spirograph.c

// Ref: https://old.reddit.com/r/C_Programming/comments/1oqxeeq
// This is free and unencumbered software released into the public domain.
#include <math.h>
#include <stdio.h>

#define lenof(a)    (int)(sizeof(a) / sizeof(*(a)))

typedef struct { float x, y;    } V2;
typedef struct { float x, y, z; } V3;

static V2 mul(V2 v, float s)
{
    v.x *= s;
    v.y *= s;
    return v;
}

static V2 add(V2 a, V2 b)
{
    a.x += b.x;
    a.y += b.y;
    return a;
}

static V2 translate(V2 p, V3 *v, int len, float t)
{
    for (int i = 0; i < len; i++) {
        p.x += v[i].x * cosf(t*v[i].y + v[i].z);
        p.y += v[i].x * sinf(t*v[i].y + v[i].z);
    }
    return p;
}

static void line(V2 a, V2 b, int color)
{
    printf(
        "  <line x1='%.3g' y1='%.3g' x2='%.3g' y2='%.3g'"
        " style='stroke: #%06x; stroke-width: 2px;'/>\n",
        (double)a.x, (double)a.y, (double)b.x, (double)b.y, color
    );
}

int main()
{
    V3 adobe[] = {  // magnitude, angular velocity, phase
        {1.0f, -1.0f, 0.0f},
        {1.0f,  2.0f, 0.0f},
        {0.5f, -1.0f, 0.0f},
    };

    enum { size=1000, end=1000, div=100 };
    V2    center = {size/2, size/2};
    float scale  = 100;

    printf(
        "<svg"
        " version='1.1'"
        " xmlns='http://www.w3.org/2000/svg'"
        " width='%d'"
        " height='%d'"
        ">\n", size, size
    );
    V2 prev = translate((V2){}, adobe, lenof(adobe), 0);
    for (int t = 0; t < end; t++) {
        V2 next = translate((V2){}, adobe, lenof(adobe), (float)t/div);
        line(
            add(center, mul(prev, scale)),
            add(center, mul(next, scale)),
            0x000000
        );
        prev = next;

    }
    puts("</svg>");
}

spirograph.svg