This provides the following enhancements & bug fixes

- Fix the chequer-board bug.
- Add proper cloud coverage function - so scattered clouds are now truly scattered.
- Add real-time control for visibility range.
- Use a limited set of clouds rather than generating a completely new Geode for each cloud. This saves sorting and display time.
- Add controls to Rendering dialog to allow fine-tuning of the number of sprites, cloud visibility and the number of different types of cloud.
- Add some variance to the sort back-off to avoid all clouds being sorted at the same time.
- Pack attributes into vectors for performance
- Re-order the cloud type determination code so that if a cloud layer could either be stratus or cumulus, cumulus is used.
- Lowered the cloud level in the standard cloud configuration slightly so a cumulus layer is generated rather than stratus.

These last two mean that you should see some 3D cumuli if disabling real weather fetch.

My thanks to Yon Uriarte for his help with performance work.

src/Environment/environment_mgr.cxx

@@ -188,6 +188,12 @@ FGEnvironmentMgr::bind ()
   fgTie("/sim/rendering/clouds3d-density", thesky,
 	  &SGSky::get_3dCloudDensity,
 	  &SGSky::set_3dCloudDensity);
+  fgTie("/sim/rendering/clouds3d-vis-range", thesky,
+        &SGSky::get_3dCloudVisRange,
+        &SGSky::set_3dCloudVisRange);
+  fgTie("/sim/rendering/clouds3d-num-flavours", thesky,
+        &SGSky::get_3dCloudNumFlavours,
+        &SGSky::set_3dCloudNumFlavours);
   fgTie("/sim/rendering/precipitation-enable", &sgEnviro,
 	  &SGEnviro::get_precipitation_enable_state, 
 	  &SGEnviro::set_precipitation_enable_state);
@@ -238,8 +244,7 @@ FGEnvironmentMgr::unbind ()
   fgUntie("/sim/rendering/clouds3d-enable");
   fgUntie("/sim/rendering/clouds3d-vis-range");
   fgUntie("/sim/rendering/clouds3d-density");
-  fgUntie("/sim/rendering/clouds3d-cache-size");
-  fgUntie("/sim/rendering/clouds3d-cache-resolution");
+  fgUntie("/sim/rendering/clouds3d-num-flavours");
   fgUntie("/sim/rendering/precipitation-enable");
   fgUntie("/environment/rebuild-layers");
   fgUntie("/sim/rendering/lightning-enable");

src/Environment/fgclouds.cxx

@@ -130,7 +130,8 @@ void FGClouds::buildCloud(SGPropertyNode *cloud_def_root, SGPropertyNode  *box_d
                         string texture = cld_def->getStringValue("texture", "cl_cumulus.rgb");
           
                         SGNewCloud *cld = 
-                                new SGNewCloud(texture_root, 
+                                new SGNewCloud(type,
+                                               texture_root, 
                                                texture, 
                                                min_width, 
                                                max_width, 
@@ -165,6 +166,7 @@ void FGClouds::buildLayer(int iLayer, const string& name, double alt, double cov
         SGCloudField *layer = thesky->get_cloud_layer(iLayer)->get_layer3D();
 
 	layer->clear();
+        
 	// when we don't generate clouds the layer is rendered in 2D
 	if( coverage == 0.0 )
 		return;
@@ -210,7 +212,6 @@ void FGClouds::buildLayer(int iLayer, const string& name, double alt, double cov
 		}
 	}
 	totalCount = 1.0 / totalCount;
-	double currCoverage = 0.0;
 
 	for(double px = 0.0; px < SGCloudField::fieldSize; px += grid_x_size) {
 		for(double py = 0.0; py < SGCloudField::fieldSize; py += grid_y_size) {
@@ -218,16 +219,11 @@ void FGClouds::buildLayer(int iLayer, const string& name, double alt, double cov
 			double y = py + grid_y_rand * (sg_random() - 0.5) - (SGCloudField::fieldSize / 2.0);
 			double z = alt + grid_z_rand * (sg_random() - 0.5);
 			double choice = sg_random();
-			currCoverage += coverage;
-			if( currCoverage < 1.0 )
-				continue;
-			currCoverage -= 1.0;
 
 			for(int i = 0; i < CloudVarietyCount ; i ++) {
 				choice -= tCloudVariety[i].count * totalCount;
 				if( choice <= 0.0 ) {
 					sgVec3 pos={x,z,y};
-
                                         buildCloud(cloud_def_root, box_def_root, tCloudVariety[i].name, pos, layer);
 					break;
 				}
@@ -235,7 +231,9 @@ void FGClouds::buildLayer(int iLayer, const string& name, double alt, double cov
 		}
 	}
 
