Made it so you can multiply 2d and 3d point transforms together.

dlib/geometry/point_transforms.h

@@ -682,6 +682,21 @@ namespace dlib
         dlib::vector<double,3> b;
     };
 
+// ----------------------------------------------------------------------------------------
+
+    inline point_transform_affine3d operator* (
+        const point_transform_affine3d& lhs,
+        const point_transform_affine& rhs
+    )
+    {
+        matrix<double,3,3> m;
+        m = 0;
+        set_subm(m, get_rect(rhs.get_m())) = rhs.get_m();
+        vector<double,3> b = rhs.get_b();
+
+        return point_transform_affine3d(lhs.get_m()*m, lhs.get_m()*b+lhs.get_b());
+    }
+
 // ----------------------------------------------------------------------------------------
 
     inline point_transform_affine3d operator* (

dlib/geometry/point_transforms_abstract.h

@@ -482,7 +482,19 @@ namespace dlib
               is, for all valid x: TFORM(x) == lhs(rhs(x)).
     !*/
 
-    // ----------------------------------------------------------------------------------------
+// ----------------------------------------------------------------------------------------
+
+    point_transform_affine3d operator* (
+        const point_transform_affine3d& lhs,
+        const point_transform_affine& rhs
+    );
+    /*!
+        ensures
+            - returns a transformation TFORM(x) that is equivalent to lhs(rhs(x)).  That
+              is, for all valid x: TFORM(x) == lhs(rhs(x)).
+    !*/
+
+// ----------------------------------------------------------------------------------------
 
     point_transform_affine3d inv (
         const point_transform_affine3d& trans

dlib/test/geometry.cpp

@@ -831,6 +831,11 @@ namespace
         tform = rotate_around_x(pi/2)*rotate_around_z(pi/2)*translate_point(x);
         DLIB_TEST(length(tform(dlib::vector<double>())-z) < 1e-12);
         DLIB_TEST(length(inv(tform)(z)) < 1e-12);
+
+        point_transform_affine tform2; 
+        tform = tform*tform2;// the default tform is the identity mapping so this shouldn't do anything different 
+        DLIB_TEST(length(tform(dlib::vector<double>())-z) < 1e-12);
+        DLIB_TEST(length(inv(tform)(z)) < 1e-12);
     }
 
 // ----------------------------------------------------------------------------------------