

Examples:
> ( text "Hello" 24 "olive")
> ( text "Goodbye" 36 "indigo")

( text/font |  | string |  |  |  |
|  | font-size |  |  |  |
|  | color |  |  |  |
|  | face |  |  |  |
|  | family |  |  |  |
|  | style |  |  |  |
|  | weight |  |  |  |
|  | underline?) |  | → |  | image?

string : string?
font-size : ( and/c integer? (<=/c 1 255))
color : image-color?
face : ( or/c string? #f)
family : ( or/c 'default 'decorative 'roman 'script 'swiss 'modern 'symbol 'system)
style : ( or/c 'normal 'italic 'slant)
weight : ( or/c 'normal 'bold 'light)
underline? : any/c

Constructs an image that draws the given string, using a complete font specification.

The face and the family combine to give the complete typeface. If face is available on the system, it is used, but if not then a default typeface based on the family is chosen. The style controls if the face is italic or not (under Windows and Mac OS X, 'slant and 'italic are the same), the weight controls if it is boldface (or light), and underline? determines if the face is underlined. For more details on these arguments, see font%, which ultimately is what this code uses to draw the font.

Examples:

> ( text/font "Hello" 24 "olive"
"Gill Sans" 'swiss 'normal 'bold #f)

> ( text/font "Goodbye" 18 "indigo"
#f 'modern 'italic 'normal #f)

> ( text/font "not really a link" 18 "blue"
#f 'roman 'normal 'normal #t)

##### 2.2.2`` Polygons

( triangle side-length mode color) → image?
side-length : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( triangle |  | side-length |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a upward-pointing equilateral triangle. The side-length argument determines the length of the side of the triangle.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Example:
> ( triangle 40 "solid" "tan")

( right-triangle |  | side-length1 |  |  |  |
|  | side-length2 |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length1 : ( and/c real? ( not/c negative? ))
side-length2 : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( right-triangle |  | side-length1 |  |  |  |
|  | side-length2 |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length1 : ( and/c real? ( not/c negative? ))
side-length2 : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a triangle with a right angle where the two sides adjacent to the right angle have lengths side-length1 and side-length2.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Example:
> ( right-triangle 36 48 "solid" "black")

( isosceles-triangle |  | side-length |  |  |  |
|  | angle |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
angle : angle?
mode : mode?
color : image-color?

( isosceles-triangle |  | side-length |  |  |  |
|  | angle |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
angle : angle?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle with two equal-length sides, of length side-length where the angle between those sides is angle. The third leg is straight, horizontally. If the angle is less than 180, then the triangle will point up and if the angle is more, then the triangle will point down.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( isosceles-triangle 200 170 "solid" "seagreen")
> ( isosceles-triangle 60 30 "solid" "aquamarine")
> ( isosceles-triangle 60 330 "solid" "lightseagreen")

To create a general triangle given known sides and angles the following family of functions are useful: triangle/sss, triangle/ass, triangle/sas, triangle/ssa, triangle/sss, triangle/sss, and, triangle/sss. They all construct a triangle oriented as follows:

( triangle/sss |  | side-length-a |  |  |  |
|  | side-length-b |  |  |  |
|  | side-length-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
side-length-b : ( and/c real? ( not/c negative? ))
side-length-c : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( triangle/sss |  | side-length-a |  |  |  |
|  | side-length-b |  |  |  |
|  | side-length-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
side-length-b : ( and/c real? ( not/c negative? ))
side-length-c : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the side lengths a, b, and, c are given by side-length-a, side-length-b, and, side-length-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/sss 40 60 80 "solid" "seagreen")
> ( triangle/sss 80 40 60 "solid" "aquamarine")
> ( triangle/sss 80 80 40 "solid" "lightseagreen")

( triangle/ass |  | angle-a |  |  |  |
|  | side-length-b |  |  |  |
|  | side-length-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

angle-a : angle?
side-length-b : ( and/c real? ( not/c negative? ))
side-length-c : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( triangle/ass |  | angle-a |  |  |  |
|  | side-length-b |  |  |  |
|  | side-length-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

angle-a : angle?
side-length-b : ( and/c real? ( not/c negative? ))
side-length-c : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the angle A and side length a and b, are given by angle-a, side-length-b, and, side-length-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/ass 10 60 100 "solid" "seagreen")
> ( triangle/ass 90 60 100 "solid" "aquamarine")
> ( triangle/ass 130 60 100 "solid" "lightseagreen")

( triangle/sas |  | side-length-a |  |  |  |
|  | angle-b |  |  |  |
|  | side-length-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
angle-b : angle?
side-length-c : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( triangle/sas |  | side-length-a |  |  |  |
|  | angle-b |  |  |  |
|  | side-length-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
angle-b : angle?
side-length-c : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the side length a, angle B, and, side length c given by side-length-a, angle-b, and, side-length-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/sas 60 10 100 "solid" "seagreen")
> ( triangle/sas 60 90 100 "solid" "aquamarine")
> ( triangle/sas 60 130 100 "solid" "lightseagreen")

( triangle/ssa |  | side-length-a |  |  |  |
|  | side-length-b |  |  |  |
|  | angle-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
side-length-b : ( and/c real? ( not/c negative? ))
angle-c : angle?
mode : mode?
color : image-color?