-        // Now we've built any clouds, enable them
+        // Now we've built any clouds, enable them and set the density (coverage)
+        layer->setCoverage(coverage);
+        layer->applyCoverage();
         thesky->get_cloud_layer(iLayer)->set_enable3dClouds(clouds_3d_enabled);
 }
 
@@ -263,7 +261,8 @@ void FGClouds::buildMETAR(void) {
 
 		double alt_ft = cloud_root->getDoubleValue("elevation-ft");
 		double alt_m = alt_ft * SG_FEET_TO_METER;
-		string coverage = cloud_root->getStringValue("coverage");
+                printf("Alt m: %.0f\n", alt_m);
+                string coverage = cloud_root->getStringValue("coverage");
 		double coverage_norm = 0.0;
 		if( coverage == "few" )
 			coverage_norm = 2.0/8.0;	// <1-2
@@ -287,13 +286,13 @@ void FGClouds::buildMETAR(void) {
 				layer_type = "ns";
 		} else {
 //			layer_type = "st|cu|cb|sc";
-			// +/- 20% from stratus probable base
-			if( stratus_base * 0.80 < alt_m && stratus_base * 1.40 > alt_m )
-				layer_type = "st";
-			// +/- 20% from cumulus probable base
-			else if( cumulus_base * 0.80 < alt_m && cumulus_base * 1.20 > alt_m )
-				layer_type = "cu";
-			else {
+                        if( cumulus_base * 0.80 < alt_m && cumulus_base * 1.20 > alt_m ) {
+                                // +/- 20% from cumulus probable base
+                                layer_type = "cu";
+                        } else if( stratus_base * 0.80 < alt_m && stratus_base * 1.40 > alt_m ) {
+                                // +/- 20% from stratus probable base
+                                layer_type = "st";
+                        } else {
 				// above formulae is far from perfect
 				if ( alt_ft < 2000 )
 					layer_type = "st";
@@ -444,12 +443,12 @@ void FGClouds::buildScenario( const string& scenario ) {
 	string station = fgGetString("/environment/metar/station-id", "XXXX");
 
 	// fetch station elevation if exists
-    if( station == "XXXX" )
-        station_elevation_ft = fgGetDouble("/position/ground-elev-m", 0.0);
-    else {
-        const FGAirport* a = globals->get_airports()->search( station );
-        station_elevation_ft = (a ? a->getElevation() : 0.0);
-    }
+        if( station == "XXXX" )
+            station_elevation_ft = fgGetDouble("/position/ground-elev-m", 0.0);
+        else {
+            const FGAirport* a = globals->get_airports()->search( station );
+            station_elevation_ft = (a ? a->getElevation() : 0.0);
+        }
 
 	for(int iLayer = 0 ; iLayer < thesky->get_cloud_layer_count(); iLayer++) {
             thesky->get_cloud_layer(iLayer)
@@ -461,7 +460,7 @@ void FGClouds::buildScenario( const string& scenario ) {
 	if( scenario == "Fair weather" ) {
 		fakeMetar = "15003KT 12SM SCT033 FEW200 20/08 Q1015 NOSIG";
 		setLayer(0, 3300.0, "scattered", "cu");
-	} else if( scenario == "Thunderstorm" ) {
+        } else if( scenario == "Thunderstorm" ) {
 		fakeMetar = "15012KT 08SM TSRA SCT040 BKN070 20/12 Q0995";
 		setLayer(0, 4000.0, "scattered", "cb");
 		setLayer(1, 7000.0, "scattered", "ns");
@@ -487,8 +486,6 @@ void FGClouds::build() {
         if(!rebuild_required && (scenario == last_scenario))
             return;
         
-        
-        
         if( last_scenario == "none" ) {
         // save clouds and weather conditions
             SGPropertyNode *param = fgGetNode("/environment/config", true);
@@ -534,8 +531,7 @@ void FGClouds::build() {
 
 void FGClouds::set_3dClouds(bool enable)
 {
-    if (enable != clouds_3d_enabled)
-    {
+    if (enable != clouds_3d_enabled) {
         clouds_3d_enabled = enable;
 
         for(int iLayer = 0 ; iLayer < thesky->get_cloud_layer_count(); iLayer++) {
@@ -546,8 +542,7 @@ void FGClouds::set_3dClouds(bool enable)
             }
         }
 
-        if (enable)
-        {
+        if (enable) {
             rebuild_required = true;
             build();
         }