►GUI: Racket Graphics Toolkit
►2 Reference
▼2.3 Drawing Classes
2.3.1 bitmap%
2.3.2 bitmap-dc%
2.3.3 brush%
2.3.4 brush-list%
2.3.5 color%
2.3.6 color-database<%>
2.3.7 dc<%>
2.3.8 dc-path%
2.3.9 font%
2.3.10 font-list%
2.3.11 font-name-directory<%>
2.3.12 gl-config%
2.3.13 gl-context<%>
2.3.14 pen%
2.3.15 pen-list%
2.3.16 point%
2.3.17 post-script-dc%
2.3.18 printer-dc%
2.3.19 ps-setup%
2.3.20 region%
On this page:
get-bounding-box
get-dc
in-region?
intersect
is-empty?
set-arc
set-ellipse
set-path
set-polygon
set-rectangle
set-rounded-rectangle
subtract
union
xor
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region% : class?

  superclass: object%

A region% object specifies a portion of a drawing area (possibly discontinuous). It is normally used for clipping drawing operations.

Each region% object is associated to a particular dc<%> object, specified when the region is created. A region can only be used with its associated dc<%> object. The origin and scale of a drawing context determine the bounding box and drawing location of a region at the time that a region is created (or set); a region is independent of the current scale and origin when the region is used. For an auto-scrolled canvas, the canvas’s current scrolling applies when the region is used (and it does not affect the region’s bounding box).

See also set-clipping-region in dc<%> and get-clipping-region in dc<%>.

(new region% [dc dc]) → (is-a?/c region%)
  dc : (is-a?/c dc<%>)
Creates an empty region.

(send a-region get-bounding-box) → 
real? real? real? real?
Returns a rectangle that encloses the region. The return values are the left, top, width, and height of the rectangle. The bounding box is precisely correct for unsmoothed drawing, but it is only approximate for smoothed drawing.

(send a-region get-dc) → (is-a?/c dc<%>)
Returns the region’s drawing context.

(send a-region in-region? x y) → boolean?
  x : real?
  y : real?
Returns #t if the given point is approximately within the region, #f otherwise. The given point is scaled and translated according to the region’s dc<%>’s current scale and translation.

The approximate in-region test represents the true result for unsmoothed drawing, but it not necessarily for smoothed drawing.

(send a-region intersect rgn) → void?
  rgn : (is-a?/c region%)
Sets the region to the intersection of itself with the given region.

The DC of rgn and a-region must be the same.

The result is always reliable for unsmoothed and smoothed drawing. For smoothed drawing, an intersect corresponds to clipping with this region’s path, and then clipping with the given region’s path. Further combining sends to this region correspond to combination with the original path before initial clip, and further combination with this region as an argument correspond to a combination with the given path after the initial clip. Thus, an intersecting region is a poor input for union, subtract, or xor, but it intersects properly in further calls to intersect.

(send a-region is-empty?) → boolean?
Returns #t if the region is approximately empty, #f otherwise. An approximately empty region is truly empty for unsmoothed drawing, but it may contain points for smoothed drawing.

(send a-region set-arc x    
  y    
  width    
  height    
  start-radians    
  end-radians) → void?
  x : real?
  y : real?
  width : (and/c real? (not/c negative?))
  height : (and/c real? (not/c negative?))
  start-radians : real?
  end-radians : real?
Sets the region to the interior of the specified wedge.

See also draw-ellipse in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the region corresponds to a clockwise path with a 'winding fill. The region is also atomic for the purposes of region combination.

(send a-region set-ellipse x y width height) → void?
  x : real?
  y : real?
  width : (and/c real? (not/c negative?))
  height : (and/c real? (not/c negative?))
Sets the region to the interior of the specified ellipse.

See also draw-ellipse in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the region corresponds to a clockwise path with a 'winding fill. The region is also atomic for the purposes of region combination.

Restrictions on the magnitude of drawing coordinates are described with dc<%>.

