Module Vg

module Vg: sig .. end
Declarative 2D vector graphics.

Vg is a declarative 2D vector graphics library. In Vg, images are values that denote functions mapping points of the cartesian plane to colors. The library provides combinators to define and compose them. Renderers for PDF, SVG and the HTML canvas are distributed with the library. An API allows to implement new renderers.

Consult the basics, the semantics and examples.

Open the module to use it, this defines only modules and types in your scope and a single composition operator.

Release 0.8.1 — Daniel Bünzli <daniel.buenzl>


module Font: sig .. end
type font = Font.t 
The type for fonts.
type glyph = int 
The type for glyphs. The integer represents a glyph identifier in a backend dependent font format.

Paths and images

type path 
The type for paths.
type image 
The type for images.
val (>>) : 'a -> ('a -> 'b) -> 'b
x >> f is f x, associates to left. Used to build paths and compose images.
module P: sig .. end
module I: sig .. end

Image renderers

type renderer 
The type for image renderers.
module Vgr: sig .. end
Image renderers.


Vg is designed to be opened in your module. This defines only types and modules in your scope and a single value, the composition operator Vg.(>>). Thus to use Vg start with :

open Gg
open Vg
Gg gives us types for points (Gg.p2), vectors (Gg.v2), 2D extents (Gg.size2), rectangles (Gg.box2) and colors (Gg.color). Later you may want to read Gg's documentation basics but for now it is sufficient to know that each of these types has a constructor v in a module named after the capitalized type name (Gg.P2.v, Gg.V2.v, etc.).

A collage model

Usual vector graphics libraries follow a painter model in which paths are filled, stroked and blended on top of each other to produce a final image. Vg departs from that, it has a collage model in which paths define 2D areas in infinite images that are cut to define new infinite images to be blended on top of each other.

The collage model maps very well to a declarative imaging model. It is also very clear from a specification point of view, both mathematically and metaphorically. This cannot be said from the painter model where the semantics of an operation like stroking a self-intersecting translucent path —  which usually applies the paint only once —  doesn't directly map to the underlying paint stroke metaphor. The collage model is also more economical from a conceptual point view since image cuts and blends naturally unify the distinct concepts of clipping paths, path strokes, path fills and compositing groups (unsupported for now in Vg) of the painter model.

The collage model introduced in the following sections was stolen and adapted from the following works.

Infinite images

Images in Vg are immutable and abstract value of type Vg.image. Conceptually, images are seen as functions mapping points of the infinite 2D plane to colors:

type Vg.image ≈  Gg.p2 -> Gg.color

The simplest image is a constant image: an image that associates the same color to every point in the plane. For a constant gray of intensity 0.5 this would be expressed by the function:

fun _ -> Color.gray 0.5
In Vg the combinator Vg.I.const represents constant infinite images and the above function is written:
let gray = I.const (Color.gray 0.5)
The module Vg.I contains all the combinators to define and compose infinite images, we will explore some of them later. But for now let's just render that fascinating image.


An infinite image alone cannot be rendered. We need a finite view rectangle and a specification of that view's physical size on the render target. These informations are coupled together with an image to form a Vg.Vgr.renderable.

Renderables can be given to a renderer for display via the function Vg.Vgr.render. Renderers are created with Vg.Vgr.create and need a render target value that defines the concrete renderer implementation used (PDF, SVG, HTML canvas etc.).

The following function outputs the unit square of gray on a 30x30 millimeters SVG target in the file /tmp/vg-basics.svg:

let svg_of_usquare i =
  let size = Size2.v 30. 30. in
  let view = Box2.unit in
    let oc = open_out "/tmp/vg-basics.svg" in
    let r = Vgr.create ( ()) (`Channel oc) in
      ignore (Vgr.render r (`Image (size, view, i)));
      ignore (Vgr.render r `End);
      close_out oc
    with e -> close_out oc; raise e
  with Sys_error e -> prerr_endline e

let () = svg_of_usquare gray
The result should be an SVG image with a gray square like this:

Coordinate space

Vg's cartesian coordinate space has its origin at the bottom left with the x-axis pointing right, the y-axis pointing up. It has no units, you define what they mean to you. However a renderable implicitely defines a physical unit for Vg's coordinate space: the corners of the specified view rectangle are mapped on a rectangular area of the given physical size on the target.

