Internal Architecture

CD - Canvas Draw

Internal Architecture


Apart from the several drivers, the CD library is composed of a few modules, the public header files cd.h and wd.h, those which implement the functions independently from drivers, cd*.c and wd.c, and the header file cd_private.h, apart from some other modules which implement non-exported specific functions. Such modules are totally independent from the implemented drivers, as well as every driver independs from one another, unless there is an intentional dependency.


Since the drivers independ from one another, we could create a library for each of them. For the drivers provided with CD it was easy to include them in their own library, thus simplifying the application's linking process. Note: Internally, the drivers are called "context".

In order to establish this dependency, when creating a canvas in a given driver the user must specify the driver to be used. This specification is done by means of a macro which is actually a function with no parameter, which passes the function table from that driver to the canvas creation function. For instance:

CD_PS (is in fact) cdContextPS()
cdCreateCanvas(CD_PS, ""); (will do) canvas->Line = context->Line

If the context function is not invoqued then that driver does not need to be linked with the application. This is usefull if the application uses a custom build of the CD library and usefull for additional drivers not included in the main library, like IUP and PDF, that have external dependencies.


The core implementation defines the structures declared in the cd.h header. But declares an undefined structure called cdCtxCanvas. This structure is defined in each driver according to their needs. But the first member of this structure must be a pointer to the cdCanvas structure.

The drivers need not to implement all functions from the function table, only a few are required.

Here is the definition of the cdContext and cdCanvas structures: 

struct _cdContext
  unsigned long caps;

  /* can NOT be NULL */
  void (*CreateCanvas)(cdCanvas* canvas, void *data);
  void (*InitTable)(cdCanvas* canvas);

  /* can be NULL */
  int (*Play)(cdCanvas* canvas, int xmin, int xmax, int ymin, int ymax, void *data); 
  int (*RegisterCallback)(int cb, cdCallback func);
struct _cdCanvas
  void (*Line)(cdCtxCanvas* ctxcanvas, int x1, int y1, int x2, int y2);
  void (*Rect)(cdCtxCanvas* ctxcanvas, int xmin, int xmax, int ymin, int ymax);
  void (*Box)(cdCtxCanvas* ctxcanvas, int xmin, int xmax, int ymin, int ymax);

  int mark_type, mark_size;
  int line_style, line_width;
  int interior_style, hatch_style;

  cdVectorFont* vector_font;
  cdSimulation* simulation;
  cdCtxCanvas* ctxcanvas;     // context dependent defintion
  cdContext* context;

Internally each driver defines its cdCtxCanvas strcuture:

struct _cdCtxCanvas 
  cdCanvas* canvas;

  char* filename; 

  int last_line_style;
  int last_fill_mode;
  FILE* file;

Then it must implement the cdcreatecanvas and cdinittable functions:

/* In the driver implementation file */

static void cdcreatecanvas(cdCanvas *canvas, void *data)
  cdCtxCanvas* ctxcanvas = (cdCtxCanvas *)malloc(sizeof(cdCtxCanvas));

  // parse data parameters

  ctxcanvas->canvas = canvas;
  canvas->ctxcanvas = ctxcanvas;

  /* update canvas context */
  canvas->w = (int)(w_mm * res);
  canvas->h = (int)(h_mm * res);
  canvas->w_mm = w_mm;
  canvas->h_mm = h_mm;
  canvas->bpp = 24;
  canvas->xres = res;
  canvas->yres = res;

static void cdinittable(cdCanvas* canvas)
  canvas->Flush = cdflush;
  canvas->Clear = cdclear;
  canvas->Pixel = cdpixel;
  canvas->Line = cdline;
  canvas->Poly = cdpoly;

static cdContext cdMetafileContext =

cdContext* cdContextMetafile(void)
  return &cdMetafileContext;

To simplify driver administration, the context structure's linking is done as follows:

/* In the header file */
#define CD_METAFILE cdContextMetafile()
cdContext* cdContextMetafile(void)


The query mechanism of an attribute is done in the core and does not depends on the driver. Due to this fact, the attributes which are modified several times for the same value are not updated in the drivers, thus saving processing. Similarly, if an attribute modification in a driver was not successful, its value is not updated. Nevertheless, the fact that a driver does not implement the attribute's modification function does not mean that it rejects that attribute - the driver just does not need to do anything with this attribute on that moment and will query it later, before drawing the primitive.

The creation of customized attributes for each driver is made generically, using string-like attributes. A structure with the attribute's name and its set and get functions must be declared, as in the example below:

static void set_fill_attrib(cdCtxCanvas* ctxcanvas, char* data)
  ctxcanvas->fill_attrib[0] = data[0];

static char* get_fill_attrib(cdCtxCanvas* ctxcanvas)
  return ctxcanvas->fill_attrib;

static cdAttribute fill_attrib =

At createcanvas in the driver:

ctxcanvas->fill_attrib[0] = '1';
ctxcanvas->fill_attrib[1] = 0;

cdRegisterAttribute(canvas, &fill_attrib);

, for instance, must exist, thus initializing the attribute and registering it in the canvas' attribute list.