This module provides efficient functions for rendering vector graphics to a number of formats, including EPS, PostScript, and PDF. It provides an abstraction for multi-page documents, as well as a set of graphics primitives for page descriptions.

The graphics model is similar to that of the PostScript and PDF languages, but we only implement a subset of their functionality. Care has been taken that graphics rendering is done efficiently and as lazily as possible; documents are rendered "on the fly", without the need to store the whole document in memory.

The provided document description model consists of two separate layers of abstraction:

• drawing is concerned with placing marks on a fixed surface, and takes place in the Draw monad;
• document structure is concerned with a sequence of pages, their bounding boxes, and other meta-data. It takes place in the Document monad.

Document description takes place in the Document monad. A basic multi-page document has the following structure:

document :: Document ()
document = do
  newpage x y $ do
    <<<drawing commands>>>
  newpage x y $ do
    <<<drawing commands>>>
  ...

Here, each newpage command describes one page of the document. The parameters x and y specify the dimensions of the page bounding box. They are expressed in units of PostScript points, i.e., multiples of 1/72 inch.

Sometimes the bounding box for a page is not known until after the page content has been generated. For this purpose, we also provide the following alternative to the newpage command:

  newpage_defer $ do
    <<<drawing commands>>>
    endpage x y

It works just like the newpage command, except that the bounding box is given at the end.

The description of the visible content of a page take place in the Draw monad. It takes the form of a sequence of drawing commands, for example:

    moveto 10 10
    lineto 10 100
    lineto 100 100
    lineto 100 10
    closepath
    stroke

The graphics model is similar to that of the PostScript and PDF languages. The basic concept is that of a path, which is a sequence of straight and curved line segments. Paths are first constructed using path construction commands, and then painted using painting commands, depending on a set of current graphics parameters and a current coordinate system.

We also provide block structure. Changes to the graphics state (color, coordinate system, etc.) that are done within a block are local to the block.

    block $ do
      <<drawing commands>>

During path construction, there is a notion of current path and current point. A path may consist of zero or more connected subpaths, and each subpath is either open or closed.

The painting commands rely on a set of graphics parameters. The graphics parameters are initially set to default values, and can be altered with the following commands.

Coordinates, lengths, widths, etc, are all interpreted relative to a current coordinate system. The initial coordinate system of each page has the origin in the lower left corner, with each unit equaling one PostScript point (1/72 inch). The following commands can be used to change the current coordinate system.

Drawing operations can be grouped into blocks with the block operator. Changes to the graphics parameters and coordinate system are local to the block. It is undefined whether changes to the current path made within a block persist after the end of the block (they do in PDF, but not in PostScript). Therefore, path construction should not be broken up across end-of-block boundaries.

The following commands can be used to render documents to various available formats. The available formats are PostScript, PDF, EPS, and an ASCII-based debugging format. Output can be written to standard output, to a file, or to a string.

The document and drawing abstractions provided by this module are purposely kept general-purpose, and do not include application-specific features. However, we provide a mechanism by which applications can provide customized drawing commands and other custom features.

It is sometimes useful to use customized drawing commands. For example, an application that draws many rectangles might like to define a custom rectangle function for appending a rectangle to the current path. Of course this can be defined as an ordinary Haskell function, using elementary drawing commands:

my_rect :: X -> Y -> X -> Y -> Draw ()
my_rect x0 y0 x1 y1 = do
  moveto x0 y0
  lineto x0 y1
  lineto x1 y1
  lineto x1 y0
  closepath

However, sometimes it is nice to make use of specialized abilities of individual backends. For example, PDF already has a built-in rectangle drawing command, and PostScript has the ability to define custom subroutines within the document text. Using these features can decrease the size of the generated documents.

We therefore provide a facility for defining new drawing commands with backend-specific implementations. For example, a more general version of the above my_rect function can be defined as follows:

my_rect :: X -> Y -> X -> Y -> Draw ()
my_rect x0 y0 x1 y1 = draw_subroutine alt $ do
  moveto x0 y0
  lineto x0 y1
  lineto x1 y1
  lineto x1 y0
  closepath
 where
  alt = [
    custom_ps $      printf "%f %f %f %f rect\n" x0 y0 x1 y1,
    custom_pdf $     printf "%f %f %f %f re\n" x0 y0 (x1-x0) (y1-y0),
    custom_ascii $   printf "My_rect %f %f %f %f\n" x0 y0 x1 y1
    ]

The idea is to provide a default definition in terms of primitive drawing commands, as well as a list of various backend specific definitions. In the case of PostScript subroutines, the PostScript file must then also be supplied with a definition for the rect subroutine, which can be done with the command render_ps_custom:

my_ps_defs = "/rect { ... } bind def\n"

my_render_ps = render_ps_custom custom { ps_defs = my_ps_defs }

Note that the draw_subroutine customization mechanism is entirely optional. Its purpose is to generate shorter output for some backends; if it is omitted, the file may be be longer but should look the same.

The following are versions of the generic rendering functions that also take a customization data structure as an additional parameter.