Rebol/AGG

AGG (Anti-Grain Geometry) extension for Rebol3 (version 3.14.1 and higher)
(This is still work in progress!)

Usage

agg: import agg
img: agg/draw 600x600 [pen red line-width 5 triangle 0x0 600x600 300x600]
save %temp.png img

Extension commands:

draw :image :commands

Draws scalable vector graphics to an image

• image [image! pair!]
• commands [block!]

AGG Draw Dialect Commands Reference

The AGG Draw dialect provides comprehensive 2D vector graphics capabilities through Rebol's block-based syntax. Each command configures state or immediately renders geometry with flexible coordinate input.

State Attributes

fill / fill-pen / fill-color

Sets fill color or toggles filling for subsequent path rendering. Supports solid colors and linear gradients.

Parameters:

Type	Description
tuple!	RGBA color value (e.g. 255.0.0.128)
logic!	true=enabled, false=disabled
linear	Linear gradient: tuple! number! (color, position)

Example:

fill 255.128.0.255    ; Semi-transparent orange fill
fill false            ; Disable filling

pen / line-color

Sets stroke color or toggles stroking for path outlines.

Parameters:

Type	Description
tuple!	RGBA stroke color
logic!	true=enabled, false=disabled

Example:

pen 0.0.255.255       ; Blue stroke
pen false             ; No outline

line-width

Sets stroke thickness for subsequent paths.

Parameters:

Type	Description
number!	Line thickness in pixels/world units

Example:

line-width 3.0

line-join

Controls corner join style between path segments.

Parameters:

Mode	Description
'miter	Sharp corners (extends until limit)
'bevel	Cut-off corners
'round	Circular corners
'miter-round	Miter with rounded truncation
'miter-revert	Miter with bevel fallback

line-cap

Defines appearance of path endpoints.

Parameters:

Style	Description
'butt	Flat end perpendicular to direction
'square	Square extension beyond endpoint
'round	Circular semicircle cap
'round-in	Inner circular cap
'triangle	Outward-pointing triangle
'triangle-in	Inward-pointing triangle
integer!	Raw cap enum value

alpha

Global transparency multiplier for all operations.

Parameters:

Type	Description
number!	0.0=transparent, 1.0=opaque

anti-alias

Controls anti-aliasing and gamma correction.

Parameters:

Type	Description
number!	Gamma value (0.5-4.0 typical)
logic!	true=normal (1.0), false=off
none!	Disable anti-aliasing

point-size

Size for subsequent point drawing commands.

Parameters:

Type	Description
number!	Point radius
pair!	Diameter/extent (non-uniform)

blend

Sets Porter-Duff/SVG compositing mode.

Modes: 'clear, 'source-copy, 'source-over, 'destination-over, 'source-in, 'destination-in, 'source-out, 'destination-out, 'source-atop, 'destination-atop, 'xor, 'add, 'multiply, 'screen, 'overlay, 'darken, 'lighten, 'color-dodge, 'color-burn, 'hard-light, 'soft-light, 'difference, 'exclusion

Basic Shapes

line

Draws open polyline (stroke only) from point sequence.

Parameters (coords):

Type	Description
pair! +	Sequential pairs (first=moveTo, rest=lineTo)
vector!	X/Y coordinates (2 values per point)
block!	Block of pair coordinates

Example:

line 10x10 50x20 80x40 120x10

polygon

Renders closed polygon from vertices.

Parameters (coords):

Type	Description
pair! 3+	3+ individual pairs
vector!	X/Y coordinates (2 per point)
block!	Block of pairs

triangle

Renders one or more triangles from vertex triplets.

Parameters (coords):

Type	Description
pair! 3+	Sequential triplets per triangle
vector!	Flattened: x1 y1 x2 y2 x3 y3...
block!	Block of 3-pair triplets

point

Draws points using current point-size.

Parameters (coords):

Type	Description
pair! +	Individual coordinates
vector!	X/Y pairs
block!	Block of pairs

box / rect

Draws rectangle (optionally rounded).

Parameters:

Param	Type	Description
top-left	pair!	Top-left corner
bottom-right/size	pair!	Opposite corner/size
radius-x	number!	Horizontal radius (opt)
radius-y	number!	Vertical radius (opt)

circle

Draws ellipse/circle by center and radii.

Parameters:

Param	Type	Description
center	pair!	Ellipse center
radius-x	number!	Horizontal radius
radius-y	number!	Vertical radius (opt)

ellipse

Ellipse from bounding rectangle.

Parameters:

Param	Type	Description
top-left	pair!	Bounding box corner
size	pair!	Width/height

arc

Elliptical arc with closure options.

Parameters:

Param	Type	Description
center	pair!	Ellipse center
radius	pair!	X/Y radii
begin	number!	Start angle (degrees)
sweep	number!	Sweep angle (degrees)
type	'open/'pie/'chord	Closure style (opt)

Curves

cubic

Cubic Bézier spline segments.

