csg.c

#include "svpng.inc"
#include <math.h> // fminf(), sinf(), cosf(), sqrt()
#include <stdlib.h> // rand(), RAND_MAX

#define TWO_PI 6.28318530718f
#define W 512
#define H 512
#define N 64
#define MAX_STEP 64
#define MAX_DISTANCE 2.0f
#define EPSILON 1e-6f

typedef struct { float sd, emissive; } Result;

unsigned char img[W * H * 3];

float circleSDF(float x, float y, float cx, float cy, float r) {
    float ux = x - cx, uy = y - cy;
    return sqrtf(ux * ux + uy * uy) - r;
}

Result unionOp(Result a, Result b) {
    return a.sd < b.sd ? a : b;
}

Result intersectOp(Result a, Result b) {
    Result r = a.sd > b.sd ? b : a;
    r.sd = a.sd > b.sd ? a.sd : b.sd;
    return r;
}

Result subtractOp(Result a, Result b) {
    Result r = a;
    r.sd = (a.sd > -b.sd) ? a.sd : -b.sd;
    return r;
}

Result complementOp(Result a) {
    a.sd = -a.sd;
    return a;
}

Result scene(float x, float y) {
#if 0
    Result r1 = { circleSDF(x, y, 0.3f, 0.3f, 0.10f), 2.0f };
    Result r2 = { circleSDF(x, y, 0.3f, 0.7f, 0.05f), 0.8f };
    Result r3 = { circleSDF(x, y, 0.7f, 0.5f, 0.10f), 0.0f };
    return unionOp(unionOp(r1, r2), r3);
#else
    Result a = { circleSDF(x, y, 0.4f, 0.5f, 0.20f), 1.0f };
    Result b = { circleSDF(x, y, 0.6f, 0.5f, 0.20f), 0.8f };
    return unionOp(a, b);
    // return intersectOp(a, b);
    // return subtractOp(a, b);
    // return subtractOp(b, a);
#endif
}

float trace(float ox, float oy, float dx, float dy) {
    float t = 0.001f;
    for (int i = 0; i < MAX_STEP && t < MAX_DISTANCE; i++) {
        Result r = scene(ox + dx * t, oy + dy * t);
        if (r.sd < EPSILON)
            return r.emissive;
        t += r.sd;
    }
    return 0.0f;
}

float sample(float x, float y) {
    float sum = 0.0f;
    for (int i = 0; i < N; i++) {
        float a = TWO_PI * (i + (float)rand() / RAND_MAX) / N;
        sum += trace(x, y, cosf(a), sinf(a));
    }
    return sum / N;
}

int main() {
    unsigned char* p = img;
    for (int y = 0; y < H; y++)
        for (int x = 0; x < W; x++, p += 3)
            p[0] = p[1] = p[2] = (int)(fminf(sample((float)x / W, (float)y / H) * 255.0f, 255.0f));
    svpng(fopen("csg.png", "wb"), W, H, img, 0);
}