Re: Extents under rotation

Uli Schlachter <[email protected]> Sun, 08 Dec 2013 18:21:25 +0100

Newsgroups	gmane.comp.lib.cairo
Message-ID	<[email protected]>

Hi again,

On 07.12.2013 08:31, Donn wrote:
> On 06/12/2013 17:13, Bill Spitzak wrote:
>> The matrices are multiplied before this is done, so this error does not
>> accumulate. For instance if you rotate by 45 degrees and then back by 45
>> degrees, it will act like a rotation of 0 and produce your expected
>> bounding box, rather than enlarging the bounding box twice.
>
> I'm sure I will face these things soon when I institute picking beyond a
> basic bbox hit approach.
>
>
>
> Bill,
> Do you have a way to draw a tight bbox in user-space around a path?
[...]

I was bored, so:

See the attached test program (written in C since I don't know vala). It uses 
three different ways for getting the extents of a circle.

1) cairo_path_extents()
2) cairo_path_extents() after cairo_identity_matrix()
3) Some slow calculations with cairo_copy_path_flat(). See get_path_extents().

The first row is painted without any rotation (I needed this since I made a typo 
at first with x1/y1/x2/y2 that I took a while to find) with the calculations as 
mentioned above.

The second row is rotated by 67° and the individual rectangles are again 
calculated as above.

Hope this helps,
Uli

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img.png (image/png, 8.8 KB) - not displayed

test.c (text/x-csrc, 2.3 KB)

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

static cairo_t *get_surface(int width, int height)
{
	cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, width, height);
	cairo_t *cr = cairo_create (surface);
	cairo_surface_destroy (surface);
	return cr;
}

static void get_path_extents(cairo_t *cr, double *x1, double *y1, double *x2, double *y2)
{
	int i, had_point = 0;
	cairo_path_t *path = cairo_copy_path_flat (cr);

	*x1 = *y1 = *x2 = *y2 = 0;

	for (i=0; i < path->num_data; i += path->data[i].header.length) {
		cairo_path_data_t *data = &path->data[i];
		switch (data->header.type) {
		case CAIRO_PATH_MOVE_TO:
		case CAIRO_PATH_LINE_TO:
			if (!had_point) {
				*x1 = *x2 = data[1].point.x;
				*y1 = *y2 = data[1].point.y;
				had_point = 1;
			} else {
				*x1 = fmin(*x1, data[1].point.x);
				*y1 = fmin(*y1, data[1].point.y);
				*x2 = fmax(*x2, data[1].point.x);
				*y2 = fmax(*y2, data[1].point.y);
			}
			break;
		case CAIRO_PATH_CLOSE_PATH:
			/* Nothing to do here */
			break;
		case CAIRO_PATH_CURVE_TO:
		default:
			assert (0);
		}
	}

	cairo_path_destroy (path);
}

static void do_row(cairo_t *cr)
{
	double x1, y1, x2, y2;
	double step_x, step_y;

	cairo_save (cr);
	step_x = 100;
	step_y = 0;
	cairo_device_to_user_distance (cr, &step_x, &step_y);

	cairo_arc (cr, 50, 50, 30, 0, 2 * M_PI);
	cairo_path_extents (cr, &x1, &y1, &x2, &y2);
	cairo_rectangle (cr, x1, y1, x2-x1, y2-y1);
	cairo_stroke (cr);
	cairo_translate (cr, step_x, step_y);

	cairo_arc (cr, 50, 50, 30, 0, 2 * M_PI);
	cairo_save (cr);
	cairo_identity_matrix (cr);
	cairo_path_extents (cr, &x1, &y1, &x2, &y2);
	cairo_rectangle (cr, x1, y1, x2-x1, y2-y1);
	cairo_stroke (cr);
	cairo_restore (cr);
	cairo_translate (cr, step_x, step_y);

	cairo_arc (cr, 50, 50, 30, 0, 2 * M_PI);
	get_path_extents (cr, &x1, &y1, &x2, &y2);
	cairo_rectangle (cr, x1, y1, x2-x1, y2-y1);
	cairo_stroke (cr);
	cairo_translate (cr, step_x, step_y);

	cairo_restore (cr);
}

int main(void)
{
	cairo_t *cr = get_surface(100 * 3, 100 * 2);

	cairo_set_source_rgb (cr, 0, 0, 0);
	cairo_paint (cr);

	cairo_set_source_rgb (cr, 1, 1, 1);
	do_row (cr);

	cairo_translate (cr, 80, 100);
	cairo_rotate (cr, 67*M_PI/180);
	do_row (cr);

	cairo_surface_write_to_png (cairo_get_target (cr), "img.png");
	cairo_destroy (cr);
	return 0;
}