This directory contains the package `mercury_cairo' which is a Mercury binding
to the Cairo 2D graphics library.  (See: <http://www.cairographics.org>.)


Building the binding
====================

(1) File in the system-specific information for your system in Cairo.options.

(2) Build and install with the following command:

   $ mmc --make libmercury_cairo.install


Overview of the Mercury Cairo Binding
=====================================

The remainder of this document outlines the mapping from the Cairo C API to
Mercury.  In general, we have followed the Cairo language binding guidelines,
<http://www.cairographics.org/manual/language-bindings.html>, where possible
although some changes had to be made due to name clashes with other Mercury
libraries.


Mapping from C API types to Mercury types
=========================================

    cairo_t                       cairo.context/1.
    cairo_pattern_t               cairo.pattern/0.
    cairo_matrix_t                cairo.matrix/0.
    cairo_path_t                  cairo.path/0.
    cairo_font_options_t          cairo.font_options/0.
    cairo_region_t                cairo.region/0.
    cairo_scaled_font_t           cairo.scaled_font/1.    
    cairo_content_t               cairo.content/0
    cairo_format_t                cairo.format/0.
    cairo_status_t                cairo.status/0.

There are no types in the Mercury API that correspond directly to the C types
cairo_surface_t and cairo_font_face_t.  In the Mercury binding these are
represented by the type classes: cairo.surface/1 and cairo.font_face/1.


Mapping from C functions to Mercury predicates
==============================================

In the Mercury binding, the Cairo API is split up amongst various submodules,
each submodule providing the operations that provide a specific piece of
functionality.  The choice has typically been governed by where a function
appears in the Cairo API manual.

The top-level module in the binding is named "cairo".  It contains

  * Bindings for the major types in Cairo.
  * Bindings for the operations on the Cairo drawing context.
  * Bindings for error handling functionality.

For example:

    cairo_create()          cairo.create_context/4.
    cairo_save()            cairo.save/3.           
    cairo_restore()         cairo.restore/2.
    ...

Operations that create and manipulate path data are contained in the
submodule "cairo.path".  For example:

    cairo_copy_path()       cairo.path.copy_path/4.
    cairo_copy_path_flat()  cairo.path.copy_path_flat/4.
    cairo_append_path()     cairo.path.append_path/4.
    ...

Operations on patterns, i.e. mainly those whose C names are begin
with cairo_pattern_, are contained in the submodule "cairo.pattern".
For example:

    cairo_pattern_add_color_stop_rgb()      cairo.pattern.add_color_stop_rgb/7
    cairo_pattern_add_color_stop_rgba()     cairo.pattern.add_color_stop_rgba/8
    cairo_pattern_create_rgb()              cairo.pattern.create_rgb/6
    ...

Operations on surfaces, i.e. those whose C name being with cairo_surface_,
are contained in the submodule "cairo.surface".
For example:

    cairo_surface_finish()              cairo.surface.finish/3.
    cairo_surface_flush()               cairo.surface.flush/3.
    cairo_surface_get_font_options()    cairo.surface.get_font_options/4.
    ...


Backends Submodules
===================

The Mercury binding has submodules that provide image, PDF, PostScript, and SVG
surfaces.  Each surface is represented by a type that is an instance of the
cairo.surface/1 type class.  For example, the interface to image surfaces is in
the submodule "cairo.image".

Image surfaces are always supported.  PDF, PostScript, and SVG surfaces are
optional.  The following predicates may be used to test whether they are are
supported by an installation:
    
    cairo.pdf.have_pdf_surface/0
    cairo.ps.have_ps_surface/0
    cairo.svg.have_svg_surface/0

(The predicate cairo.png.png_is_supported/0 performs the same function for
determining whether the PNG reading and writing capabilities in the submodule
"cairo.png" are supported by an installation.)

Other surface types, e.g. X, Quartz, Win32, are not provided by the Mercury
Cairo binding.  The intention is that they can be provided by separate
libraries that would provide an instance of the cairo.surface/1 type class as
well as bindings to the required backend specific Cairo functions.
