feat(sky): dim/brighten the moon highlight through its phase

assets/shaders/sky_frag.glsl

@@ -20,11 +20,14 @@ uniform vec4 skySettings;
 #define skyDaylightBrightness skySettings.z
 #define skyNightBrightness skySettings.w
 
+// 0.0 at new moon, 1.0 at full moon - see CelestialSystem#getMoonPhase(). Modulates the moon
+// highlight's peak brightness so the "moon" dims and brightens through its cycle (#94).
+uniform float moonPhaseIntensity = 1.0;
+
 const vec4 eyePos = vec4(0.0, 0.0, 0.0, 1.0);
 
 #define HIGHLIGHT_BLEND_START 0.1
 #define SUN_HIGHLIGHT_INTENSITY_FACTOR 1.0
-#define MOON_HIGHLIGHT_INTENSITY_FACTOR 1.0
 
 layout(location = 0) out vec4 outColor;
 
@@ -45,7 +48,7 @@ void main () {
 
     float moonHighlight = 0.0;
     if (negLDotV >= 0.0 && -l.y >= 0.0) {
-       moonHighlight = pow(negLDotV, moonExponent) * MOON_HIGHLIGHT_INTENSITY_FACTOR;
+       moonHighlight = pow(negLDotV, moonExponent) * moonPhaseIntensity;
     }
     if (-l.y < HIGHLIGHT_BLEND_START && -l.y >= 0.0) {
        moonHighlight *= 1.0 - (HIGHLIGHT_BLEND_START + l.y) / HIGHLIGHT_BLEND_START;

src/main/java/org/terasology/corerendering/rendering/dag/nodes/BackdropNode.java

@@ -166,6 +166,12 @@
         skyMaterial.setFloat("colorExp", backdropProvider.getColorExp(), true);
         skyMaterial.setFloat4("skySettings", sunExponent, moonExponent, skyDaylightBrightness, skyNightBrightness, true);
 
+        // Moon phase in [0, 1): 0/1 is new moon, 0.5 is full moon. Fold it into a triangle wave so
+        // the highlight peaks at full moon and fades to nothing at new moon, rather than jumping. #94
+        float moonPhase = backdropProvider.getMoonPhase();
+        float moonPhaseIntensity = 1.0f - Math.abs(moonPhase * 2.0f - 1.0f);
+        skyMaterial.setFloat("moonPhaseIntensity", moonPhaseIntensity, true);
+
         Camera camera = worldRenderer.getActiveCamera();
         skyMaterial.setMatrix4("projectionMatrix", camera.getProjectionMatrix());
         skyMaterial.setMatrix4("modelViewMatrix", camera.getNormViewMatrix());