diff --git a/geometry.cpp b/geometry.cpp
index a68dc452..450dc357 100644
--- a/geometry.cpp
+++ b/geometry.cpp
@@ -927,7 +927,7 @@ EX int last_texture_step;
 
 int ntimestamp;
 
-EX void check_cgi() {
+EX string cgi_string() {
   string s;
   auto V = [&] (string a, string b) { s += a; s += ": "; s += b; s += "; "; };
   V("GEO", its(int(geometry)));
@@ -977,6 +977,12 @@ EX void check_cgi() {
 
   V("LQ", its(vid.linequality));
   
+  return s;
+  }
+
+EX void check_cgi() {
+  string s = cgi_string();
+  
   cgip = &cgis[s];
   cgi.timestamp = ++ntimestamp;
   if(hybri) hybrid::underlying_cgip->timestamp = ntimestamp;