Added STK atmosphere improvements

CHANGES.md

@@ -8,6 +8,7 @@ Change Log
 * Fixed issue where the `Model` glTF cache loses reference to the model's buffer data. [#5720](https://github.com/AnalyticalGraphicsInc/cesium/issues/5720)
 * Fixed some issues with `disableDepthTestDistance` [#5501](https://github.com/AnalyticalGraphicsInc/cesium/issues/5501) [#5331](https://github.com/AnalyticalGraphicsInc/cesium/issues/5331) [#5621](https://github.com/AnalyticalGraphicsInc/cesium/issues/5621)
 * Added several new Bing Maps styles: `CANVAS_DARK`, `CANVAS_LIGHT`, and `CANVAS_GRAY`.
+* Added small improvements to the atmosphere. [#5741](https://github.com/AnalyticalGraphicsInc/cesium/pull/5741)
 * Fixed a bug that caused imagery splitting to work incorrectly when CSS pixels were not equivalent to WebGL drawing buffer pixels, such as on high DPI displays in Microsoft Edge and Internet Explorer.
 * Added `Cesium3DTileset.loadJson` to support overriding the default tileset loading behavior. [#5685](https://github.com/AnalyticalGraphicsInc/cesium/pull/5685)
 

Source/Shaders/SkyAtmosphereFS.glsl

@@ -37,6 +37,8 @@
 uniform vec3 u_hsbShift; // Hue, saturation, brightness
 #endif
 
+uniform vec4 u_cameraAndRadiiAndDynamicAtmosphereColor; // Camera height, outer radius, inner radius, dynamic atmosphere color flag
+
 const float g = -0.95;
 const float g2 = g * g;
 
@@ -73,5 +75,12 @@ void main (void)
     l = min(l, czm_luminance(rgb));
 #endif
 
-    gl_FragColor = vec4(rgb, min(smoothstep(0.0, 0.1, l), 1.0) * smoothstep(0.0, 1.0, czm_morphTime));
+    // Alter alpha based on how close the viewer is to the ground (1.0 = on ground, 0.0 = at edge of atmosphere)
+    float atmosphereAlpha = clamp((u_cameraAndRadiiAndDynamicAtmosphereColor.y - u_cameraAndRadiiAndDynamicAtmosphereColor.x) / (u_cameraAndRadiiAndDynamicAtmosphereColor.y - u_cameraAndRadiiAndDynamicAtmosphereColor.z), 0.0, 1.0);
+
+    // Alter alpha based on time of day (0.0 = night , 1.0 = day)
+    float nightAlpha = (u_cameraAndRadiiAndDynamicAtmosphereColor.w > 0.0) ? clamp(dot(normalize(czm_viewerPositionWC), normalize(czm_sunPositionWC)), 0.0, 1.0) : 1.0;
+    atmosphereAlpha *= pow(nightAlpha, 0.5);
+
+    gl_FragColor = vec4(rgb, mix(rgb.b, 1.0, atmosphereAlpha) * smoothstep(0.0, 1.0, czm_morphTime));
 }

Source/Shaders/SkyAtmosphereVS.glsl

@@ -2,11 +2,11 @@
  * @license
  * Copyright (c) 2000-2005, Sean O'Neil ([email protected])
  * All rights reserved.
- * 
+ *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
- * 
+ *
  * * Redistributions of source code must retain the above copyright notice,
  *   this list of conditions and the following disclaimer.
  * * Redistributions in binary form must reproduce the above copyright notice,
@@ -15,7 +15,7 @@
  * * Neither the name of the project nor the names of its contributors may be
  *   used to endorse or promote products derived from this software without
  *   specific prior written permission.
- * 
+ *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
@@ -29,10 +29,10 @@
  *
  * Modifications made by Analytical Graphics, Inc.
  */
- 
+
  // Code:  http://sponeil.net/
  // GPU Gems 2 Article:  http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter16.html
-  
+
 attribute vec4 position;
 
 uniform vec4 u_cameraAndRadiiAndDynamicAtmosphereColor; // Camera height, outer radius, inner radius, dynamic atmosphere color flag
@@ -49,7 +49,7 @@ const vec3 InvWavelength = vec3(
     9.473284437923038, // Green = 1.0 / Math.pow(0.570, 4.0)
     19.643802610477206); // Blue = 1.0 / Math.pow(0.475, 4.0)
 const float rayleighScaleDepth = 0.25;
-          
+
 const int nSamples = 2;
 const float fSamples = 2.0;