From f732ea1ab553b92e04557e9a2252a8b36eb1af9f Mon Sep 17 00:00:00 2001
From: ifv <ifv@opencascade.com>
Date: Thu, 20 Feb 2020 14:42:47 +0300
Subject: [PATCH] 0031381: Foundation Classes -wrong evaluations for rational
 BSpline curves using Adaptor curves

BSplCLib_Cache.cxx - it is forbidden zeroing derivatives for rational splines when derivative order more than degree

bug31381 - test case added
---
 src/BSplCLib/BSplCLib_Cache.cxx |  2 +-
 tests/bugs/fclasses/bug31381    | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 tests/bugs/fclasses/bug31381

diff --git a/src/BSplCLib/BSplCLib_Cache.cxx b/src/BSplCLib/BSplCLib_Cache.cxx
index f835c9b9e3..2075211c4e 100644
--- a/src/BSplCLib/BSplCLib_Cache.cxx
+++ b/src/BSplCLib/BSplCLib_Cache.cxx
@@ -111,7 +111,7 @@ void BSplCLib_Cache::CalculateDerivative(const Standard_Real&    theParameter,
   // When the degree of curve is lesser than the requested derivative,
   // nullify array cells corresponding to greater derivatives
   Standard_Integer aDerivative = theDerivative;
-  if (myParams.Degree < theDerivative)
+  if (!myIsRational && myParams.Degree < theDerivative)
   {
     aDerivative = myParams.Degree;
     for (Standard_Integer ind = myParams.Degree * aDimension; ind < (theDerivative + 1) * aDimension; ind++)
diff --git a/tests/bugs/fclasses/bug31381 b/tests/bugs/fclasses/bug31381
new file mode 100644
index 0000000000..fbc313f208
--- /dev/null
+++ b/tests/bugs/fclasses/bug31381
@@ -0,0 +1,15 @@
+puts "# ============"
+puts "# 0031381: Foundation Classes - wrong evaluations for rational BSpline curves using Adaptor curves "
+puts "# ============"
+puts ""
+puts "Eliminate output of approxcurve command"
+puts "REQUIRED ALL: Error   4.32754e-05" 
+
+circle c 0 0 1
+puts "Get rational BSpline curve of degree 2"
+convert c c
+puts "Get offset curve to force calculation of D3 for base curve when algorithm needs D2 for offset"
+offset oc c -.5
+puts "Approximate offset curve with continuity C2"
+approxcurve ac oc 1.e-4 2 5 25
+checklength ac -l 3.1415850499856011
\ No newline at end of file