( triangle/ssa |  | side-length-a |  |  |  |
|  | side-length-b |  |  |  |
|  | angle-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
side-length-b : ( and/c real? ( not/c negative? ))
angle-c : angle?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the side length a, side length b, and, angle c given by side-length-a, side-length-b, and, angle-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/ssa 60 100 10 "solid" "seagreen")
> ( triangle/ssa 60 100 90 "solid" "aquamarine")
> ( triangle/ssa 60 100 130 "solid" "lightseagreen")

( triangle/aas |  | angle-a |  |  |  |
|  | angle-b |  |  |  |
|  | side-length-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

angle-a : angle?
angle-b : angle?
side-length-c : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( triangle/aas |  | angle-a |  |  |  |
|  | angle-b |  |  |  |
|  | side-length-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

angle-a : angle?
angle-b : angle?
side-length-c : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the angle A, angle B, and, side length c given by angle-a, angle-b, and, side-length-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/aas 10 40 200 "solid" "seagreen")
> ( triangle/aas 90 40 200 "solid" "aquamarine")
> ( triangle/aas 130 40 40 "solid" "lightseagreen")

( triangle/asa |  | angle-a |  |  |  |
|  | side-length-b |  |  |  |
|  | angle-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

angle-a : angle?
side-length-b : ( and/c real? ( not/c negative? ))
angle-c : angle?
mode : mode?
color : image-color?

( triangle/asa |  | angle-a |  |  |  |
|  | side-length-b |  |  |  |
|  | angle-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

angle-a : angle?
side-length-b : ( and/c real? ( not/c negative? ))
angle-c : angle?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the angle A, side length b, and, angle C given by angle-a, side-length-b, and, angle-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/asa 10 200 40 "solid" "seagreen")
> ( triangle/asa 90 200 40 "solid" "aquamarine")
> ( triangle/asa 130 40 40 "solid" "lightseagreen")

( triangle/saa |  | side-length-a |  |  |  |
|  | angle-b |  |  |  |
|  | angle-c |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
angle-b : angle?
angle-c : angle?
mode : mode?
color : image-color?

( triangle/saa |  | side-length-a |  |  |  |
|  | angle-b |  |  |  |
|  | angle-c |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length-a : ( and/c real? ( not/c negative? ))
angle-b : angle?
angle-c : angle?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Creates a triangle where the side length a, angle B, and, angle C given by side-length-a, angle-b, and, angle-c respectively.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( triangle/saa 200 10 40 "solid" "seagreen")
> ( triangle/saa 200 90 40 "solid" "aquamarine")
> ( triangle/saa 40 130 40 "solid" "lightseagreen")

( square side-len mode color) → image?
side-len : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?
( square side-len outline-mode pen-or-color) → image?
side-len : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a square.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( square 40 "solid" "slateblue")
> ( square 50 "outline" "darkmagenta")

( rectangle width height mode color) → image?
width : real?
height : real?
mode : mode?
color : image-color?

( rectangle |  | width |  |  |  |
|  | height |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

width : real?
height : real?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a rectangle with the given width, height, mode, and color.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( rectangle 40 20 "outline" "black")
> ( rectangle 20 40 "solid" "blue")

( rhombus side-length angle mode color) → image?
side-length : ( and/c real? ( not/c negative? ))
angle : angle?
mode : mode?
color : image-color?

( rhombus |  | side-length |  |  |  |
|  | angle |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
angle : angle?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a four sided polygon with all equal sides and thus where opposite angles are equal to each other. The top and bottom pair of angles is angle and the left and right are ( - 180 angle).

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( rhombus 40 45 "solid" "magenta")
> ( rhombus 80 150 "solid" "mediumpurple")

( star side-length mode color) → image?
side-length : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?
( star side-length outline-mode color) → image?
side-length : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
color : ( or/c pen? image-color? )

Constructs a star with five points. The side-length argument determines the side length of the enclosing pentagon.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Example:
> ( star 40 "solid" "gray")

( star-polygon |  | side-length |  |  |  |
|  | side-count |  |  |  |
|  | step-count |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
side-count : side-count?
step-count : step-count?
mode : mode?
color : image-color?

( star-polygon |  | side-length |  |  |  |
|  | side-count |  |  |  |
|  | step-count |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
side-count : side-count?
step-count : step-count?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs an arbitrary regular star polygon (a generalization of the regular polygons). The polygon is enclosed by a regular polygon with side-count sides each side-length long. The polygon is actually constructed by going from vertex to vertex around the regular polgon, but skipping over every step-count vertices.

For examples, if side-count is 5 and step-count is 2, then this function produces a shape just like star.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( star-polygon 40 5 2 "solid" "seagreen")
> ( star-polygon 40 7 3 "outline" "darkred")
> ( star-polygon 20 10 3 "solid" "cornflowerblue")

( radial-star |  | point-count |  |  |  |
|  | inner-radius |  |  |  |
|  | outer-radius |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

point-count : ( and/c integer? (>=/c 2))
inner-radius : ( and/c real? ( not/c negative? ))
outer-radius : ( and/c real? ( not/c negative? ))
mode : mode?
color : image-color?

( radial-star |  | point-count |  |  |  |
|  | inner-radius |  |  |  |
|  | outer-radius |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

point-count : ( and/c integer? (>=/c 2))
inner-radius : ( and/c real? ( not/c negative? ))
outer-radius : ( and/c real? ( not/c negative? ))
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a star-like polygon where the star is specified by two radii and a number of points. The first radius determines where the points begin, the second determines where they end, and the point-count argument determines how many points the star has.

