Fixed aerial perspective being applied at wrong location in clouds

It was applied at the enter point of the walking algorithm, which
was the camera when it was inside a cloud layer.

Now it is applied at the first found segment, which is still not
optimal but better.

The bruneton model was also fixed to not produce black results for
aerial perspective exactly at the camera location.

src/interface/commandline/tests.cpp

@@ -210,6 +210,23 @@ static void testNearFrustum()
     startTestRender(&renderer, "near_frustum");
 }
 
+static void testCloudsNearGround()
+{
+    Scenery scenery;
+    scenery.autoPreset(8);
+    scenery.getAtmosphere()->setDayTime(6, 20);
+
+    SoftwareCanvasRenderer renderer(&scenery);
+    renderer.setSize(400, 300);
+    renderer.setQuality(0.3);
+
+    startTestRender(&renderer, "clouds_near_ground", 1);
+
+    scenery.getCamera()->strafeUp(6.0);
+    scenery.getCamera()->copy(renderer.render_camera);
+    startTestRender(&renderer, "clouds_near_ground", 2);
+}
+
 void runTestSuite()
 {
     testGroundShadowQuality();
@@ -217,4 +234,5 @@ void runTestSuite()
     testCloudQuality();
     testGodRays();
     testNearFrustum();
+    testCloudsNearGround();
 }

src/render/software/AtmosphereModelBruneton.cpp

@@ -1191,6 +1191,12 @@ AtmosphereResult AtmosphereModelBruneton::applyAerialPerspective(Vector3 locatio
     Vector3 sun_position = parent->getAtmosphereRenderer()->getSunDirection().scale(SUN_DISTANCE);
 
     Vector3 direction = location.sub(eye).scale(WORLD_SCALING);
+    double t = direction.getNorm();
+    if (t < 0.000001)
+    {
+        direction = parent->getCameraDirection(location).scale(0.001 * WORLD_SCALING);
+        t = direction.getNorm();
+    }
 
     Vector3 x = {0.0, Rg + WORKAROUND_OFFSET + eye.y * WORLD_SCALING, 0.0};
     Vector3 v = direction.normalize();
@@ -1203,7 +1209,6 @@ AtmosphereResult AtmosphereModelBruneton::applyAerialPerspective(Vector3 locatio
 
     double r = x.getNorm();
     double mu = x.dotProduct(v) / r;
-    double t = direction.getNorm();
 
     AtmosphereResult result;
     Vector3 attenuation;

src/render/software/CloudBasicLayerRenderer.cpp

@@ -11,6 +11,8 @@
 #include "SurfaceMaterial.h"
 #include "Logs.h"
 
+#include <cassert>
+
 struct CloudSegment
 {
     Vector3 start;
@@ -196,9 +198,16 @@ Color CloudBasicLayerRenderer::getColor(BaseCloudsModel *model, const Vector3 &e
         result.a += (1.0 - result.a) * ((inside_length - transparency_depth * 0.8) / (transparency_depth * 0.2));
     }
 
-    double a = result.a;
-    result = parent->getAtmosphereRenderer()->applyAerialPerspective(start, result).final;
-    result.a = a;
+    // Apply aerial perspective
+    if (result.a > 0.00001)
+    {
+        assert(segment_count > 0);
+
+        double a = result.a;
+        // TODO Don't apply it only at first segment
+        result = parent->getAtmosphereRenderer()->applyAerialPerspective(segments[0].start, result).final;
+        result.a = a;
+    }
 
     return result;
 }