drawing a shaded sphere (using https://cidscropt.org/)

sphere.c

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

_sqrt(a)
{
	prev = 0;
	for(i=0.0;i<10000;i = i + 0.1)
	{
		if(i*i>a)
			return prev;
		prev = i;
	}
	return 0;
}

len(v)
{
	return _sqrt(dot(v,v));
}

dot_n(a,b)
{
	return dot(a,b) / (len(a) * len(b));
}

clamp(v)
{
	x = v.x;
	y = v.y;
	z = v.z;
	if(x > 1)
	{
		x = 1;
	}
	if(y > 1)
	{
		y = 1;
	}
	if(z > 1)
	{
		z = 1;
	}
	return (x,y,z);
}

draw_circle_filled2(at,start,end,color)
{
	targ=(at+start/4);
	tlen=len(targ);
	end2=end*end;
	for(x = start;x<end;x++)
	{
		for(y = start;y<end;y++)
		{
			if(x*x+y*y<=end2)
			{
				pos = (at.x+x,at.y+y,0);
				d = pos - targ;
				s = len(d) / tlen;
				s *= 3;
				v = ((1,1,1) - clamp((s,s,s))) * 255;
				set_pixel(pos,v);
			}
		}
	}
}

main()
{
	printf("hello graphics\n");
	draw_circle_filled2((150,150,0),-50,50,(255,0,0));
}