Examples:
> ( radial-star 8 8 64 "solid" "darkslategray")
> ( radial-star 32 30 40 "outline" "black")

( regular-polygon |  | side-length |  |  |  |
|  | side-count |  |  |  |
|  | mode |  |  |  |
|  | color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
side-count : side-count?
mode : mode?
color : image-color?

( regular-polygon |  | side-length |  |  |  |
|  | side-count |  |  |  |
|  | outline-mode |  |  |  |
|  | pen-or-color) |  | → |  | image?

side-length : ( and/c real? ( not/c negative? ))
side-count : side-count?
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a regular polygon with side-count sides.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:
> ( regular-polygon 50 3 "outline" "red")
> ( regular-polygon 40 4 "outline" "blue")
> ( regular-polygon 20 8 "solid" "red")

( polygon vertices mode color) → image?
vertices : (listof posn? )
mode : mode?
color : image-color?
( polygon vertices outline-mode pen-or-color) → image?
vertices : (listof posn? )
outline-mode : ( or/c 'outline "outline")
pen-or-color : ( or/c pen? image-color? )

Constructs a polygon connecting the given vertices.

If the mode is 'outline or "outline", then the last argument can be a pen struct or an image-color?, but if the mode is 'solid or "solid", then the last argument must be an image-color?.

Examples:

> ( polygon ( list ( make-posn 0 0)
( make-posn -10 20)
( make-posn 60 0)
( make-posn -10 -20))
"solid"
"burlywood")

> ( polygon ( list ( make-posn 0 0)
( make-posn 0 40)
( make-posn 20 40)
( make-posn 20 60)
( make-posn 40 60)
( make-posn 40 20)
( make-posn 20 20)
( make-posn 20 0))
"solid"
"plum")

> ( underlay
( rectangle 80 80 "solid" "mediumseagreen")
( polygon
( list ( make-posn 0 0)
( make-posn 50 0)
( make-posn 0 50)
( make-posn 50 50))
"outline"
( make-pen "darkslategray" 10 "solid" "round" "round")))

> ( underlay
( rectangle 90 80 "solid" "mediumseagreen")
( polygon
( list ( make-posn 0 0)
( make-posn 50 0)
( make-posn 0 50)
( make-posn 50 50))
"outline"
( make-pen "darkslategray" 10 "solid" "projecting" "miter")))

##### 2.2.3`` Overlaying Images

( overlay i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Overlays all of its arguments building a single image. The first argument goes on top of the second argument, which goes on top of the third argument, etc. The images are all lined up on their centers.

Examples:

> ( overlay ( rectangle 30 60 "solid" "orange")
( ellipse 60 30 "solid" "purple"))

> ( overlay ( ellipse 10 10 "solid" "red")
( ellipse 20 20 "solid" "black")
( ellipse 30 30 "solid" "red")
( ellipse 40 40 "solid" "black")
( ellipse 50 50 "solid" "red")
( ellipse 60 60 "solid" "black"))

> ( overlay ( regular-polygon 20 5 "solid" ( make-color 50 50 255))
( regular-polygon 26 5 "solid" ( make-color 100 100 255))
( regular-polygon 32 5 "solid" ( make-color 150 150 255))
( regular-polygon 38 5 "solid" ( make-color 200 200 255))
( regular-polygon 44 5 "solid" ( make-color 250 250 255)))

( overlay/align x-place y-place i1 i2 is ...) → image?
x-place : x-place?
y-place : y-place?
i1 : image?
i2 : image?
is : image?

Overlays all of its image arguments, much like the overlay function, but using x-place and y-place to determine where the images are lined up. For example, if x-place and y-place are both "middle", then the images are lined up on their centers.

Examples:

> ( overlay/align "left" "middle"
( rectangle 30 60 "solid" "orange")
( ellipse 60 30 "solid" "purple"))

> ( overlay/align "right" "bottom"
( rectangle 20 20 "solid" "silver")
( rectangle 30 30 "solid" "seagreen")
( rectangle 40 40 "solid" "silver")
( rectangle 50 50 "solid" "seagreen"))

( overlay/xy i1 x y i2) → image?
i1 : image?
x : real?
y : real?
i2 : image?

Constructs an image by overlaying i1 on top of i2 after shifting i2 over by x pixels to the right and y pixels down.

Examples:

> ( overlay/xy ( rectangle 20 20 "outline" "black")
20 0
( rectangle 20 20 "outline" "black"))

> ( overlay/xy ( rectangle 20 20 "solid" "red")
20 20
( rectangle 20 20 "solid" "black"))

> ( overlay/xy ( rectangle 20 20 "solid" "red")
-20 -20
( rectangle 20 20 "solid" "black"))

> ( overlay/xy
( overlay/xy ( ellipse 40 40 "outline" "black")
10
15
( ellipse 10 10 "solid" "forestgreen"))
20
15
( ellipse 10 10 "solid" "forestgreen"))

( underlay i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Underlays all of its arguments building a single image.

It behaves like overlay, but with the arguments in the reverse order. That is, the first argument goes underneath of the second argument, which goes underneath the third argument, etc. The images are all lined up on their centers.

