Shaders

usa_flag - A procedural shader for a U.S. Flag

 
/* shader usa_flag - dsward@webnation.com
 * based on Steve May's Renderman notes at http://www.cgrg.ohio-state.edu/~smay/RManNotes
 * and _Texturing and Modeling, A Procedural Approach_ by Darwyn Peachey, et al, ISBN 0-12-228760-6
*/
 
#define whichtile(x,freq) (floor((x) * (freq)))
#define repeat(x,freq)    (mod((x) * (freq), 1.0))
#define pulse(a,b,fuzz,x) (smoothstep((a)-(fuzz),(a),(x)) - \
			   smoothstep((b)-(fuzz),(b),(x)))
#define odd(x)            (mod((x), 2) == 1)
#define even(x)           (mod((x), 2) == 0)
 
surface
usa_flag
(
	float fuzz = 0.01;
	float Ka = 0.5;
	float Kd = 1;
	float Ks = 0.0;
	float roughness = 0.1;
	color specular_color = 1;
)
{
	color surface_color = color(1, 0 ,0);
	color surface_opac = color(1);
     float ss, tt, angle, r, a;
	float in_out = 0;
     float cell_s, cell_t;
     point p0, p1;
     point d0, d1;
     float star_angle;
	float mix_ratio;
	float offset = 0;
	float spin = 0;
	float r_min = 0.36;
	float r_max = 0.84;
     float star_points = 5;
	float blue_right = 0.4;
	float blue_bottom = 7 / 13;
	float cell_star = 1;
	point Nf, V;
	float star_fuzz;
	float stripe_fuzz;
	float blue_fuzz_s, blue_fuzz_t;
	float stripe_idx;
 
	/* overlay red stripes */
	stripe_fuzz = fuzz * 13;
	tt = repeat(t, 13);
	stripe_idx = whichtile(t, 13);
	mix_ratio = pulse(0, 1, stripe_fuzz, tt);
	if (stripe_idx == 12) mix_ratio = 1.0;
	if odd(stripe_idx)
	{
	    surface_color = mix(surface_color, color(1, 1, 1), mix_ratio);
	}
	else
	{
	    surface_color = mix( color(1, 1, 1), surface_color, mix_ratio);
	}
 
	/* overlay blue field */
	blue_fuzz_s = fuzz / 3;
	blue_fuzz_t = fuzz * 1.6;
	mix_ratio = pulse(0, blue_right + blue_fuzz_s, blue_fuzz_s, s) * 
			pulse(0, blue_bottom + (blue_fuzz_t * 0.25), blue_fuzz_t, t);
	surface_color = mix(surface_color, color(0, 0, 0.5), mix_ratio);
 
	/* overlay stars */
    cell_s = whichtile(s, 32);
    cell_t = whichtile(t, 20);
	
	if (cell_s < 1) cell_star = 0;
	if (cell_t < 1) cell_star = 0;
	if (cell_s > 11) cell_star = 0;
	if (cell_t > 9) cell_star = 0;
	
	if (cell_star == 1)
	{
		if even(cell_s)
		{
			if odd(cell_t) cell_star = 0;
		}
		else
		{
			if even(cell_t) cell_star = 0;
		}
	}
	
	if (cell_star == 1)
	{
		ss = repeat(s, 32) - 0.5;
		tt = repeat(t, 20) - 0.5;
 
		star_angle = 2 * PI / star_points;
		star_fuzz = 0.15;
 
		p0 = r_max * (cos(0), sin(0), 0);
		p1 = r_min * (cos(star_angle / 2), sin(star_angle / 2), 0);
		d0 = p1 - p0;
 
		angle = atan(ss, tt) + PI;
		r = sqrt(ss * ss + tt * tt);
		a = mod(angle, star_angle) / star_angle;
		if (a >= 0.5) a = 1 - a;	
		d1 = r * (cos(a), sin(a), 0) - p0;
		in_out += smoothstep(0, star_fuzz, zcomp(d0 ^ d1));
 
		mix_ratio = smoothstep(0.5, 0.5 + star_fuzz, in_out);
		surface_color = mix(surface_color, color(1, 1, 1), mix_ratio);
		surface_opac = surface_opac + mix_ratio;
	}
 
	/* plastic method */
	Nf = faceforward(normalize(N), I);
	V = -normalize(I);
	surface_color = surface_opac * (surface_color * (Ka * ambient() + Kd * diffuse(Nf)) + 
			specular_color * Ks * specular(Nf, V, roughness));
		
    Oi = surface_opac;
    Ci = surface_color;
}