[cairo] Radii of radial gradients

[KUROSAWA, Takeshi]

Hi,

I tried the radial gradients with cairo 1.3.13.
The following code generates an image(*1) I unexpected.

  pat = cairo_pattern_create_radial (45.0, 45.0, 10.0, 50.0, 50.0, 30.0);
  cairo_pattern_add_color_stop_rgba (pat, 0, 1, 1, 1, 1);
  cairo_pattern_add_color_stop_rgba (pat, 0.9, 0, 0, 0, 1);
  cairo_pattern_add_color_stop_rgba (pat, 1, 0, 0, 0, 0);

  cairo_rectangle (cr, 0, 0, 150.0, 150.0);
  cairo_set_source (cr, pat);
  cairo_fill (cr);
  cairo_destroy_pattern (pat);

I expected the radius arguments of the |cairo_pattern_create_radial|
(i.e. 10.0 and 30.0) conform to the radii of the circles.
But, in the result image, the each radius seems to conform to a side
of cyclic quadrilateral of the each circle.
# but not exactly

Therefore, I have a question.
Is it a behavior as per spec of a bug?


Full of the code is following

#include <cairo.h>
#define WIDTH 150
#define HEIGHT 150
#define STRIDE (WIDTH * 4)

unsigned char image[STRIDE*HEIGHT];

int
main (void)
{
  cairo_surface_t *surface;
  cairo_t *cr;
  cairo_pattern_t *pat;

  surface = cairo_image_surface_create_for_data (image, CAIRO_FORMAT_ARGB32,
						 WIDTH, HEIGHT, STRIDE);
  cr = cairo_create (surface);

  pat = cairo_pattern_create_radial (45.0, 45.0, 10.0, 50.0, 50.0, 30.0);
  cairo_pattern_add_color_stop_rgba (pat, 0, 1, 1, 1, 1);
  cairo_pattern_add_color_stop_rgba (pat, 0.9, 0, 0, 0, 1);
  cairo_pattern_add_color_stop_rgba (pat, 1, 0, 0, 0, 0);

  cairo_rectangle (cr, 0, 0, 150.0, 150.0);
  cairo_set_source (cr, pat);
  cairo_fill (cr);
  cairo_pattern_destroy (pat);

  /* draw rects that expected circumscribed quadrangles of the
gradient circles. */
  cairo_set_source_rgba (cr, 0, 0, 1, 0.5);
  cairo_rectangle (cr, 35, 35, 20, 20);
  cairo_rectangle (cr, 20, 20, 60, 60);
  cairo_stroke (cr);

  cairo_surface_write_to_png (surface, "cairo_radial_gradient.png");
  cairo_destroy (cr);
  cairo_surface_destroy (surface);
  return 0;
}


*1: I've attached it to this mail.
-- 
KUROSAWA, Takeshi - http://taken.s101.xrea.com/
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