Examples:

> ( underlay ( rectangle 30 60 "solid" "orange")
( ellipse 60 30 "solid" "purple"))

> ( underlay ( ellipse 10 60 "solid" "red")
( ellipse 20 50 "solid" "black")
( ellipse 30 40 "solid" "red")
( ellipse 40 30 "solid" "black")
( ellipse 50 20 "solid" "red")
( ellipse 60 10 "solid" "black"))

( underlay/align x-place y-place i1 i2 is ...) → image?
x-place : x-place?
y-place : y-place?
i1 : image?
i2 : image?
is : image?

Underlays all of its image arguments, much like the underlay function, but using x-place and y-place to determine where the images are lined up. For example, if x-place and y-place are both "middle", then the images are lined up on their centers.

Examples:

> ( underlay/align "left" "middle"
( rectangle 30 60 "solid" "orange")
( ellipse 60 30 "solid" "purple"))

> ( underlay/align "right" "top"
( rectangle 50 50 "solid" "seagreen")
( rectangle 40 40 "solid" "silver")
( rectangle 30 30 "solid" "seagreen")
( rectangle 20 20 "solid" "silver"))

( underlay/xy i1 x y i2) → image?
i1 : image?
x : real?
y : real?
i2 : image?

Constructs an image by underlaying i1 underneath of i2 after shifting i2 over by x pixels to the right and y pixels down.

This is the same as ( overlay/xy i2 ( - x) ( - y) i1).

Examples:

> ( underlay/xy ( rectangle 20 20 "outline" "black")
20 0
( rectangle 20 20 "outline" "black"))

> ( underlay/xy ( rectangle 20 20 "solid" "red")
20 20
( rectangle 20 20 "solid" "black"))

> ( underlay/xy ( rectangle 20 20 "solid" "red")
-20 -20
( rectangle 20 20 "solid" "black"))

> ( underlay/xy
( underlay/xy ( ellipse 40 40 "solid" "gray")
10
15
( ellipse 10 10 "solid" "forestgreen"))
20
15
( ellipse 10 10 "solid" "forestgreen"))

( beside i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Constructs an image by placing all of the argument images in a horizontal row, aligned along their centers.

Example:

> ( beside ( ellipse 20 70 "solid" "gray")
( ellipse 20 50 "solid" "darkgray")
( ellipse 20 30 "solid" "dimgray")
( ellipse 20 10 "solid" "black"))

( beside/align y-place i1 i2 is ...) → image?
y-place : y-place?
i1 : image?
i2 : image?
is : image?

Constructs an image by placing all of the argument images in a horizontal row, lined up as indicated by the y-place argument. For example, if y-place is "middle", then the images are placed side by side with their centers lined up with each other.

Examples:

> ( beside/align "bottom"
( ellipse 20 70 "solid" "lightsteelblue")
( ellipse 20 50 "solid" "mediumslateblue")
( ellipse 20 30 "solid" "slateblue")
( ellipse 20 10 "solid" "navy"))

> ( beside/align "top"
( ellipse 20 70 "solid" "mediumorchid")
( ellipse 20 50 "solid" "darkorchid")
( ellipse 20 30 "solid" "purple")
( ellipse 20 10 "solid" "indigo"))

> ( beside/align "baseline"
( text "ijy" 18 "black")
( text "ijy" 24 "black"))

( above i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Constructs an image by placing all of the argument images in a vertical row, aligned along their centers.

Example:

> ( above ( ellipse 70 20 "solid" "gray")
( ellipse 50 20 "solid" "darkgray")
( ellipse 30 20 "solid" "dimgray")
( ellipse 10 20 "solid" "black"))

( above/align x-place i1 i2 is ...) → image?
x-place : x-place?
i1 : image?
i2 : image?
is : image?

Constructs an image by placing all of the argument images in a vertical row, lined up as indicated by the x-place argument. For example, if x-place is "middle", then the images are placed above each other with their centers lined up.

Examples:

> ( above/align "right"
( ellipse 70 20 "solid" "gold")
( ellipse 50 20 "solid" "goldenrod")
( ellipse 30 20 "solid" "darkgoldenrod")
( ellipse 10 20 "solid" "sienna"))

> ( above/align "left"
( ellipse 70 20 "solid" "yellowgreen")
( ellipse 50 20 "solid" "olivedrab")
( ellipse 30 20 "solid" "darkolivegreen")
( ellipse 10 20 "solid" "darkgreen"))

##### 2.2.4`` Placing Images & Scenes

Placing images into scenes is particularly useful when building worlds and universes using 2htdp/universe.

( empty-scene width height) → image?
width : ( and/c real? ( not/c negative? ))
height : ( and/c real? ( not/c negative? ))

Creates an empty scene, i.e., a rectangle with a black outline.

Example:
> ( empty-scene 160 90)

( place-image image x y scene) → image?
image : image?
x : real?
y : real?
scene : image?

Places image onto scene with its center at the coordinates (x,y) and crops the resulting image so that it has the same size as scene. The coordinates are relative to the top-left of scene.

