Frederic Bouvier:

Move the rendering stage for upper clouds from preDraw
 to drawUpperClouds. Rename postDraw to drawLowerClouds.

simgear/scene/sky/sky.cxx

@@ -168,15 +168,9 @@ bool SGSky::reposition( SGSkyState &st, double dt )
 
 // draw background portions of the sky ... do this before you draw the
 // rest of your scene.
-void SGSky::preDraw( float alt, float fog_exp2_density ) {
+void SGSky::preDraw( float alt ) {
     ssgCullAndDraw( pre_root );
 
-    // FIXME: This should not be needed, but at this time (08/15/2003)
-    //        certain NVidia drivers don't seem to implement
-    //        glPushAttrib(FG_FOG_BIT) properly. The result is that
-    //        there is not fog when looking at the sun.
-    glFogf ( GL_FOG_DENSITY, fog_exp2_density );
-
     	// if we are closer than this to a cloud layer, don't draw clouds
     static const float slop = 5.0;
     int i;
@@ -206,9 +200,17 @@ void SGSky::preDraw( float alt, float fog_exp2_density ) {
     {
        ++cur_layer_pos;
     }
+}
+
+void SGSky::drawUpperClouds( float fog_exp2_density ) {
+    // FIXME: This should not be needed, but at this time (08/15/2003)
+    //        certain NVidia drivers don't seem to implement
+    //        glPushAttrib(FG_FOG_BIT) properly. The result is that
+    //        there is not fog when looking at the sun.
+    glFogf ( GL_FOG_DENSITY, fog_exp2_density );
 
     // draw the cloud layers that are above us, top to bottom
-    for ( i = (int)cloud_layers.size() - 1; i >= cur_layer_pos; --i ) {
+    for ( int i = (int)cloud_layers.size() - 1; i >= cur_layer_pos; --i ) {
         if ( i != in_cloud ) {
             cloud_layers[i]->draw();
         }
@@ -218,10 +220,9 @@ void SGSky::preDraw( float alt, float fog_exp2_density ) {
 
 // draw translucent clouds ... do this after you've drawn all the
 // oapaque elements of your scene.
-void SGSky::postDraw( float alt ) {
+void SGSky::drawLowerClouds() {
 
     // draw the cloud layers that are below us, bottom to top
-
     for ( int i = 0; i < cur_layer_pos; ++i ) {
         if ( i != in_cloud ) {
             cloud_layers[i]->draw();

simgear/scene/sky/sky.hxx

@@ -143,22 +143,27 @@ typedef struct {
 
  * Rendering the Sky 
 
- * The sky is designed to be rendered in two stages. The first stage
+ * The sky is designed to be rendered in three stages. The first stage
  * renders the parts that form your back drop - the sky dome, the
  * stars and planets, the sun, and the moon.  These should be rendered
  * before the rest of your scene by calling the preDraw() method. The
- * second stage renders the clouds which are likely to be translucent
+ * second stage renders the clouds that are above the viewer. This stage 
- * (depending on type) and should be drawn after your scene has been
+ * is done before translucent objects in the main scene are drawn. It 
- * rendered.  Use the postDraw() method to draw the second stage of
+ * is seperated from the preDraw routine to enable to implement a 
- * the sky.
+ * multi passes technique and is located in the drawUpperClouds() method.
+ * The third stage renders the clouds that are below the viewer an which 
+ * are likely to be translucent (depending on type) and should be drawn 
+ * after your scene has been rendered.  Use the drawLowerClouds() method 
+ * to draw the second stage of the sky.
 
  * A typical application might do the following: 
 
- * <li> thesky->preDraw();
+ * <li> thesky->preDraw( my_altitude );
+ * <li> thesky->drawUpperClouds();
  * <li> ssgCullAndDraw ( myscene ) ;
- * <li> thesky->postDraw( my_altitude );
+ * <li> thesky->drawLowerClouds();
 
- * The current altitude in meters is passed to the postDraw() method
+ * The current altitude in meters is passed to the preDraw() method
  * so the clouds layers can be rendered correction from most distant
  * to closest.
 
@@ -329,15 +334,22 @@ public:
      * class description.
      * @param alt current altitude
      */
-    void preDraw( float alt, float fog_exp2_density );
+    void preDraw( float alt );
 
     /**
-     * Draw translucent clouds ... do this after you've drawn all the
+     * Draw upper translucent clouds ... do this before you've drawn 
-     * oapaque elements of your scene.  See discussion in detailed
+     * all the translucent elements of your scene.  See discussion in 
-     * class description.
+     * detailed class description.
-     * @param alt current altitude
+     * @param fog_exp2_density fog density of the current cloud layer
      */
-    void postDraw( float alt );
+    void drawUpperClouds( float fog_exp2_density );
+
+    /**
+     * Draw lower translucent clouds ... do this after you've drawn 
+     * all the opaque elements of your scene.  See discussion in detailed
+     * class description.
+     */
+    void drawLowerClouds();
 
     /** 
      * Specify the texture path (optional, defaults to current directory)