backandfront projection option

config.cpp

@@ -2164,6 +2164,10 @@ EX void show3D() {
         XLAT("Rotate the camera. Can be used to obtain a first person perspective, "
         "or third person perspective when combined with Y shift.")
         );
+      dialog::extra_options = [] {
+        dialog::addBoolItem(XLAT("render behind the camera"), vpconf.back_and_front, 'R');
+        dialog::add_action([] { vpconf.back_and_front = !vpconf.back_and_front; });
+        };
       });
     }
   if(GDIM == 2) {

drawing.cpp

@@ -2277,6 +2277,16 @@ EX void set_vr_sphere() {
 EX void draw_main() {
   DEBBI(DF_GRAPH, ("draw_main"));
   
+  if(pconf.back_and_front == 1 && vid.consider_shader_projection) {
+    dynamicval<int> pa(pconf.back_and_front);
+    pconf.back_and_front = 0;
+    draw_main();
+    pconf.back_and_front = 2;
+    reset_projection();
+    draw_main();
+    return;
+    }
+
   set_vr_sphere();  
   
   if(sphere && GDIM == 3 && pmodel == mdPerspective && !stretch::in() && !ray::in_use) {

models.cpp

@@ -1040,6 +1040,8 @@ EX namespace models {
       param_f(p.xposition, sp+"xposition", 0);
       param_f(p.yposition, sp+"yposition", 0);
 
+      param_i(p.back_and_front, sp+"backandfront", 0);
+
       addsaver(p.alpha, sp+"projection", 1);
       param_custom(p.alpha, sp+"projection", menuitem_projection_distance, 'p')
       ->help_text = "projection distance|Gans Klein Poincare orthographic stereographic";

shaders.cpp

@@ -652,6 +652,8 @@ void display_data::set_projection(int ed, ld shift) {
     GLfloat sc = current_display->radius / (cd->ysize/2.);
     glhr::projection_multiply(glhr::frustum(cd->xsize / cd->ysize, 1));
     glhr::projection_multiply(glhr::scale(sc, -sc, -1));
+    if(pconf.back_and_front)
+      glhr::projection_multiply(glhr::scale(-1,-1,-1));
     u_alpha = true;
     }