cairo-a8.c

#include <cairo/cairo.h>
#include <math.h>

static void
draw_region (cairo_t        *cr,
             cairo_region_t *region)
{
  cairo_rectangle_int_t box;
  int n_boxes, i;

  n_boxes = cairo_region_num_rectangles (region);

  for (i = 0; i < n_boxes; i++)
    {
      cairo_region_get_rectangle (region, i, &box);
      cairo_rectangle (cr, box.x, box.y, box.width, box.height);
    }
}

static void
see_region (cairo_region_t *region,
            int             width,
            int             height,
            char           *filename)
{
  cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_A8, width, height);
  cairo_t *cr = cairo_create (surface);

  draw_region (cr, region);
  cairo_fill (cr);

  cairo_surface_write_to_png (surface, filename);
  cairo_destroy (cr);
  cairo_surface_destroy (surface);
}

static void
scan_region (cairo_surface_t *surface,
             cairo_region_t  *union_against)
{
  unsigned char *pixels = cairo_image_surface_get_data (surface);
  int width = cairo_image_surface_get_width (surface);
  int height = cairo_image_surface_get_width (surface);
  int stride = cairo_image_surface_get_stride (surface);
  int x, y;

  for (y = 0; y < height; y++)
    {
      for (x = 0; x < width; x++)
        {
          int w = x;
          while (pixels[y * stride + w] == 255 && w < width)
            w++;

          if (w > 0)
            {
              cairo_rectangle_int_t tmp = { x, y, w - x, 1 };
              cairo_region_union_rectangle (union_against, &tmp);
              x = w;
            }
        }
    }
}

int
main (void)
{
  cairo_surface_t *surface;
  cairo_region_t *circle_region, *clip_region;
  cairo_rectangle_int_t rect = { 0, 0, 500, 500 };
  cairo_t *cr;

  surface = cairo_image_surface_create (CAIRO_FORMAT_A8, 500, 500);
  cr = cairo_create (surface);

  cairo_arc (cr, 250, 250, 100, 0, 2 * M_PI);
  cairo_fill (cr);

  cairo_set_operator (cr, CAIRO_OPERATOR_CLEAR);
  cairo_set_source_rgba (cr, 0, 0, 0, 0);
  cairo_paint (cr);

  cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
  cairo_set_source_rgba (cr, 1, 1, 1, 0.5);

  circle_region = cairo_region_create ();
  cairo_surface_flush (surface);
  scan_region (surface, circle_region);
  see_region (circle_region, 500, 500, "cairo-a8-circle-region.png");

  clip_region = cairo_region_create_rectangle (&rect);
  cairo_region_subtract (clip_region, circle_region);

  see_region (clip_region, 500, 500, "cairo-a8-clip-region.png");

  draw_region (cr, clip_region);
  cairo_clip (cr);

  cairo_rectangle (cr, 50, 50, 400, 400);
  cairo_fill (cr);

  cairo_surface_write_to_png (surface, "cairo-a8.png");

  cairo_region_destroy (circle_region);
  cairo_region_destroy (clip_region);
  cairo_destroy (cr);
  cairo_surface_destroy (surface);
  
  return 0;
}