Scissors and glue

Constant images can be boring. To make things more interesting Vg gives you scissors: the Vg.I.cut combinator.

This combinator takes a finite area of the plane defined by a path path (more on paths later) and a source image img to define the image I.cut path img that has the color of the source image in the area defined by the path and the invisible transparent black color (Gg.Color.void) everywhere else. In other words I.cut path img represents this function:

fun pt -> if inside path pt then img pt else Color.void
The following code cuts a circle of radius 0.4 centered in the unit square in the gray image defined before.
let circle = P.empty >> (P2.v 0.5 0.5) 0.4
let gray_circle = I.cut circle gray
Rendered by svg_of_usquare the result is:

Note that the background white color surrounding the circle does not belong to the image itself, it is the color of the webpage background against which the image is composited. Your eyes require a wavelength there and Gg.Color.void cannot provide it.

Vg.I.cut has an optional area argument of type Vg.P.area that determines how a path should be interpreted as an area of the plane. The default value is `Anz, which means that it uses the non-zero winding number rule and for circle that defines its interior.

But the circle path can also be seen as defining a thin outline area around the ideal mathematical circle of circle. This can be specified by using an outline area `O o. The value o of type Vg.P.outline defines various parameters that define the outline area; for example its width. The following code cuts the circle outline area of width 0.04 in an infinite black image.

let circle_outline =
  let area = `O { P.o with P.width = 0.04 } in
  let black = I.const in
  I.cut ~area circle black

Below is the result and again, the white you see here is in fact Gg.Color.void.

Vg.I.cut gives us scissors but to combine the results of cuts we need some glue: the Vg.I.blend combinator. This combinator takes two infinite images front and back and defines an image I.blend front back that has the colors of front alpha blended on top of those of back. I.blend front back represents this function:

let i' = fun pt -> Color.blend (front pt) (back pt)
If we blend circle_outline on top of gray_circle:
let dot = I.blend circle_outline gray_circle
We get:

The order of arguments in Vg.I.blend is defined so that images can be blended using the left-associative composition operator Vg.(>>). That is dot can also be written as follows:

let dot = gray_circle >> I.blend circle_outline

This means that with Vg.(>>) and Vg.I.blend left to right order in code maps to back to front image blending.

Transforming images

The combinators Vg.I.move, Vg.I.rot, Vg.I.scale, and allow to perform arbitrary affine transformations on an image. For example the image I.move v i is i but translated by the vector v, that is the following function:

fun pt -> img (V2.(pt - v))
The following example uses I.move. The function scatter_plot takes a list of points and returns a scatter plot of the points. First we define a dot around the origin, just a black circle of diameter pt_width. Second we define the function mark that given a point returns an image with dot at that point and blend_mark that blends a mark at a point on an image. Finally we blend all the marks toghether.
let scatter_plot pts pt_width =
  let dot =
    let circle = P.empty >> P2.o (0.5 *. pt_width) in
    I.const >> I.cut circle
  let mark pt = dot >> I.move pt in
  let blend_mark acc pt = acc >> I.blend (mark pt) in
  List.fold_left blend_mark I.void pts
Note that dot is defined outside mark, this means that all marks share the same dot, doing so allows renderers to perform space and time optimizations. For example the SVG renderer will output a single circle path shared by all marks.

Here's the result of scatter_point on 800 points with coordinates on independent normal distributions.


Paths are used to define areas of the plane. A path is an immutable value of type Vg.path which is a list of disconnected subpaths. A subpath is a list of directed and connected curved segments.

