diff --git a/LuaLibs/CalcPaths.lua b/LuaLibs/CalcPaths.lua
index d3d884f..235d159 100644
--- a/LuaLibs/CalcPaths.lua
+++ b/LuaLibs/CalcPaths.lua
@@ -511,7 +511,6 @@ end
 local function TrimRibs( vRibs, nRibsGrp, nSrf, dSrfOffs, vtSlicing)
    
    -- costruisco le superfici con cui fare il trim grossolano dei setti
-   EgtCopyGlob( nSrf, nRibsGrp)
    local nSrfInt = EgtCopyGlob( nSrf, nRibsGrp)
    local nSrfExt = EgtCopyGlob( nSrf, nRibsGrp)
    EgtSurfFrOffset( nSrfInt, dSrfOffs)
@@ -553,7 +552,7 @@ end
 
 --------------------------------------------------------------------
 local function ComputeRibsOrientedOffset( nRibsGrp, dStrand)
-   
+
    local nOffsGrp = EgtGroup( EgtGetParent( nRibsGrp))
    
    local nCurrRib = EgtGetFirstNameInGroup( nRibsGrp, RIBS_CRV .. '*')
@@ -592,12 +591,14 @@ local function ComputeRibsOrientedOffset( nRibsGrp, dStrand)
          end
          
          for k = 0, nShellsNbr - 1 do
+            local dOffsCurr = dOffs - k * dStrand
             local nNewId
-            if bPlusToMinus then
-               nNewId = EgtOffsetCurveAdv( vIds[i], dOffs - k * dStrand)
+            if abs( dOffsCurr) < GEO.EPS_SMALL then
+               nNewId = EgtCopyGlob( vIds[i], nOffsGrp)
             else
-               nNewId = EgtOffsetCurveAdv( vIds[i], - dOffs + k * dStrand)
+               nNewId = EgtOffsetCurveAdv( vIds[i], EgtIf( bPlusToMinus, dOffsCurr, - dOffsCurr))
             end
+            
             if nNewId then
                EgtSetName( nNewId, sName)
                EgtSetInfo( nNewId, KEY_TYPE, TYPE.RIB)