Examples:

> ( place-image
( triangle 32 "solid" "red")
24 24
( rectangle 48 48 "solid" "gray"))

> ( place-image
( triangle 64 "solid" "red")
24 24
( rectangle 48 48 "solid" "gray"))

> ( place-image
( circle 4 "solid" "white")
18 20
( place-image
( circle 4 "solid" "white")
0 6
( place-image
( circle 4 "solid" "white")
14 2
( place-image
( circle 4 "solid" "white")
8 14
( rectangle 24 24 "solid" "goldenrod")))))

( place-image/align |  | image |  |  |  |
|  | x |  |  |  |
|  | y |  |  |  |
|  | x-place |  |  |  |
|  | y-place |  |  |  |
|  | scene) |  | → |  | image?

image : image?
x : real?
y : real?
x-place : x-place?
y-place : y-place?
scene : image?

Like place-image, but uses image’s x-place and y-place to anchor the image. Also, like place-image, place-image/align crops the resulting image so that it has the same size as scene.

Examples:

> ( place-image/align ( triangle 48 "solid" "yellowgreen")
64 64 "right" "bottom"
( rectangle 64 64 "solid" "mediumgoldenrod"))

> ( beside
( place-image/align ( circle 8 "solid" "tomato")
0 0 "center" "center"
( rectangle 32 32 "outline" "black"))
( place-image/align ( circle 8 "solid" "tomato")
8 8 "center" "center"
( rectangle 32 32 "outline" "black"))
( place-image/align ( circle 8 "solid" "tomato")
16 16 "center" "center"
( rectangle 32 32 "outline" "black"))
( place-image/align ( circle 8 "solid" "tomato")
24 24 "center" "center"
( rectangle 32 32 "outline" "black"))
( place-image/align ( circle 8 "solid" "tomato")
32 32 "center" "center"
( rectangle 32 32 "outline" "black")))

( scene+line image x1 y1 x2 y2 color) → image?
image : image?
x1 : real?
y1 : real?
x2 : real?
y2 : real?
color : image-color?

Adds a line to the image scene, starting from the point (x1,y1) and going to the point (x2,y2); unlike add-line, this function crops the resulting image to the size of scene.

Examples:

> ( scene+line ( ellipse 40 40 "outline" "maroon")
0 40 40 0 "maroon")

> ( scene+line ( rectangle 40 40 "solid" "gray")
-10 50 50 -10 "maroon")

> ( scene+line
( rectangle 100 100 "solid" "darkolivegreen")
25 25 100 100
( make-pen "goldenrod" 30 "solid" "round" "round"))

( scene+curve |  | scene |  |  |  |
|  | x1 |  |  |  |
|  | y1 |  |  |  |
|  | angle1 |  |  |  |
|  | pull1 |  |  |  |
|  | x2 |  |  |  |
|  | y2 |  |  |  |
|  | angle2 |  |  |  |
|  | pull2 |  |  |  |
|  | color) |  | → |  | image?

scene : image?
x1 : real?
y1 : real?
angle1 : angle?
pull1 : real?
x2 : real?
y2 : real?
angle2 : angle?
pull2 : real?
color : image-color?

Adds a curve to scene, starting at the point (x1,y1), and ending at the point (x2,y2).

The angle1 and angle2 arguments specify the angle that the curve has as it leaves the initial point and as it reaches the final point, respectively.

The pull1 and pull2 arguments control how long the curve tries to stay with that angle. Larger numbers mean that the curve stays with the angle longer.

Unlike add-curve, this function crops the curve, only showing the parts that fit onto scene.

Examples:

> ( scene+curve ( rectangle 100 100 "solid" "black")
20 20 0 1/3
80 80 0 1/3
"white")

> ( scene+curve ( rectangle 100 100 "solid" "black")
20 20 0 1
80 80 0 1
"white")

> ( scene+curve
( add-curve
( rectangle 40 100 "solid" "black")
20 10 180 1/2
20 90 180 1/2
"white")
20 10 0 1/2
20 90 0 1/2
"white")

> ( scene+curve ( rectangle 100 100 "solid" "black")
-20 -20 0 1
120 120 0 1
"red")

##### 2.2.5`` Rotating, Scaling, Flipping, Cropping, and Framing Images

( rotate angle image) → image?
angle : angle?
image : image?

Rotates image by angle degrees in a counter-clockwise direction.

Examples:
> ( rotate 45 ( ellipse 60 20 "solid" "olivedrab"))
> ( rotate 5 ( rectangle 50 50 "outline" "black"))

> ( rotate 45
( beside/align
"center"
( rectangle 40 20 "solid" "darkseagreen")
( rectangle 20 100 "solid" "darkseagreen")))

( scale factor image) → image?
factor : ( and/c real? positive? )
image : image?

Scales image by factor.

The pen sizes are also scaled and thus draw thicker (or thinner) lines than the original image, unless the pen was size 0. That pen size is treated specially to mean “the smallest available line” and thus it always draws a one pixel wide line; this is also the case for 'outline and "outline" shapes that are drawn with an image-color? instead of a pen.

Examples:
> ( scale 2 ( ellipse 20 30 "solid" "blue"))
> ( ellipse 40 60 "solid" "blue")

( scale/xy x-factor y-factor image) → image?
x-factor : ( and/c real? positive? )
y-factor : ( and/c real? positive? )
image : image?

Scales image by x-factor horizontally and by y-factor vertically.

Examples:

> ( scale/xy 3
2
( ellipse 20 30 "solid" "blue"))

> ( ellipse 60 60 "solid" "blue")

( flip-horizontal image) → image?
image : image?

