The Render Method (Antialiased Mode)

The render method is shared between GnomeCanvasRect and GnomeCanvasEllipse; all it does is stamp the two paths created in the update method into the RGB buffer:

static void
gnome_canvas_re_render (GnomeCanvasItem *item,
                        GnomeCanvasBuf *buf)
{
  GnomeCanvasRE *re;
  guint32 fg_color, bg_color;

  re = GNOME_CANVAS_RE (item);

  if (re->fill_svp != NULL) {
    gnome_canvas_render_svp (buf, re->fill_svp, re->fill_color);
  }

  if (re->outline_svp != NULL) {
    gnome_canvas_render_svp (buf, re->outline_svp, re->outline_color);
  }
}

      

As you can see, most of the work takes place in gnome_canvas_render_svp(), another function from libgnomeui/gnome-canvas-util.h; here is its implementation:

void
gnome_canvas_render_svp (GnomeCanvasBuf *buf, ArtSVP *svp, guint32 rgba)
{
  guint32 fg_color, bg_color;

  if (buf->is_bg) {
    bg_color = buf->bg_color;
    fg_color = rgba >> 8;
    art_rgb_svp_aa (svp,
                    buf->rect.x0, buf->rect.y0, buf->rect.x1, buf->rect.y1,
                    fg_color, bg_color,
                    buf->buf, buf->buf_rowstride,
                    NULL);
    buf->is_bg = 0;
    buf->is_buf = 1;
  } else {
    art_rgb_svp_alpha (svp,
                       buf->rect.x0, buf->rect.y0, buf->rect.x1, buf->rect.y1,
                       rgba,
                       buf->buf, buf->buf_rowstride,
                       NULL);
  }
}

      

To understand gnome_canvas_render_svp(), or to do your own RGB buffer drawing (without using libart_lgpl), you will need to know what a GnomeCanvasBuf is:

typedef struct {
  guchar *buf;

  int buf_rowstride;

  ArtIRect rect;

  guint32 bg_color;

  unsigned int is_bg : 1;
  unsigned int is_buf : 1;
} GnomeCanvasBuf;
      

The buf member is an RGB buffer, as explained in the section called RGB Buffers in the chapter called GDK Basics. The buf_rowstride is the buffer's rowstride, also explained in the section called RGB Buffers in the chapter called GDK Basics. An ArtIRect is an integer rectangle; rect defines the redraw region in canvas pixel coordinates that this buffer represents. rect.x0 and rect.y0 are the buffer offsets and correspond to row 0, column 0 in the RGB buffer; you can convert from canvas pixel coordinates to RGB buffer coordinates by subtracting these values.

As an optimization, the canvas does not initialize the RGB buffer with the background color, because the first canvas item might cover the entire background anyway. Thus, if your canvas item is the first one to render, you must put some pixel value in every pixel of the redraw region defined by the buffer's rect. If your item does not cover a pixel, you should fill that pixel with the bg_color; bg_color is a packed RGB value (no alpha). If you do this manually, unpack an RGB value rgb like this:

    guchar r, g, b;

    r = (rgb >> 16) & 0xff;
    g = (rgb >> 8) & 0xff;
    b = rgb & 0xff;

      

However, a convenience function is provided to fill a GnomeCanvasBuf with its bg_color:

void
gnome_canvas_buf_ensure_buf (GnomeCanvasBuf *buf)
{
  guchar *bufptr;
  int y;

  if (!buf->is_buf) {
    bufptr = buf->buf;
    for (y = buf->rect.y0; y < buf->rect.y1; y++) {
      art_rgb_fill_run (bufptr,
                        buf->bg_color >> 16,
                        (buf->bg_color >> 8) & 0xff,
                        buf->bg_color & 0xff,
                        buf->rect.x1 - buf->rect.x0);
      bufptr += buf->buf_rowstride;
    }
    buf->is_buf = 1;
  }
}
      

As you can see from the implementation of gnome_canvas_buf_ensure_buf(), is_bg is a flag indicating that the RGB buffer still contains random memory garbage; it has not been initialized with RGB pixels. is_buf indicates that the buffer has been initialized, and subsequent items should only draw themselves, ignoring background pixels. These two flags are mutually exclusive; if your item receives a buffer with is_bg set, it should take steps to fill the buffer, unset is_bg, and set is_buf:

  if (buf->is_bg)
    {
      gnome_canvas_buf_ensure_buf(buf);
      buf->is_bg = FALSE;
    }