How to fuse TopoDS_Shapes in OpenCASCADE (boolean OR)

The OCCUtils library provides an easy way to fuse two shapes in OpenCASCADE.

First, let’s generate a box and a cylinder using OCCUtil’s primitive generation functions:

#include <occutils/Primitive.hxx>

using namespace OCCUtils;
TopoDS_Shape solid1 = Primitive::MakeBox(18 /* X */, 8 /* Y */, 14 /* Z */,
    Primitive::CenterX | Primitive::CenterY);

TopoDS_Shape solid2 = Primitive::MakeCylinder(5 /* d */, 35 /* L */,

Now we can fuse the primitives using this snippet:

#include <occutils/Boolean.hxx>

using namespace OCCUtils;

TopoDS_Shape fused = Boolean::Fuse({solid1, solid2});

Boolean::Fuse() can take a number of different argument types. In this example, we’are using a std::initializer_list (this is the curly-bracket syntax) – but it can also take a TopTools_ListOfShape or a std::vector<TopoDS_Shape> (and also most other STL- or non-STL containers).

If you want to perform a fuse without using OCCUtils, you might be tempted to use this syntax:

BRepAlgoAPI_Fuse fuse(solid1, solid2);
TopoDS_Shape result = fuse.Shape();

However, the BRepAlgoAPI_Fuse(TopoDS, TopoDS) constructor is deprecated and should therefore not be used. Instead, the following syntax should be used:

// Configure fuse
BRepAlgoAPI_Fuse fuse;
// Run fuse
TopoDS_Shape shape = fuse.Shape(); // Raises NotDone if not done.

where arguments and tools are two TopTools_ListOfShape instances. For other boolean algorithms like BRepAlgoAPI_Cut (boolean difference) the two have distinct meanings, but for BRepAlgoAPI_Fuse it’s just important that both arguments and tools have at least one element each!

You can use ListUtils::SplitIntoHeadAndTail from OCCUtils to generate a std::pair of lists (arguments and tools), with arguments having one element and tools having the rest like this:

auto toolsAndArgs = ListUtils::SplitIntoHeadAndTail(shapes, 1);
auto tools = toolsAndArgs.first;
auto arguments = toolsAndArgs.second;

However, if you use ListUtils::SplitIntoHeadAndTail(), you can just use Boolean::Fuse() right away.