The project grew up around cairo and the GObject library. But although the ADG canvas is developed in plain C using the object-oriented approach supplied by GObject, this does not mean that the final application must be developed in C. Care has been taken in order to have an easily bindingable project, so it can be consumed from higher level languages.

This simple idea is shared throughout the whole GNOME ecosystem, that is providing a common low-level base from where a set of bindings for higher level languages (above all the garbage-collected ones) can be implemented, so the end-user applications can be developed in any of the provided languages.

adg-lua is an example that shows how ADG can be easily accessed from the Lua language.

Using the library

Basically, an application based on ADG must fulfill the following tasks:

• define the model
  instantiate and settle an AdgModel derived class, that is the virtual object representing the mechanical part or whatever you need to render;
• populate the canvas
  stroke the model inside an AdgCanvas instance, completing the drawing with decoration entities (title block, quotes, labels, axis and whatever else needed);
• customize the rendering
  by default the ADG library provides and uses some predefined style but you still have the opportunity to change the way the drawing will be rendered, such as changing line thickness, colors, fonts, spacing, quote style, hatching mode and so on;
• render to a cairo surface
  once you have the canvas ready, just render it on a cairo surface: the ADG provides the AdgGtkArea facility for rendering on a GtkDrawingArea; checkout the list of cairo backends to know the availables options and how to use them.

Define the model

The model is the abstract representation of the drawing subject (or part of it). The most common one is probably the AdgPath class, a model built around the cairo_path_t struct. A model can be rendered in the canvas more than once, with different styles and matrices.

The units used in the model definition are not relevant, as they will became signicative only while rendering it to the canvas (that is, when drawing a model into a view). It is common practice to use obvious units: for instance, in Italy you will use meters for plant projects and millimeters for mechanical parts. When adding the model to the canvas, you should set a proper local matrix to scale the drawing as needed and eventually get the proper ending scale.

You can have as many models as you need, depending on the application complexity. For instance, if the part you want to manage changes a lot only on some specific detail, you can consider to move this everchanging portion on its own model leaving the rest in another one, and join the models just before rendering them to the canvas.

Populate the canvas

With this operation the model is transposed to the view. Just instantiate an AdgCanvas and add entities inside it in the same way you construct a user interface in GTK+. Together with the model you will probably would like to add other decoration entities such as title block, quotes, hatches and whatever needed to get the final drawing.

Drawing by hand or with a CAD gives you full control: you must choose the best position for every quotes avoiding overlapping by hand. An automatic drawing system doesn't have the cognition of proper position and a collision detection system is beyond the scope of this project.

The ADG way to solve this issue is by providing everything needed to escape from (hopefully) any situation, allowing to customize almost everything. It is up to the application to put the quotes in a better place or to choose a proper scale.

Global and local space

One key concept of this approach is the use of two different matrices: the global matrix and the local matrix.

The global matrix is calculated by piling up (that is, multiplying) every global map defined by an AdgEntity and its ancestors. How the local matrix is calculated, instead, can be changed set by setting a specific property.

The big difference between these matrices is the global matrix is applied to everything while the local matrix only to some parts, usually the ones belonging in some way to a model. As a side note, the local matrix is always applied (when used) before the global matrix.

To grasp this concept, here it is a screenshot: applying a similar scale factor using a global map or a local map brings two really different results.

As you can see, when zooming in local space only the model shape is scaled and the start points of the dimensions follow this change (that is, they are scaled too). Other stuff, such as line thicknesses, text and arrow sizes, the distance from the extension lines to the shape and the spacing between the baselines, kept the original scale.

When zooming in global space, instead, you get the traditional zoom: everything is scaled.

By default global and local maps are initialized to the identity matrix, that is a matrix that, when used in multiplication, doesn't change the source.

Customize the rendering

The rendering customization is provided throught the interaction of the following ADG components:

• AdgStyle and derived
  In its basic form, the style is an GObject that implements the apply() method. This function should change the passed-in cairo context to fulfill a customization request.
• AdgDress
  This is an abstraction layer to virtualize the styles. The entities will always refer to dresses instead of styles, allowing to do some quite advanced operation such as overriding a dress only in a specific branch of the entity hierarchy.
• AdgEntity
  The entities are the natural consumers of styles. They provide APIs to resolve dresses to styles. Above all, adg_entity_set_style() provides a way to override the style of a dress in a specific entity.

Styles

The style is the brick over which all the customization is done. Being a huge task that could grow beyond any guess, the ADG approach is to implement the styles in an opened way, without knowing or guessing anything about them.

This means adding a new style is a matter of subclassing AdgStyle: the only requirement is to implement the apply() virtual method. You could put all the stuff you consider style stuff inside this new class: it is handled as an opaque object by the ADG library itsself. A new style will be likely used by a new entity, so you'd probably access the style instance only from the render() method of this new entity.

Dresses

When an entity needs to access a style instance (usually inside its render() method), the AdgDress value should be resolved to its AdgStyle equivalent. The main getter function is adg_entity_style(), which executes the following operations up to when a style is found:

• adg_entity_get_style(entity, dress);
• check if the style is defined directly by this entity type, that is if it was explicitely set with adg_entity_set_style();
• adg_entity_style(parent, dress);
• if the subject entity has a parent returns its style by recursively call adg_entity_style() on the parent entity;
• adg_dress_get_style(dress);
• return the default style for the given AdgDressvalue.

In the default implementation all the styles will be resolved by adg_dress_get_style(), but this implementation gives a good level of freedom, allowing the cascade overriding of a dress style with a single adg_entity_set_style() call. When required, new AdgDress values can be easily added with adg_dress_new(). Every dress always requires a default style, just to be sure to resolve any request as stated above.

Render to a cairo surface

To be useful, the canvas needs to be rendered somewhere. For convenience, the ADG library provides AdgGtkArea and AdgGtkLayout widgets if the library has been built with GTK+ support enabled. These are a GtkDrawingArea derived widgets that can be embedded inside a GTK+ user interface. The former provides a way to embed an AdgCanvas in a specific portion of the screen while the latter adds scrollbars and zooming facilities.

In general you should use plain cairo to create the surface. The following example shows a quite basic way to render a canvas to a pdf file:

void
render_to_pdf(AdgCanvas *canvas)
{
    cairo_surface_t *surface;
    cairo_t *cr;

    surface = cairo_pdf_surface_create("test.pdf", 841, 595);
    cr = cairo_create(surface);
    cairo_surface_destroy(surface);

    adg_entity_render(ADG_ENTITY(canvas), cr);

    cairo_show_page(cr);
    cairo_destroy(cr);
}

You can check the demo/adg-demo.c program present in the ADG sources for a more in-depth example.