julia set generator for /r/dailyprogramming challenge 277 https://redd.it/4v5h3u

challenge277.c

// challenge 277 hard 2016-07-29 /r/dailyprogrammer https://redd.it/4v5h3u
// compile: $ gcc -std=c99 challenge277.c -lm
// outputs to: fractal.tga

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

typedef struct {
	unsigned char level;
} pixel_t;

typedef struct {
	pixel_t *pixels;
	unsigned short width;
	unsigned short height;
} bitmap_t;

typedef double complex complex_t;

void write_tga(bitmap_t *bitmap, char *fname){
	int datalen = 19 + bitmap->width*bitmap->height*3;
	unsigned char *data = (unsigned char*) calloc(1, datalen);
	data[2] = 2;
	memcpy(data+12, &bitmap->width, 2);
	memcpy(data+14, &bitmap->height, 2);
	data[16] = 24;
	unsigned char *offset = data+19;
	int i=0;
	for(int y=0;y<bitmap->height;y+=1){
		for(int x=0;x<bitmap->width;x+=1){
			offset[i] = bitmap->pixels[y*bitmap->width + bitmap->width-x-1].level;
			offset[i+1] = bitmap->pixels[y*bitmap->width + bitmap->width-x-1].level;
			offset[i+2] = bitmap->pixels[y*bitmap->width + bitmap->width-x-1].level;
			i += 3;
		}
	}
	FILE *file = fopen(fname,"wb");
	fwrite(data, 1, datalen, file);
	fclose(file);
	free(data);
}

void set_pixel(bitmap_t* bitmap, int x, int y, unsigned char level){
	bitmap->pixels[bitmap->width*y + x].level = level;
}

int main(int argc, char **argv){
	if( argc < 3 ){
		printf("usage: %s width height a b scale\n", argv[0]);
		printf("a b are constants in f(z) = z^2 + a + bi\n");
		printf("scale adjusts the zoom of the fractal. start with 0.25.\n");
		printf("outputs to fractal.tga\n");
		return 1;
	}
	
	// prepare TGA for writing
	bitmap_t output;
	output.width = atof(argv[1]);
	output.height = atof(argv[2]);
	output.pixels = (pixel_t*) calloc(sizeof(pixel_t), output.width*output.height);
	
	// declare variables
	int centerX = output.width/2;
	int centerY = output.height/2;
	int iterations = 128;
	
	double scale = 0.4;
	if( argc == 6 ) scale = atof(argv[5]);
	
	complex_t complex_constant = -0.221 - 0.713*I;
	if( argc >= 5 ) complex_constant = atof(argv[3]) + atof(argv[4]) * I;
	
	printf("Using function: f(z) = z^2 + %f + %fi\n", creal(complex_constant), cimag(complex_constant));
	
	
	// loop through pixels and generate intensity map
	for( int y=0; y < output.height; y+=1 ){
		for( int x=0; x < output.width; x+=1 ){
			
			complex_t z = (x-centerX)/(output.width*scale) +
				(y - centerY) * I / (output.width*scale);
			
			int count;
			for( count=0; count<iterations; count+=1 ){
				z = z*z + complex_constant;
				if( abs(creal(z)+1) > 2 || abs(cimag(z)+1) > 2 ){
					set_pixel(&output, x, y, 0);
					break;
				}
			}
			set_pixel(&output, x, y, (unsigned char) floor(((double)count/iterations)*255));
			
		}
	}
	
	write_tga(&output, "fractal.tga");
	free(output.pixels);
	return 0;
}