Trac #7232: fix tachyon segfault introduced by #6542

This pretty much says it all:

{{{
sage: a = 1
sage: b = 0
sage: c = 0
sage:
sage: f = lambda t: ((a*c*cos(t) - b*sin(t))/sqrt(a^2+b^2), (b*c*cos(t)
+
....: a*sin(t))/sqrt(a^2+b^2), cos(t)*sqrt(a^2+b^2))
sage: t = Tachyon(camera_center=(0,0,0))
sage: t.texture('t')
sage: t.light((-20,-20,40), 0.2, (1,1,1))
sage: t.parametric_plot(f,0,2*pi,'t',min_depth=6)
sage:
sage: t.show()
sh: line 1:  2214 Segmentation fault      tachyon
/Users/wstein/.sage//temp/flat.local/2161//tmp_2.dat -format PNG -o
/Users/wstein/.sage//temp/flat.local/2161//tmp_1.png > /dev/null
}}}

Note, that this was I think caused by #6542.

URL: http://trac.sagemath.org/7232
Reported by: was
Ticket author(s): Frédéric Chapoton
Reviewer(s): Karl-Dieter Crisman, Niles Johnson

src/sage/plot/plot3d/tachyon.py

@@ -42,14 +42,13 @@
 
 - Marshall Hampton: improved doctests, rings, axis-aligned boxes.
 
-TODO:
+.. TODO:
 
-- clean up trianglefactory stuff
+    - clean up trianglefactory stuff
 """
 
 from tri_plot import Triangle, SmoothTriangle, TriangleFactory, TrianglePlot
 
-
 from sage.interfaces.tachyon import tachyon_rt
 
 from sage.structure.sage_object import SageObject
@@ -63,6 +62,7 @@
 
 from math import sqrt
 
+
 class Tachyon(SageObject):
     r"""
     Create a scene the can be rendered using the Tachyon ray tracer.
@@ -78,7 +78,7 @@ class Tachyon(SageObject):
     - ``camera_center`` - (default (-3, 0, 0))
     - ``updir`` - (default (0, 0, 1))
     - ``look_at`` - (default (0,0,0))
-    - ``viewdir`` - (default None)
+    - ``viewdir`` - (default ``None``)
     - ``projection`` - (default 'PERSPECTIVE')
 
     OUTPUT: A Tachyon 3d scene.
@@ -192,20 +192,29 @@ class Tachyon(SageObject):
         sage: t.texture('white', color=(1,1,1), opacity=1, specular=1, diffuse=1)
         sage: t.plane((0,0,-100), (0,0,-100), 'white')
         sage: t.show()
+
+    If the optional parameter ``viewdir`` is not set, the camera
+    center should not coincide with the point which
+    is looked at (see :trac:`7232`)::
+
+        sage: t = Tachyon(xres=80,yres=80, camera_center=(2,5,2), look_at=(2,5,2))
+        Traceback (most recent call last):
+        ...
+        ValueError: camera_center and look_at coincide
     """
     def __init__(self,
                  xres=350, yres=350,
-                 zoom = 1.0,
-                 antialiasing = False,
-                 aspectratio = 1.0,
-                 raydepth = 8,
-                 camera_center = (-3, 0, 0),
-                 updir = (0, 0, 1),
-                 look_at = (0,0,0),
-                 viewdir = None,
-                 projection = 'PERSPECTIVE'):
+                 zoom=1.0,
+                 antialiasing=False,
+                 aspectratio=1.0,
+                 raydepth=8,
+                 camera_center=(-3, 0, 0),
+                 updir=(0, 0, 1),
+                 look_at=(0, 0, 0),
+                 viewdir=None,
+                 projection='PERSPECTIVE'):
         r"""
-        Creates an instance of the Tachyon class.
+        Create an instance of the Tachyon class.
 
         EXAMPLES::
 
@@ -224,15 +233,19 @@ def __init__(self,
         self._projection = projection
         self._objects = []
         if viewdir is None:
-            self._viewdir = [look_at[i] - camera_center[i] for i in range(3)]
+            if look_at != camera_center:
+                self._viewdir = [look_at[i] - camera_center[i]
+                                 for i in range(3)]
+            else:
+                raise ValueError('camera_center and look_at coincide')
         else:
             self._viewdir = viewdir
 
-
-
     def save_image(self, filename=None, *args, **kwds):
         r"""
-        Save an image representation of self.  The image type is
+        Save an image representation of ``self``.
+
+        The image type is
         determined by the extension of the filename.  For example,
         this could be ``.png``, ``.jpg``, ``.gif``, ``.pdf``,
         ``.svg``.  Currently this is implemented by calling the
@@ -276,7 +289,6 @@ def save(self, filename='sage.png', verbose=0, block=True, extra_opts=''):
         r"""
         INPUT:
 
-
         -  ``filename`` - (default: 'sage.png') output
            filename; the extension of the filename determines the type.
            Supported types include:
@@ -320,7 +332,7 @@ def save(self, filename='sage.png', verbose=0, block=True, extra_opts=''):
 
     def show(self, verbose=0, extra_opts=''):
         r"""
-        Creates a PNG file of the scene.
+        Create a PNG file of the scene.
 
         EXAMPLES::
 
@@ -381,7 +393,7 @@ def _camera(self):
 
     def str(self):
         r"""
-        Returns the complete tachyon scene file as a string.
+        Return the complete tachyon scene file as a string.
 
         EXAMPLES::
 
@@ -409,7 +421,7 @@ def str(self):
 
     def light(self, center, radius, color):
         r"""
-        Creates a light source of the given center, radius, and color.
+        Create a light source of the given center, radius, and color.
 
         EXAMPLES::
 
@@ -512,7 +524,7 @@ def texture(self, name, ambient=0.2, diffuse=0.8,
 
     def texture_recolor(self, name, colors):
         r"""
-        Recolors default textures.
+        Recolor default textures.
 
         EXAMPLES::
 
@@ -542,7 +554,7 @@ def texture_recolor(self, name, colors):
 
     def sphere(self, center, radius, texture):
         r"""
-        Creates the scene information for a sphere with the given
+        Create the scene information for a sphere with the given
         center, radius, and texture.
 
         EXAMPLES::
@@ -1207,13 +1219,14 @@ def str(self):
             %s
         """%(tostr(self._a), tostr(self._b),tostr(self._c), self._color)
 
+
 class TachyonSmoothTriangle(SmoothTriangle):
     r"""
     A triangle along with a normal vector, which is used for smoothing.
     """
     def str(self):
         r"""
-        Returns the scene string for a smoothed triangle.
+        Return the scene string for a smoothed triangle.
 
         EXAMPLES::
 
@@ -1230,7 +1243,6 @@ def str(self):
              tostr(self._da), tostr(self._db), tostr(self._dc), self._color)
 
 
-
 class TachyonTriangleFactory(TriangleFactory):
     r"""
     A class to produce triangles of various rendering types.