Re: Drawing a multicolor spline

Andrea Canciani <[email protected]> Fri, 27 Apr 2012 10:30:43 +0200

Newsgroups	gmane.comp.lib.cairo
Message-ID	<CAN_5=BBAC204WciopnLad1u7gBE0m4ZZoVLdu0x28+H32f2PZw@mail.gmail.com>

On Fri, Apr 27, 2012 at 9:45 AM, Andrea Canciani <[email protected]> wrote:
> On Wed, Apr 25, 2012 at 8:35 PM, Davide Calaminici <[email protected]> wrote:
>> Hello,
>>
>> i'm trying to learn cairo graphics doing some test here and there.
>>
>> I want to draw a spline or bezier, simulating a Windows screen saver like
>> Mistify or Ribbons.
>> Anyone have a simple sample code in c for this?

Some sample code which might be somewhat similar to Mystify is attached.

Andrea

>
> Some sample code on how to draw Bezier curves can be found here:
> http://cairographics.org/samples/curve_to/
>
> If you want the spline to be multicolor, you will need to draw it with
> a source that is not solid black, but some kind of pattern.
>
>
> A different approach might be that of painting a gradient mesh pattern
> (possibly without even using a mask).
> Sample code can be found here:
> http://cairographics.org/manual/cairo-cairo-pattern-t.html#cairo-pattern-create-mesh
>
> Andrea
>
>>
>> Thank you very much
>>
>> --
>> cairo mailing list
>> [email protected]
>> http://lists.cairographics.org/mailman/listinfo/cairo

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mystify.c (text/x-csrc, 2.4 KB)

#include <stdio.h>
#include <stdlib.h>
#include <cairo.h>

#define WIDTH 1024
#define HEIGHT 768

#define RING_SIZE 16
#define MAX_POINTS 19

static double x[MAX_POINTS], y[MAX_POINTS];
static double r, g, b;

static double dx[MAX_POINTS], dy[MAX_POINTS];
static double dr, dg, db;

static double randdouble (double a, double b)
{
    return (random () / (1. + RAND_MAX)) * (b - a) + a;
}

static void nextvalues (void)
{
    int i;

    for (i = 0; i < MAX_POINTS; i++) {
	x[i] += dx[i];
	y[i] += dy[i];
    }

    r += dr;
    g += dg;
    b += db;
}

static void init_values (void)
{
    int i;

    srandomdev ();

    for (i = 0; i < MAX_POINTS; i++) {
	x[i] = randdouble (0, WIDTH);
	y[i] = randdouble (0, HEIGHT);
	dx[i] = randdouble (-15., 15.);
	dy[i] = randdouble (-15., 15.);
    }

    r = randdouble (0., 1.);
    g = randdouble (0., 1.);
    b = randdouble (0., 1.);

    dr = randdouble (-0.1, 0.1);
    dg = randdouble (-0.1, 0.1);
    db = randdouble (-0.1, 0.1);
}

static void my_line(cairo_surface_t *surface)
{
    cairo_t *cr;
    int i;

    cr = cairo_create (surface);
    cairo_set_operator (cr, CAIRO_OPERATOR_CLEAR);
    cairo_paint (cr);

    nextvalues ();

    for (i = 0; i < MAX_POINTS; i++)
	cairo_line_to (cr, x[i], y[i]);
    cairo_close_path (cr);

    cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
    cairo_set_source_rgb (cr, r, g, b);
    cairo_stroke (cr);

    cairo_destroy (cr);
}

int main (int argc, char **argv)
{
    char filename[256];
    cairo_surface_t *ring[RING_SIZE];
    cairo_surface_t *output;
    cairo_t *cr;
    int i, j, num_outs;

    if (argc > 2)
	num_outs = atoi (argv[1]);
    else
	num_outs = 1;

    init_values ();

    output = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, WIDTH, HEIGHT);

    for (i = 0; i < RING_SIZE; i++) {
	ring[i] = cairo_surface_create_similar (output,
						CAIRO_CONTENT_COLOR_ALPHA,
						WIDTH, HEIGHT);
	my_line (ring[i]);
    }

    for (j = 0; j < num_outs; j++) {
	cr = cairo_create (output);
	cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
	cairo_set_source_surface (cr, ring[j % RING_SIZE], 0, 0);
	cairo_paint (cr);

	cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
	for (i = 1; i < RING_SIZE; i++) {
	    cairo_set_source_surface (cr, ring[(i + j) % RING_SIZE], 0, 0);
	    cairo_paint (cr);
	}

	cairo_destroy (cr);
	sprintf (filename, "output%d.png", j);
	cairo_surface_write_to_png (output, filename);

	my_line (ring[j % RING_SIZE]);
    }

    return 0;
}