Flips image left to right.

Flipping images with text is not supported (so passing flip-horizontal an image that contains a text or text/font image inside somewhere signals an error).

Example:

> ( beside
( rotate 30 ( square 50 "solid" "red"))
( flip-horizontal
( rotate 30 ( square 50 "solid" "blue"))))

( flip-vertical image) → image?
image : image?

Flips image top to bottom.

Flipping images with text is not supported (so passing flip-horizontal an image that contains a text or text/font image inside somewhere signals an error).

Example:

> ( above
( star 40 "solid" "firebrick")
( scale/xy 1 1/2 ( flip-vertical ( star 40 "solid" "gray"))))

( crop x y width height image) → image?
x : ( and/c real? (between/c 0 ( image-width image)))
y : ( and/c real? (between/c 0 ( image-height image)))
width : ( and/c real? ( not/c negative? ))
height : ( and/c real? ( not/c negative? ))
image : image?

Crops image to the rectangle with the upper left at the point (x,y) and with width and height.

The x and y arguments must be between 0 and the width or height, respectively.

Examples:
> ( crop 0 0 40 40 ( circle 40 "solid" "chocolate"))
> ( crop 40 60 40 60 ( ellipse 80 120 "solid" "dodgerblue"))

> ( above
( beside ( crop 40 40 40 40 ( circle 40 "solid" "palevioletred"))
( crop 0 40 40 40 ( circle 40 "solid" "lightcoral")))
( beside ( crop 40 0 40 40 ( circle 40 "solid" "lightcoral"))
( crop 0 0 40 40 ( circle 40 "solid" "palevioletred"))))

( frame image) → image?
image : image?

Returns an image just like image, except with a black, single pixel frame drawn around the bounding box of the image.

Example:
> ( frame ( ellipse 40 40 "solid" "gray"))

Generally speaking, this function is useful to debug image constructions, i.e., to see where certain sub-images appear within some larger image.

Example:

> ( beside
( ellipse 20 70 "solid" "lightsteelblue")
( frame ( ellipse 20 50 "solid" "mediumslateblue"))
( ellipse 20 30 "solid" "slateblue")
( ellipse 20 10 "solid" "navy"))

##### 2.2.6`` Bitmaps

DrRacket’s Insert Image ... menu item allows you to insert images into your program text, and those images are treated as images for this library.

Unlike all of the other images in this library, those images (and the other images created by functions in this section of the documentation) are represented as bitmaps, i.e., an array of colors (that can be quite large in some cases). This means that scaling and rotating them loses fidelity in the image and is significantly more expensive than with the other shapes.

( bitmap bitmap-spec)

bitmap-spec |  | = |  | rel-string
|  | | |  | id

Loads the bitmap specified by bitmap-spec. If bitmap-spec is a string, it is treated as a relative path. If it is an identifier, it is treated like a require spec and used to refer to a file in a collection.

Examples:
> ( bitmap icons/stop-16x16.png)
> ( bitmap icons/b-run.png)

( image->color-list image) → (listof color? )
image : image?

Returns a list of colors that correspond to the colors in the image, reading from left to right, top to bottom.

Examples:
> ( image->color-list ( rectangle 2 2 "solid" "black"))
(list (color 0 0 0) (color 0 0 0) (color 0 0 0) (color 0 0 0))

> ( image->color-list
( above ( beside ( rectangle 1 1 "solid" ( make-color 1 1 1))
( rectangle 1 1 "solid" ( make-color 2 2 2)))
( beside ( rectangle 1 1 "solid" ( make-color 3 3 3))
( rectangle 1 1 "solid" ( make-color 4 4 4)))))

(list (color 1 1 1) (color 2 2 2) (color 3 3 3) (color 4 4 4))

( color-list->bitmap colors width height) → image?
colors : (listof image-color? )
width : ( and/c real? ( not/c negative? ))
height : ( and/c real? ( not/c negative? ))

Constructs a bitmap from the given colors, with the given width and height.

Example:

> ( scale
40
( color-list->bitmap
( list "red" "green" "blue")
3 1))

##### 2.2.7`` Image Properties

( image-width i) → ( and/c integer? ( not/c negative? ) exact? )
i : image?

Returns the width of i.

Examples:
> ( image-width ( ellipse 30 40 "solid" "orange"))
30
> ( image-width ( circle 30 "solid" "orange"))
60

> ( image-width ( beside ( circle 20 "solid" "orange")
( circle 20 "solid" "purple")))

80
> ( image-width ( rectangle 0 10 "solid" "purple"))
0

( image-height i) → ( and/c integer? ( not/c negative? ) exact? )
i : image?

Returns the height of i.

Examples:
> ( image-height ( ellipse 30 40 "solid" "orange"))
40
> ( image-height ( circle 30 "solid" "orange"))
60

> ( image-height ( overlay ( circle 20 "solid" "orange")
( circle 30 "solid" "purple")))

60
> ( image-height ( rectangle 10 0 "solid" "purple"))
0

( image-baseline i) → ( and/c integer? ( not/c negative? ) exact? )
i : image?

Returns the distance from the top of the image to its baseline. Unless the image was constructed with text or text/font, this will be the same as its height.

