diff -urN rt_v3/cg.py rt_v4/cg.py
--- rt_v3/cg.py	2018-03-18 10:31:07.000000000 +0900
+++ rt_v4/cg.py	2018-03-18 17:56:56.000000000 +0900
@@ -20,11 +20,15 @@
 	copen = 'copen-090419.jpg'
 	beat = 'beat.jpg'
 
+	rtd = { 'reflect': 0.3 }
+
 	data = [ {
 		'kind': 'square',
+		'rtd': rtd,
 		'l2g': [ ax.zoom_all(50) ],
 	},{
 		'kind': 'ball',
+		'rtd': rtd,
 		'def_col': [128,0,0],
 		'l2g': [ ax.zoom_all(20) ],
 	} ]
diff -urN rt_v3/rt.py rt_v4/rt.py
--- rt_v3/rt.py	2018-03-18 10:57:44.000000000 +0900
+++ rt_v4/rt.py	2018-03-18 18:31:18.000000000 +0900
@@ -10,41 +10,66 @@
 	def_col = d.get( 'def_col', v.all(128) )
 	return def_col
 
-def diffusivity(lights, crsp, nv, eyev_nega):
-	ang_nv_eye = v.dot_product( nv, eyev_nega )
-
-	def f(ltd):
-		ltv = [0,0,1]
-		if 'v' in ltd:
-			ltv = v.nega( ltd.get('v') )
-		elif 'p' in ltd:
-			ltv = v.sub_unit( ltd.get('p'), crsp )
-		ang_nv_lt = v.dot_product( nv, ltv )
-		if ang_nv_eye * ang_nv_lt < 0:
-			return 0
-		return max( ang_nv_lt, 0 ) * ltd.get('e', 1.0)
-
-	return sum( map( f, lights ) )
+def get_col(data, lights, l_g, nest_rate=1.0, ref_len=None):
+	if nest_rate < 0.01:
+		return []
 
-def get_col(data, lights, l_g):
 	lst = map( lambda d: ( d, cross.cross_g(l_g, d) ), data )
 	lst = filter( lambda (d, crs): 't' in crs, lst )
 	if not lst:
 		return []
+
+	if ref_len:
+		lst = filter( lambda (d, crs): crs.get('t') > ref_len * 0.000001, lst )
+		if not lst:
+			return []
+
 	(d, crs) = min( lst, key=lambda (d, crs): crs.get('t') )
 	col = map_col(d, crs)
 
-	rt = d.get('rt', {})
-	base = rt.get('base', 0.3)
+	rtd = d.get('rtd', {})
+	base = rtd.get('base', 0.3)
 
 	crsp = crs.get('p')
 	nv = v.unit( crs.get('nv') )
 	eyev = v.unit(l_g.v)
 	eyev_nega = v.nega(eyev)
+	ang_nv_eye = v.dot_product( nv, eyev_nega )
+
+	def diffusivity():
+		diff = rtd.get('diff', 0.9)	
+		if not diff:
+			return 0
 
-	diff = diffusivity(lights, crsp, nv, eyev_nega) * rt.get('diff', 0.9)
+		def f(ltd):
+			ltv = [0,0,1]
+			if 'v' in ltd:
+				ltv = v.nega( ltd.get('v') )
+			elif 'p' in ltd:
+				ltv = v.sub_unit( ltd.get('p'), crsp )
+			ang_nv_lt = v.dot_product( nv, ltv )
+			if ang_nv_eye * ang_nv_lt < 0:
+				return 0
+			return max( ang_nv_lt, 0 ) * ltd.get('e', 1.0)
+
+		return sum( map( f, lights ) ) * diff
+
+	diff = diffusivity()
 	col = v.op1( '*', col, base + diff )
 
+	def reflect_col():
+		reflect = rtd.get('reflect', 0)
+		if not reflect:
+			return []
+		ref_v = v.op2( '+', v.op1( '*', nv, 2 * ang_nv_eye ), eyev )
+		ref_l = line.new( crsp, ref_v )
+		col = get_col( data, lights, ref_l, nest_rate * reflect, crs.get('t') )
+		return v.op1( '*', col, reflect )
+
+	col_ = reflect_col()
+	if col_:
+		col = v.op2( '+', col, col_ )
+
 	return map( lambda v: min( int(v), 255 ), col )
 
 def draw(data, lights, eye2g, wh2eye, sc_sz, video):