To build a path you start with the empty path Vg.P.empty, give it to Vg.P.sub to start a new subpath and give the result to Vg.P.line, Vg.P.qcurve, Vg.P.ccurve, Vg.P.earc or Vg.P.close to add a new segment and so forth.

Path combinators take the path they act upon as the last argument so that the left-associative operator Vg.(>>) can be used to construct paths.

The image below is made by cutting the outline of the single path p defined hereafter.

let p =
  let rel = true in
  P.empty >>
  P.sub (P2.v 0.1 0.5) >>
    P.line (P2.v 0.3 0.5) >>
    P.qcurve ~rel (P2.v 0.2 0.5) (P2.v 0.2 0.0) >>
    P.ccurve ~rel (P2.v 0.0 (-. 0.5)) (P2.v 0.1 (-. 0.5)) (P2.v 0.3 0.0) >>
    P.earc ~rel (Size2.v 0.1 0.2) (P2.v 0.15 0.0) >>
  P.sub (P2.v 0.18 0.26) >>
    P.qcurve ~rel (P2.v (0.01) (-0.1)) (P2.v 0.1 (-. 0.05)) >>
    P.close >>
  P.sub (P2.v 0.65 0.8) >>
    P.line ~rel (P2.v 0.1 (-. 0.05))
let area = `O { P.o with P.width = 0.01 } in
I.const >> I.cut ~area p

Except for Vg.P.close which has no other argument but a path, the last point argument before the path argument is always the concrete end point of the segment. When true the optional rel argument indicates that the coordinates given to the constructor are expressed relative to end point of the last segment (or P2.o if there is no such segment).

Note that after a P.close or on the P.empty path, the call to Vg.P.sub can be omitted. In that case an implicit P.sub P2.o is introduced.

For more information about how paths are intepreted as areas, consult their semantics.

Remarks and tips


The following notations and definitions are used to give precise meaning to the images and the combinators.


The semantics of colors is the one ascribed to Gg.color: colors are in a linearized sRGBA space.

Color stops

A value of type Gg.Color.stops specifies a color at each point of the 1D unit space. It is defined by a list of pairs (ti, ci) where ti is a value from 0 to 1 and ci the corresponding color at that value. Colors at points between ti and ti+1 are linearly interpolated between ci and ci+1. If ti lies outside 0 to 1 or if ti-1 >= ti the semantics is undefined.

Given a stops value stops = [(t0, c0); (t1,c1); ... (tn, cn)] and any point t of 1D space, the semantic function:

[] : Gg.Color.stops -> float -> Gg.color

maps them to a color value written [stops]t as follows.


Values of type Vg.image represent maps from the infinite 2D euclidian space to colors. Given an image i and a point pt of the plane the semantic function

[]: image -> Gg.p2 -> Gg.color

maps them to a color value written [i]pt representing the image's color at this point.

Paths and areas

A value of type Vg.path is a list of subpaths. A subpath is a list of directed and connected curved segments. Subpaths are disconnected from each other and may (self-)intersect.

A path and a value of type Vg.P.area defines a finite area of the 2D euclidian space. Given an area specification a, a path p and a point pt, the semantic function:

[]: P.area -> path -> Gg.p2 -> bool

maps them to a boolean value written [a, p]pt that indicates whether pt belongs to the area or not.

The semantics of area rules is as follows:


Many examples of images and their source can be found in the online version of Vg's test image database. Clicking on the title of an image brings you to its definition.

The following examples show for each renderer the minimal code needed to output an image. This code can also be found in the test directory of the distribution.