Examples:
> ( image-baseline ( text "Hello" 24 "black"))
18
> ( image-height ( text "Hello" 24 "black"))
24
> ( image-baseline ( rectangle 100 100 "solid" "black"))
100
> ( image-height ( rectangle 100 100 "solid" "black"))
100

##### 2.2.8`` Image Predicates

This section lists predicates for the basic structures provided by the image library.

( image? x) → boolean?
x : any/c

Determines if x is an image. Images are returned by functions like ellipse and rectangle and accepted by functions like overlay and beside.

Additionally, images inserted into a DrRacket window are treated as bitmap images, as are instances of image-snip% and bitmap%.

( mode? x) → boolean?
x : any/c

Determines if x is a mode suitable for constructing images. It can be one of 'solid, "solid", 'outline, or "outline", indicating if the shape is filled in or not.

( image-color? x) → boolean?
x : any/c

Determines if x represents a color. Strings, symbols, and color structs are allowed as colors.

For example, "magenta", "black", 'orange, and 'purple are allowed. Colors are not case-sensitive, so "Magenta", "Black", 'Orange, and 'Purple are also allowed, and are the same colors as in the previous sentence. If a string or symbol color name is not recognized, black is used in its place.

The complete list of colors is available in the documentation for color-database<%>.

( 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))

The color struct defines a color with red, green, and blue components that range from 0 to 255.

( y-place? x) → boolean?
x : any/c

Determines if x is a placement option for the vertical direction. It can be one of "top", 'top, "bottom", 'bottom, "middle", 'middle, "center", 'center, "baseline", 'baseline, "pinhole", or 'pinhole.

The baseline of an image is the place where the bottoms any letters line up, not counting descenders, e.g. the tail on “y” or “g” or “j”.

Using "pinhole" or 'pinhole is only allowed when all of the image arguments have pinholes.

( x-place? x) → boolean?
x : any/c

Determines if x is a placement option for the horizontal direction. It can be one of "left", 'left, "right", 'right, "middle", 'middle, "center", 'center, "pinhole", or 'pinhole.

Using "pinhole" or 'pinhole is only allowed when all of the image arguments have pinholes.

( angle? x) → boolean?
x : any/c

Determines if x is an angle, namely a real number between 0 (inclusive) and 360 (exclusive).

( side-count? x) → boolean?
x : any/c

Determines if x is an integer greater than or equal to 3.

( step-count? x) → boolean?
x : any/c

Determines if x is an integer greater than or equal to 1.

( struct |  | pen (color width style cap join)
|  | #:extra-constructor-name make-pen )

color : image-color?
width : ( and/c real? (<=/c 0 255))
style : pen-style?
cap : pen-cap?
join : pen-join?

The pen struct specifies how the drawing library draws lines.

A good default for style is "solid", and good default values for the cap and join fields are "round".

Using 0 as a width is special; it means to always draw the smallest possible, but visible, pen. This means that the pen will always be one pixel in size, no matter how the image is scaled.

( pen-style? x) → boolean?
x : any/c

Determines if x is a valid pen style. It can be one of "solid", 'solid, "dot", 'dot, "long-dash", 'long-dash, "short-dash", 'short-dash, "dot-dash", or 'dot-dash.

( pen-cap? x) → boolean?
x : any/c

Determines if x is a valid pen cap. It can be one of "round", 'round, "projecting", 'projecting, "butt", or 'butt.

( pen-join? x) → boolean?
x : any/c

Determines if x is a valid pen join. It can be one of "round", 'round, "bevel", 'bevel, "miter", or 'miter.

##### 2.2.9`` Equality Testing of Images

Two images are equal if they draw exactly the same way at their current size (not neccessarily at all sizes) and, if there are pinholes, the pinholes are in the same place.

##### 2.2.10`` Pinholes

A pinhole is an optional property of an image that identifies a point somewhere in the image. The pinhole can then be used to facilitate overlaying images by lining them up on the their pinholes.

When an image has a pinhole, the pinhole is drawn with crosshairs on the image. The crosshairs are drawn with a two one pixel wide black lines (one horizontal and one vertical) and two one pixel wide white lines, where the black lines is drawn .5 pixels to the left and above the pinhole, and the white lines are drawn .5 pixels to the right and below the pinhole. Accordingly, when the pixel is on an integral coordinate, then black and white lines all take up a single pixel and in the center of their intersections is the actual pinholes. See The nitty gritty of pixels, pens, and lines for more details about pixels.

When images are overlay’d, underlay’d (or the variants of those functions), placed beside, or above each other, the pinhole of the resulting image is the pinhole of the first image argument passed to the combining operation. When images are combined with place-image (or the variants of place-image), then the scene argument’s pinhole is preserved.

( center-pinhole image) → image?
image : image?

Creates a pinhole in image at its center.

Examples:
> ( center-pinhole ( rectangle 40 20 "solid" "red"))
> ( rotate 30 ( center-pinhole ( rectangle 40 20 "solid" "orange")))

( put-pinhole x y image) → image?
x : integer?
y : integer?
image : image?

Creates a pinhole in image at the point (x,y).

Example:
> ( put-pinhole 2 18 ( rectangle 40 20 "solid" "forestgreen"))

( pinhole-x image) → ( or/c integer? #f)
image : image?

Returns the x coordinate of image’s pinhole.

Example:
> ( pinhole-x ( center-pinhole ( rectangle 10 10 "solid" "red")))
5

( pinhole-y image) → ( or/c integer? #f)
image : image?

