glow sphere

glow.cpp

#include <stdio.h>
#include <math.h>
#include <vector>
#include <stdint.h>
#include <time.h>

typedef uint8_t u8;

struct vec3
{
float x,y,z;
};

float dot(const vec3& a, const vec3& b)
{
	return a.x * b.x + a.y * b.y + a.z * b.z;
}

vec3 subv(const vec3& a, const vec3& b)
{
	return vec3 {
		a.x - b.x,
		a.y - b.y,
		a.z - b.z
	};
}

float len(const vec3& v)
{
	return sqrtf(dot(v,v));
}

vec3 unit(const vec3& v)
{
	float l = len(v);
	return vec3 {
		v.x / l,
		v.y / l,
		v.z / l
	};
}

float dot_n(const vec3& a, const vec3& b)
{
	return dot(a,b) / len(a) * len(b);
}

float dist(const vec3& a, const vec3& b)
{
	return len(subv(a,b));
}

float clamp(float v)
{
	if(v > 1.f)
		return 1.f;
	if(v < 0.f)
		return 0.f;
	return v;
}

//rgb (3 bytes * w * h)
using buffer = std::vector<u8>;

void write_image(const buffer& buf, int w, int h)
{
	FILE *fp = fopen("test.ppm", "wb");
	if(!fp) return;
	fprintf(fp, "P3\n%d %d\n255\n", w, h);
	for(int x = 0; x < w; x++)
	{
		for(int y = 0; y < h; y++)
		{
			int index = (y * w) + x;
			fprintf(fp, "%d %d %d\n", buf[index], buf[index+1], buf[index+2]);
		}
	}
	fclose(fp);
}

int main()
{
	srand(time(0));
	buffer buf;
	
	vec3 light { 0,1,0 };
	int w = 512;
	int h = 512;
	buf.resize(w*h*3);
	vec3 mid { w/2,h/2,0 };
	for(int x = 0; x < w; x++)
	{
		for(int y = 0; y < h; y++)
		{
			int index = (y * w) + x;
			vec3 pos { x, y, 0};
			vec3 dir = subv(pos, mid);
			float s = 1.f;
			s -= (dot(dir, dir) / dot(mid, mid));
			//s -= sin(dir.x) * cos(dir.y);
			s = pow(s,16);
			s *= 255.f;
			while(s<0.f)
				s+=255.f;
			while(s>255.f)
				s-=255.f;
			//printf("s = %f\n", s);
			for(int i = 0; i < 3; ++i)
				buf[index + i] = s;
		}
	}
	write_image(buf, w, h);
	printf("len = %f\n", len(vec3{1,1,0}));
}