Fix flickering edges on iOS

Li: "rq * rq - qq might be negative, and produce nan in this case,
which got translated into the largest float point number to displayed
as white"

Author: 375gnu

src/celengine/shadermanager.cpp

@@ -1373,7 +1373,7 @@ AtmosphericEffects(const ShaderProperties& props)
     // Compute the intersection of the view direction and the cloud layer (currently assumed to be a sphere)
     source += "    float rq = dot(eyePosition, eyeDir);\n";
     source += "    float qq = dot(eyePosition, eyePosition) - atmosphereRadius.y;\n";
-    source += "    float d = sqrt(rq * rq - qq);\n";
+    source += "    float d = sqrt(max(rq * rq - qq, 0.0));\n";
     source += "    vec3 atmEnter = eyePosition + min(0.0, (-rq + d)) * eyeDir;\n";
     source += "    vec3 atmLeave = in_Position.xyz;\n";
 
@@ -1384,7 +1384,7 @@ AtmosphericEffects(const ShaderProperties& props)
     source += "    vec3 atmSamplePointSun = atmEnter * 0.5 + atmLeave * 0.5;\n";
     source += "    rq = dot(atmSamplePointSun, " + LightProperty(0, "direction") + ");\n";
     source += "    qq = dot(atmSamplePointSun, atmSamplePointSun) - atmosphereRadius.y;\n";
-    source += "    d = sqrt(rq * rq - qq);\n";
+    source += "    d = sqrt(max(rq * rq - qq, 0.0));\n";
     source += "    float distSun = -rq + d;\n";
     source += "    float distAtm = length(atmEnter - atmLeave);\n";
 
@@ -1465,7 +1465,7 @@ AtmosphericEffects(const ShaderProperties& props, unsigned int nSamples)
     // Compute the intersection of the view direction and the cloud layer (currently assumed to be a sphere)
     source += "    float rq = dot(eyePosition, eyeDir);\n";
     source += "    float qq = dot(eyePosition, eyePosition) - atmosphereRadius.y;\n";
-    source += "    float d = sqrt(rq * rq - qq);\n";
+    source += "    float d = sqrt(max(rq * rq - qq, 0.0));\n";
     source += "    vec3 atmEnter = eyePosition + min(0.0, (-rq + d)) * eyeDir;\n";
     source += "    vec3 atmLeave = in_Position.xyz;\n";
 
@@ -1480,7 +1480,7 @@ AtmosphericEffects(const ShaderProperties& props, unsigned int nSamples)
     // Compute the distance through the atmosphere from the sample point to the sun
     source += "        rq = dot(atmSamplePoint, " + LightProperty(0, "direction") + ");\n";
     source += "        qq = dot(atmSamplePoint, atmSamplePoint) - atmosphereRadius.y;\n";
-    source += "        d = sqrt(rq * rq - qq);\n";
+    source += "        d = sqrt(max(rq * rq - qq, 0.0));\n";
     source += "        float distSun = -rq + d;\n";
 
     // Compute the density of the atmosphere at the sample point; it falls off exponentially
@@ -1987,7 +1987,7 @@ ShaderManager::buildVertexShader(const ShaderProperties& props)
                 // Compute the intersection of the sun direction and the cloud layer (currently assumed to be a sphere)
                 source += "    float rq = dot(" + LightProperty(j, "direction") + ", in_Position.xyz);\n";
                 source += "    float qq = dot(in_Position.xyz, in_Position.xyz) - cloudHeight * cloudHeight;\n";
-                source += "    float d = sqrt(rq * rq - qq);\n";
+                source += "    float d = sqrt(max(rq * rq - qq, 0.0));\n";
                 source += "    vec3 cloudSpherePos = (in_Position.xyz + (-rq + d) * " + LightProperty(j, "direction") + ");\n";
                 //source += "    vec3 cloudSpherePos = in_Position.xyz;\n";