(send a-region set-path path    
  [xoffset    
  yoffset    
  fill-style]) → void?
  path : (is-a?/c dc-path%)
  xoffset : real? = 0
  yoffset : real? = 0
  fill-style : (one-of/c 'odd-even 'winding) = 'odd-even
Sets the region to the content of the given path.

See also draw-path in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the fill style affects how well the region reliably combines with other regions (via union, xor, and subtract). The region is also atomic for the purposes of region combination.

(send a-region set-polygon points    
  [xoffset    
  yoffset    
  fill-style]) → void?
  points : (listof (is-a?/c point%))
  xoffset : real? = 0
  yoffset : real? = 0
  fill-style : (one-of/c 'odd-even 'winding) = 'odd-even
Sets the region to the interior of the specified polygon.

See also draw-polygon in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the fill style affects how well the region reliably combines with other regions (via union, xor, and subtract). The region is also atomic for the purposes of region combination.

(send a-region set-rectangle x    
  y    
  width    
  height) → void?
  x : real?
  y : real?
  width : (and/c real? (not/c negative?))
  height : (and/c real? (not/c negative?))
Sets the region to the interior of the specified rectangle.

See also draw-rectangle in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the region corresponds to a clockwise path with a 'winding fill. The region is also atomic for the purposes of region combination.

Restrictions on the magnitude of drawing coordinates are described with dc<%>.

(send a-region set-rounded-rectangle x    
  y    
  width    
  height    
  [radius]) → void?
  x : real?
  y : real?
  width : (and/c real? (not/c negative?))
  height : (and/c real? (not/c negative?))
  radius : real? = -0.25
Sets the region to the interior of the specified rounded rectangle.

See also draw-rounded-rectangle in dc<%>, since the region content is determined the same way as brush-based filling in a dc<%>.

The result is reliable for both unsmoothed and smoothed drawing. For smoothed drawing, the region corresponds to a clockwise path with a 'winding fill. The region is also atomic for the purposes of region combination.

Restrictions on the magnitude of drawing coordinates are described with dc<%>.

(send a-region subtract rgn) → void?
  rgn : (is-a?/c region%)
Sets the region to the subtraction of itself minus the given region. In other words, a point is removed from the region if it is included in the given region. (The given region may contain points that are not in the current region; such points are ignored.)

This region’s DC and given region’s DC must be the same.

The result is always reliable for unsmoothed drawing. For smoothed drawing, the result is consistent across platforms and devices, but it is never a true subtraction. A subtraction corresponds to combining the sub-paths of this region with the reversed sub-paths of the given region, then intersecting the result with this region. This fails as a true subtraction, because the boundary of loops (with either 'odd-even or 'winding filling) is ambiguous.

(send a-region union rgn) → void?
  rgn : (is-a?/c region%)
Sets the region to the union of itself with the given region.

This region’s DC and given region’s DC must be the same.

The result is always reliable for unsmoothed drawing. For smoothed drawing, a union corresponds to combining the sub-paths of each region into one path, using an 'odd-even fill if either of the region uses an 'odd-even fill (otherwise using a 'winding fill). Consequently, while the result is consistent across platforms and devices, it is a true union only for certain input regions. For example, it is a true union for non-overlapping atomic and union regions. It is also a true union for atomic and union regions (potentially overlapping) that are all clockwise and use 'winding fill.

(send a-region xor rgn) → void?
  rgn : (is-a?/c region%)
Sets the region to the xoring of itself with the given region (i.e., contains points that are enclosed by exactly one of the two regions).

This region’s DC and given region’s DC must be the same.

The result is always reliable for unsmoothed drawing. For smoothed drawing, the result is consistent across platforms and devices, but it is not necessarily a true xoring. An xoring corresponds to combining the sub-paths of this region with the reversed sub-paths of the given region. The result uses an 'odd-even fill if either of the region uses an 'odd-even fill (otherwise using a 'winding fill). Consequently, the result is a reliable xoring only for certain input regions. For example, it is reliable for atomic and xoring regions that all use 'even-odd fill.

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