diff --git a/src/graphics/rtts.cpp b/src/graphics/rtts.cpp
index f83be6b13..590972bb8 100644
--- a/src/graphics/rtts.cpp
+++ b/src/graphics/rtts.cpp
@@ -56,6 +56,7 @@ RTT::RTT(size_t width, size_t height)
     m_RH_FBO = NULL;
     m_RSM = NULL;
     m_RH_FBO = NULL;
+    m_diffuse_coefficients_calculated = false;
     using namespace video;
     using namespace core;
 
@@ -294,7 +295,12 @@ void RTT::prepareRender(scene::ICameraSceneNode* camera)
 {
     irr_driver->setRTT(this);
     irr_driver->getSceneManager()->setActiveCamera(camera);
-    irr_driver->generateDiffuseCoefficients();
+
+    if (!m_diffuse_coefficients_calculated)
+    {
+        irr_driver->generateDiffuseCoefficients();
+        m_diffuse_coefficients_calculated = true;
+    }
 }
 
 FrameBuffer* RTT::render(scene::ICameraSceneNode* camera, float dt)
diff --git a/src/graphics/rtts.hpp b/src/graphics/rtts.hpp
index a37ba1df7..ecc85c5c1 100644
--- a/src/graphics/rtts.hpp
+++ b/src/graphics/rtts.hpp
@@ -62,6 +62,8 @@ private:
     int m_width;
     int m_height;
 
+    bool m_diffuse_coefficients_calculated;
+
     unsigned shadowColorTex, shadowNormalTex, shadowDepthTex;
     unsigned RSM_Color, RSM_Normal, RSM_Depth;
     unsigned RH_Red, RH_Green, RH_Blue;