julia.c

/* $ cc -std=c99 -Ofast -fopenmp -o julia julia.c -lm
 * $ ./julia > output.ppm
 * $ ppmtoy4m -F 60:1 < output.ppm > output.y4m
 * $ x264 --qp 0 -o output.mp4 output.y4m
 *
 * Due to parallel compute, stdout must be seekable. Therefore it
 * cannot be piped directly into ppmtoy4m.
 */
#define IMAGE_SIZE  600
#define IMAX        512
#define FRAMES      540
#define RADIUS      1.0
#define HEADER_LEN  21

#ifdef _WIN32
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
static void
write_frame(const void *buf, int n)
{
    size_t len = HEADER_LEN + 3UL * IMAGE_SIZE * IMAGE_SIZE;
    DWORD out;
    OVERLAPPED o = {.Offset = len * n};
    WriteFile(GetStdHandle(STD_OUTPUT_HANDLE), buf, len, &out, &o);
}
#else /* POSIX */
#define _POSIX_C_SOURCE 200809L
#include <unistd.h>
static void
write_frame(const void *buf, int n)
{
    size_t len = HEADER_LEN + 3UL * IMAGE_SIZE * IMAGE_SIZE;
    pwrite(1, buf, len, len * n);
}
#endif

#include <math.h>
#include <stdio.h>
#include <complex.h>

int
main(void)
{
    #pragma omp parallel for schedule(dynamic, 1)
    for (int f = 0; f < FRAMES; f++) {
        double theta = f * 3.141592653589793 * 2 / FRAMES;
        unsigned char buf[HEADER_LEN + 3UL * IMAGE_SIZE * IMAGE_SIZE];
        sprintf((char *)buf, "P6 % 6d % 6d 255\n", IMAGE_SIZE, IMAGE_SIZE);
        unsigned char *p = buf + HEADER_LEN;
        for (int y = 0; y < IMAGE_SIZE; y++) {
            for (int x = 0; x < IMAGE_SIZE; x++) {
                complex double z =
                    ((x / (IMAGE_SIZE - 1.0) - 0.5) * RADIUS * 2) +
                    ((y / (IMAGE_SIZE - 1.0) - 0.5) * RADIUS * 2) * I;
                complex double c =
                    (-0.80 + 0.1 * sin(theta)) +
                    (+0.00 + 0.1 * cos(theta)) * I;
                int i = 0;
                for (; i < IMAX && cabs(z) <= 2.0; i++)
                    z = z * z + c;
                int v = powf(i / (float)IMAX, 1 / 2.2f) * 255;
                *p++ = v;
                *p++ = v;
                *p++ = v;
            }
        }
        write_frame(buf, f);
    }
    return 0;
}