Returns the y coordinate of image’s pinhole.

Example:
> ( pinhole-y ( center-pinhole ( rectangle 10 10 "solid" "red")))
5

( clear-pinhole image) → image?
image : image?

Removes a pinhole from image (if the image has a pinhole).

( overlay/pinhole i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Overlays all of the image arguments on their pinholes. If any of the arguments do not have pinholes, then the center of the image is used instead.

Examples:

> ( overlay/pinhole
( put-pinhole 25 10 ( ellipse 100 50 "solid" "red"))
( put-pinhole 75 40 ( ellipse 100 50 "solid" "blue")))

> (let ([petal ( put-pinhole
20 20
( ellipse 100 40 "solid" "purple"))])
( clear-pinhole
( overlay/pinhole
( circle 30 "solid" "yellow")
( rotate ( * 60 0) petal)
( rotate ( * 60 1) petal)
( rotate ( * 60 2) petal)
( rotate ( * 60 3) petal)
( rotate ( * 60 4) petal)
( rotate ( * 60 5) petal))))

( underlay/pinhole i1 i2 is ...) → image?
i1 : image?
i2 : image?
is : image?

Underlays all of the image arguments on their pinholes. If any of the arguments do not have pinholes, then the center of the image is used instead.

Examples:

> ( underlay/pinhole
( put-pinhole 25 10 ( ellipse 100 50 "solid" "red"))
( put-pinhole 75 40 ( ellipse 100 50 "solid" "blue")))

> (let* ([t ( triangle 40 "solid" "orange")]
[w ( image-width t)]
[h ( image-height t)])
( clear-pinhole
( overlay/pinhole
( put-pinhole ( / w 2) 0 t)
( put-pinhole w h t)
( put-pinhole 0 h t))))

##### 2.2.11`` The nitty gritty of pixels, pens, and lines

The image library treats coordinates as if they are in the upper-left corner of each pixel, and infinitesimally small (unlike pixels which have some area).

Thus, when drawing a solid square of whose side-length is 10, the image library colors in all of the pixels enclosed by the square starting at the upper left corner of (0,0) and going down to the upper left corner of (10,10), so the pixel whose upper left at (9,9) is colored in, but the pixel at (10,10) is not. All told, 100 pixels get colored in, just as expected for a square with a side length of 10.

When drawing lines, however, things get a bit more complex. Specifically, imagine drawing the outline of that rectangle. Since the border is between the pixels, there really isn’t a natural pixel to draw to indicate the border. Accordingly, when drawing an outline square (without a pen specification, but just a color as the last argument), the image library uses a pen whose width is 1 pixel, but draws a line centered at the point (0.5,0.5) that goes down and around to the point (10.5,10.5). This means that the outline slightly exceeds the bounding box of the shape. Specifically, the upper and left-hand lines around the square are within the bounding box, but the lower and right-hand lines are just outside.

This kind of rectangle is useful when putting rectangles next to each other and avoiding extra thick lines on the interior. For example, imagine building a grid like this:

Example:

> (let* ([s ( rectangle 20 20 "outline" "black")]
[r ( beside s s s s s s)])
( above r r r r r r))

The reason interior lines in this grid are the same thickness as the lines around the edge is because the rectangles overlap with each other. That is, the upper-left rectangle’s right edge is right on top of the next rectangle’s left edge.

The special case of adding 0.5 to each coordinate when drawing the square applies to all outline polygon-based shapes that just pass color, but does not apply when a pen is passed as the last argument to create the shape. For example, if using a pen of thickness 2 to draw a rectangle, we get a shape that has a border drawing the row of pixels just inside and just outside the shape. One might imagine that a pen of thickness 1 would draw an outline around the shape with a 1 pixel thick line, but this would require 1/2 of each pixel to be illuminated, something that is not possible. Instead, the same pixels are lit up as with the 2 pixel wide pen, but with only 1/2 of the intensity of the color. So a 1 pixel wide black pen object draws a 2 pixel wide outline, but in gray.

When combining pens and cropping, we can make a rectangle that has a line that is one pixel wide, but where the line is drawn entirely within the rectangle

Example:

> ( crop
0 0 20 20
( rectangle
20 20 "outline"
( make-pen "black" 2 "solid" "round" "round")))

and we can use that to build a grid now too, but this grid has doubled lines on the interior.

Example:

> (let* ([s ( crop
0 0 20 20
( rectangle
20 20 "outline"
( make-pen "black" 2 "solid" "round" "round")))]
[r ( beside s s s s s s)])
( above r r r r r r))

While this kind of rectangle is not useful for building grids, it is important to be able to build rectangles whose drawing does not exceed its bounding box. Specifically, this kind of drawing is used by frame and empty-scene so that the extra drawn pixels are not lost if the image is later clipped to its bounding box.

##### 2.2.12`` Exporting Images to Disk

In order to use an image as an input to another program (Photoshop, e.g., or a web browser), it is necessary to represent it in a format that these programs can understand. The save-image function provides this functionality, writing an image to disk using the PNG format. Since this format represents an image using a set of pixel values, an image written to disk generally contains less information than the image that was written, and cannot be scaled or manipulated as cleanly (by any image program).

( save-image image filename) → boolean?
image : image?
filename : path-string?

writes an image to the path specified by filename, using the PNG format.
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