diff -urN rt_v1/cg.py rt_v2/cg.py
--- rt_v1/cg.py	2018-03-16 21:06:29.000000000 +0900
+++ rt_v2/cg.py	2018-03-17 19:20:42.000000000 +0900
@@ -13,6 +13,7 @@
 import img
 import way
 import wf
+import rt
 
 if __name__ == "__main__":
 
@@ -24,6 +25,7 @@
 		'l2g': [ ax.zoom_all(50) ],
 	},{
 		'kind': 'ball',
+		'def_col': [128,0,0],
 		'l2g': [ ax.zoom_all(20) ],
 	} ]
 
@@ -53,7 +55,10 @@
 
 		eye2g = ax.new_line( eye_f( cnt.cur_sec() ) )
 
-		wf.draw(data, eye2g, wh2eye, sc_sz, video)
+		f = rt.draw
+		if 'wf' in sys.argv:
+			f = wf.draw
+		f(data, eye2g, wh2eye, sc_sz, video)
 
 		video.add_img()
 		fn = ( name if n == 1 else name+'0000{}'.format(i)[-5:] ) + '.jpg'
diff -urN rt_v1/cross.py rt_v2/cross.py
--- rt_v1/cross.py	1970-01-01 09:00:00.000000000 +0900
+++ rt_v2/cross.py	2018-03-16 20:38:43.000000000 +0900
@@ -0,0 +1,111 @@
+#!/usr/bin/env python
+
+import math
+import mt
+import v
+import line
+import ax
+import lstx
+
+# return dic keys 't', 'p', 'nv'
+
+def plane_z0(l, d):
+	# z=0 (x-y)
+
+	(vx, vy, vz) = l.v
+	(px, py, pz) = l.p
+
+	if vz == 0:
+		return {}
+
+	t = -float(pz) / vz
+	if t <= 0:
+		return {}
+
+	p = l.on_line_p(t)
+	return { 't': t, 'nv': v.z1, 'p': p }
+
+def plane_z0_xy_cond(l, d):
+	xy_cond_f = d.get( 'xy_cond_f', lambda x, y: True )
+	r = plane_z0(l, d)
+	if not r:
+		return {}
+	(x, y, _) = r.get('p')
+	return r if xy_cond_f(x, y) else {}
+
+def square(l, d):
+	# [-1,-1,0] --- [1,1,0]
+	d['xy_cond_f'] = lambda x, y: abs(x) <= 1 and abs(y) <= 1
+	return plane_z0_xy_cond(l, d)
+
+def circle(l, d):
+	# center zero, r=1, z=0
+	d['xy_cond_f'] = lambda x, y: x*x + y*y <= 1
+	return plane_z0_xy_cond(l, d)
+
+def triangle(l, d):
+	# (0,1,0),(-1,0,0),(1,0,0)
+	d['xy_cond_f'] = lambda x, y: x+y<=1 and -x+y<=1 and y>=0
+	return plane_z0_xy_cond(l, d)
+
+def poly_n(l, d):
+	n = d.get('n', 3)
+	deg = 360.0 / n
+	rad = mt.deg_to_rad( deg * 0.5 )
+	h = math.cos(rad)
+	w = math.sin(rad)
+	for i in range(n):
+		axi = ax.slide_y(-1).compo( ax.zoom([w,h,1]) ).compo( ax.rot_z(i*deg) )
+		r = triangle( axi.tr('g2l', l), d )
+		if r:
+			return r
+	return {}
+
+def ball(l, d):
+	# r = 1, c = zero
+	# l = p + v t
+	# v^2 t^2 + 2 v p t + p^2 = 1^2
+	a = sum( v.op2('*', l.v, l.v) )
+	b = 2 * sum( v.op2('*', l.v, l.p) )
+	c = sum( v.op2('*', l.p, l.p) ) - 1
+	ts = mt.quadratic_formula(a, b, c)
+	ts = sorted( filter( lambda v: v > 0, ts ) )
+	if not ts:
+		return {}
+	t = ts[0]
+	p = l.on_line_p(t)
+	nv = v.unit(p)
+	return { 't': t, 'nv': nv, 'p': p }
+
+def cross(l, d):
+	dic = {
+		'square': square,
+		'circle': circle,
+		'triangle': triangle,
+		'poly_n': poly_n,
+		'ball': ball,
+	}
+	f = dic.get( d.get('kind') )
+	if not f:
+		return {}
+	crs = f(l, d)
+	if not crs:
+		return {}
+	if 'p' not in crs:
+		crs['p'] = l.on_line_p( crs.get('t') )
+	if 'l' not in crs:
+		crs['l'] = line.new( crs.get('p'), crs.get('nv') )
+	return crs
+
+def cross_g(l_g, d):
+	l2g = d.get('l2g')
+	l = lstx.tr( l2g, 'g2l', l_g )
+	crs = cross(l, d)
+	if not crs:
+		return {}
+	crs['l'] = nvl_g = lstx.tr( l2g, 'l2g', crs.get('l') )
+	crs['p'] = nvl_g.p
+	crs['nv'] = nvl_g.v
+	return crs
+
+# EOF
diff -urN rt_v1/rt.py rt_v2/rt.py
--- rt_v1/rt.py	1970-01-01 09:00:00.000000000 +0900
+++ rt_v2/rt.py	2018-03-17 19:19:33.000000000 +0900
@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+
+import ut
+import v
+import line
+import lstx
+import cross
+
+def map_col(d, crs):
+	def_col = d.get( 'def_col', v.all(128) )
+	return def_col
+
+def get_col(data, 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 []
+	(d, crs) = min( lst, key=lambda (d, crs): crs.get('t') )
+	return map_col(d, crs)
+
+def draw(data, eye2g, wh2eye, sc_sz, video):
+	(w, h, d_) = sc_sz
+	wh2g = lstx.opt( wh2eye + [ eye2g ] )
+	cnt = ut.new_cnt_wh(w, h, 'wh')
+	while cnt.up():
+		(ix, iy) = cnt.cur()
+		p = lstx.tr( wh2g, 'l2g', [ix,iy,-d_] )
+		l_g = line.new_p2(eye2g.p, p)
+		col = get_col(data, l_g)
+		if col:
+			video.set(ix, iy, col)
+# EOF