Parameters:

Param	Type	Description
start	pair!	Initial position (moveTo)
segments	pair! 3+	control1 control2 end (per segment)

Example:

cubic 10x10 30x50 70x50 90x10

Transformations

translate

Shifts coordinate origin.

Parameters: offset pair! (dx, dy)

scale

Scales coordinate system.

Parameters: number! (uniform) or pair! (sx, sy)

skew

Applies shear transformation.

Parameters: angles pair!/number!, optional pivot pair!

rotate

Rotates around center.

Parameters: angle number! (degrees), optional center pair!

transform

Compound affine transform.

Parameters: center pair! (opt), then rotate/scale-x/scale-y/translate

matrix

Full 2D affine matrix.

Parameters: vector!/block! (6 values: sx shy shx sy tx ty)

reset-matrix

Returns identity transform.

Clipping & Clearing

clip

Rectangular clipping region.

Parameters: top-left pair!, bottom-right pair!

clear-all

Clears entire surface to transparent black.

clear-clip

Clears clip region (optional fill color).

Parameters: tuple! color or unset! (transparent)

Scoping

push

State isolation context.

Parameters: block! of drawing commands

Example:

push [pen red line 10x10 100x100]
; Original pen restored

Text

font

Loads font resource.

Parameters: file! path or string! name

text

Renders text string.

Parameters:

Param	Type	Description
position	pair!	Baseline position
height	number!	Font height (opt)
content	string!	Text to render

Paths

shape

Renders complex path from sub-dialect.

Parameters: path-data block! with path commands (move, line, arc, curve, close, etc.)

Images

image

Blits image with optional scaling.

Parameters:

Param	Type	Description
img	image!	Source image
position	pair!	Destination top-left
scale	pair!/percent!	Size or scale factor (opt)

AGG Path Dialect Reference

The AGG Path DSL defines a sub-dialect for constructing complex vector paths within the shape command of the draw dialect. Paths are built sequentially from a current "pen" position, supporting absolute and relative coordinates for maximum flexibility.

Paths support filling, stroking, or both based on the outer draw state's fill/pen settings. Multiple subpaths can be defined before closing.

Path Movement

move

Moves the path pen to an absolute position (starts new subpath).

Parameters:

Type	Description
position	pair! XY coordinates

Example:

shape [move 10x10 ...]

move-by

Moves pen by relative offset (starts new subpath).

Parameters:

Type	Description
offset	pair! Delta (dx, dy)

Line Commands

line

Adds one or more absolute line segments from current position.

Parameters:

Type	Description
points	pair! + Absolute endpoint coordinates (batched)

Example:

shape [move 0x0 line 50x0 50x50 0x50]

line-by

Adds relative line segments.

Parameters:

Type	Description
deltas	pair! + Relative deltas from current position

hline

Horizontal line to absolute X (Y unchanged).

Parameters: x number!

hline-by

Horizontal relative delta X.

Parameters: dx number!

vline

Vertical line to absolute Y (X unchanged).

Parameters: y number!

vline-by

Vertical relative delta Y.

Parameters: dy number!

Arcs

arc

Adds elliptical arc (SVG-style parameters).

Parameters:

Param	Type	Description
end-point	pair!	Arc endpoint coordinates
rx	number!	X-axis radius
ry	number!	Y-axis radius
rotation	number!	Ellipse rotation (degrees)
sweep	logic!	true=positive sweep direction
large	logic!	true=large-arc (>180°)

arc-by

Relative endpoint version.

Parameters:

Param	Type	Description
end-delta	pair!	Relative endpoint delta

Cubic Bézier Curves

curve

Absolute cubic Bézier (C1, C2, end).

Parameters:

Param	Type	Description
control1	pair!	First control point
control2	pair!	Second control point
end-point	pair!	Endpoint coordinates

Example:

shape [move 10x10 curve 30x50 70x50 90x10]

curve-by

All-relative cubic Bézier.

Parameters: Relative control1, control2, end-delta pair!

curv

Smooth cubic (first control relative/reflected).

Parameters: control1 pair! (relative), end-point pair!

curv-by

Fully relative smooth cubic.

Parameters: control1 pair! (relative), end-delta pair!

Quadratic Bézier Curves

qcurve

Absolute quadratic Bézier (single control).

Parameters:

Param	Type	Description
control	pair!	Control point
end-point	pair!	Endpoint

qcurve-by

Relative quadratic.

Parameters: Relative control, end-delta pair!

qcurv

Smooth quadratic (reflected control).

Parameters: control pair! (relative/reflected)

qcurv-by

Relative smooth quadratic.

Parameters: end-delta pair!

Path Closure

close

Closes current subpath to its starting point with a straight line.

Parameters: None

Example (Closed Rectangle with Curve):

shape [
    move 10x10
    line 90x10 90x50 10x50
    curv 30x30 50x70
    close
]

Usage in Draw: Paths are consumed by shape in the main draw dialect, inheriting fill/pen state. All commands support batching multiple segments for efficiency.