Support odd-length dash patterns in Agg.

anntzer · anntzer · commit 98ff4b4d0f57 · 2020-05-18T22:57:00.000+02:00
All vector backends already support passing an odd-length dash pattern,
duplicating the on/off array (so that on the second repetition "on"
becomes "off" and vice-versa).  This is also explicitly supported by
the svg spec for `stroke-dasharray` ("If an odd number of values is
provided, then the list of values is repeated to yield an even number of
values. Thus, 5,3,2 is equivalent to 5,3,2,5,3,2.") and less explicitly
by the pdf and ps specs, and by cairo.  So the backend needing support
is Agg; iterating twice over the sequence is likely simplest.
diff --git a/lib/matplotlib/tests/test_lines.py b/lib/matplotlib/tests/test_lines.py
@@ -217,3 +217,9 @@ def test_marker_as_markerstyle():
 
     assert_array_equal(line2.get_marker().vertices, triangle1.vertices)
     assert_array_equal(line3.get_marker().vertices, triangle1.vertices)
+
+
+@check_figures_equal()
+def test_odd_dashes(fig_test, fig_ref):
+    fig_test.add_subplot().plot([1, 2], dashes=[1, 2, 3])
+    fig_ref.add_subplot().plot([1, 2], dashes=[1, 2, 3, 1, 2, 3])
diff --git a/src/py_converters.cpp b/src/py_converters.cpp
@@ -226,7 +226,6 @@ int convert_dashes(PyObject *dashobj, void *dashesp)
     PyObject *dash_offset_obj = NULL;
     double dash_offset = 0.0;
     PyObject *dashes_seq = NULL;
-    Py_ssize_t nentries;
 
     if (!PyArg_ParseTuple(dashobj, "OO:dashes", &dash_offset_obj, &dashes_seq)) {
         return 0;
@@ -256,18 +255,17 @@ int convert_dashes(PyObject *dashobj, void *dashesp)
         return 0;
     }
 
-    nentries = PySequence_Size(dashes_seq);
-    if (nentries % 2 != 0) {
-        PyErr_Format(PyExc_ValueError, "dashes sequence must have an even number of elements");
-        return 0;
-    }
+    Py_ssize_t nentries = PySequence_Size(dashes_seq);
+    // If the dashpattern has odd length, iterate through it twice (in
+    // accordance with the pdf/ps/svg specs).
+    Py_ssize_t dash_pattern_length = (nentries % 2) ? 2 * nentries : nentries;
 
-    for (Py_ssize_t i = 0; i < nentries; ++i) {
+    for (Py_ssize_t i = 0; i < dash_pattern_length; ++i) {
         PyObject *item;
         double length;
         double skip;
 
-        item = PySequence_GetItem(dashes_seq, i);
+        item = PySequence_GetItem(dashes_seq, i % nentries);
         if (item == NULL) {
             return 0;
         }
@@ -280,7 +278,7 @@ int convert_dashes(PyObject *dashobj, void *dashesp)
 
         ++i;
 
-        item = PySequence_GetItem(dashes_seq, i);
+        item = PySequence_GetItem(dashes_seq, i % nentries);
         if (item == NULL) {
             return 0;
         }