Minimal PDF output

The file contains the following mostly self-explanatory code. We first define an image and then render it. For the latter step we define some meta-data for the image, a function to print rendering warnings and then render the image on stdout.

open Gg
open Vg

(* 1. Define your image *)

let aspect = 1.618
let size = Size2.v (aspect *. 100.) 100. (* mm *)
let view = Box2.v P2.o (Size2.v aspect 1.)
let image = I.const (Color.v_srgb 0.314 0.784 0.471)

(* 2. Render *)

let () =
  let title = "Vgr_pdf minimal example" in
  let description = "Emerald Color" in
  let xmp = Vgr.xmp ~title ~description () in
  let warn w = Vgr.pp_warning Format.err_formatter w in
  let r = Vgr.create ~warn ( ~xmp ()) (`Channel stdout) in
  ignore (Vgr.render r (`Image (size, view, image)));
  ignore (Vgr.render r `End)

This can be compiled with:

> ocamlfind ocamlopt -package gg,vg,vg.pdf \
                     -linkpkg -o min_pdf.native

Minimal code for SVG output

The file contains the following mostly self-explanatory code. We first define an image and then render it. For the latter step we define some meta-data for the image, a function to print rendering warnings and then render the image on stdout.

open Gg
open Vg

(* 1. Define your image *)

let aspect = 1.618
let size = Size2.v (aspect *. 100.) 100. (* mm *)
let view = Box2.v P2.o (Size2.v aspect 1.)
let image = I.const (Color.v_srgb 0.314 0.784 0.471)

(* 2. Render *)

let () =
  let title = "Vgr_svg minimal example" in
  let description = "Emerald Color" in
  let xmp = Vgr.xmp ~title ~description () in
  let warn w = Vgr.pp_warning Format.err_formatter w in
  let r = Vgr.create ~warn ( ~xmp ()) (`Channel stdout) in
  ignore (Vgr.render r (`Image (size, view, image)));
  ignore (Vgr.render r `End)

This can be compiled with:

> ocamlfind ocamlopt -package gg,vg,vg.svg \
                     -linkpkg -o min_svg.native

Minimal code for HTML canvas output

The file contains the following code. Step by step we have:

  1. Define an image.
  2. Create and add to the DOM an anchor a that will parent the canvas. This will allow to download a (usually PNG) file of the image.
  3. Create a canvas element c and add it as a child of a.
  4. Create a renderer r targeting the canvas c.
  5. Render the image.
  6. Ask the canvas for an image data URL and set it as the the link of the anchor.
open Gg
open Vg

(* 1. Define your image *)

let aspect = 1.618
let size = Size2.v (aspect *. 100.) 100. (* mm *)
let view = Box2.v P2.o (Size2.v aspect 1.)
let image = I.const (Color.v_srgb 0.314 0.784 0.471)

(* Browser bureaucracy. *)

let main _ =
  let d = Dom_html.window ## document in
  let a = (* 2 *)
    let a = Dom_html.createA d in
    a ## title <- Js.string "Download PNG file";
    a ## href <- Js.string "#";
    a ## setAttribute (Js.string "download"Js.string "min_htmlc.png");
    Dom.appendChild (d ## body) a; a
  let c = (* 3 *)
    let c = Dom_html.createCanvas d in
    Dom.appendChild a c; c
  let r = Vgr.create ( c) `Other in   (* 4 *)
  ignore (Vgr.render r (`Image (size, view, image))); (* 5 *)
  ignore (Vgr.render r `End);
  a ## href <- (c ## toDataURL ()); (* 6 *)

let () = Dom_html.window ## onload <- Dom_html.handler main

This file needs to be compiled to byte code and then js_of_ocaml must be applied. This can be achieved with:

> ocamlfind ocamlc \
  -package js_of_ocaml,js_of_ocaml.syntax \
  -package gg,vg,vg.htmlc \
  -syntax camlp4o -linkpkg -o min_htmlc.byte \
  && js_of_ocaml min_htmlc.byte

Finally we need a minimal HTML file that references our final javascript min_htmlc.js. The following one will do:

<!DOCTYPE html>
<html lang="en">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width,
  <script type="text/javascript" defer="defer" src="min_htmlc.js"></script>
  <style type="text/css"> body { background-color: black; margin: 3em; }</style>
  <title>Vgr_htmlc minimal example</title>
  <noscript>Sorry, you need to enable JavaScript to see this page.</noscript>