Fix polyline clipping against the near plane.

CHANGES.md

@@ -6,6 +6,9 @@ Change Log
 ##### Additions :tada:
 * Added `Scene.invertClassificationSupported` for checking if invert classification is supported.
 
+##### Fixes :wrench:
+* Fixed issue causing polyline to look wavy depending on the position of the camera [#7209](https://github.com/AnalyticalGraphicsInc/cesium/pull/7209)
+
 ### 1.51 - 2018-11-01
 
 ##### Additions :tada:

Source/Scene/PolylineCollection.js

@@ -13,6 +13,7 @@ define([
         '../Core/destroyObject',
         '../Core/DeveloperError',
         '../Core/EncodedCartesian3',
+        '../Core/FeatureDetection',
         '../Core/IndexDatatype',
         '../Core/Intersect',
         '../Core/Math',
@@ -52,6 +53,7 @@ define([
         destroyObject,
         DeveloperError,
         EncodedCartesian3,
+        FeatureDetection,
         IndexDatatype,
         Intersect,
         CesiumMath,
@@ -1182,6 +1184,10 @@ define([
             defines.push('POLYLINE_DASH');
         }
 
+        if (!FeatureDetection.isInternetExplorer()) {
+            defines.push('CLIP_POLYLINE');
+        }
+
         var fs = new ShaderSource({
             defines : defines,
             sources : ['varying vec4 v_pickColor;\n', this.material.shaderSource, PolylineFS]

Source/Scene/PolylineColorAppearance.js

@@ -1,6 +1,7 @@
 define([
         '../Core/defaultValue',
         '../Core/defineProperties',
+        '../Core/FeatureDetection',
         '../Core/VertexFormat',
         '../Shaders/Appearances/PerInstanceFlatColorAppearanceFS',
         '../Shaders/Appearances/PolylineColorAppearanceVS',
@@ -9,6 +10,7 @@ define([
     ], function(
         defaultValue,
         defineProperties,
+        FeatureDetection,
         VertexFormat,
         PerInstanceFlatColorAppearanceFS,
         PolylineColorAppearanceVS,
@@ -19,6 +21,10 @@ define([
     var defaultVertexShaderSource = PolylineCommon + '\n' + PolylineColorAppearanceVS;
     var defaultFragmentShaderSource = PerInstanceFlatColorAppearanceFS;
 
+    if (!FeatureDetection.isInternetExplorer()) {
+        defaultVertexShaderSource = '#define CLIP_POLYLINE \n' + defaultVertexShaderSource;
+    }
+
     /**
      * An appearance for {@link GeometryInstance} instances with color attributes and
      * {@link PolylineGeometry} or {@link GroundPolylineGeometry}.

Source/Scene/PolylineMaterialAppearance.js

@@ -2,6 +2,7 @@ define([
         '../Core/defaultValue',
         '../Core/defined',
         '../Core/defineProperties',
+        '../Core/FeatureDetection',
         '../Core/VertexFormat',
         '../Shaders/Appearances/PolylineMaterialAppearanceVS',
         '../Shaders/PolylineCommon',
@@ -12,6 +13,7 @@ define([
         defaultValue,
         defined,
         defineProperties,
+        FeatureDetection,
         VertexFormat,
         PolylineMaterialAppearanceVS,
         PolylineCommon,
@@ -23,6 +25,10 @@ define([
     var defaultVertexShaderSource = PolylineCommon + '\n' + PolylineMaterialAppearanceVS;
     var defaultFragmentShaderSource = PolylineFS;
 
+    if (!FeatureDetection.isInternetExplorer()) {
+        defaultVertexShaderSource = '#define CLIP_POLYLINE \n' + defaultVertexShaderSource;
+    }
+
     /**
      * An appearance for {@link PolylineGeometry} that supports shading with materials.
      *

Source/Scene/Vector3DTilePolylines.js

@@ -8,6 +8,7 @@ define([
         '../Core/defineProperties',
         '../Core/destroyObject',
         '../Core/Ellipsoid',
+        '../Core/FeatureDetection',
         '../Core/IndexDatatype',
         '../Core/Matrix4',
         '../Core/Rectangle',
@@ -35,6 +36,7 @@ define([
         defineProperties,
         destroyObject,
         Ellipsoid,
+        FeatureDetection,
         IndexDatatype,
         Matrix4,
         Rectangle,
@@ -410,7 +412,7 @@ define([
         var fsSource = batchTable.getFragmentShaderCallback()(PolylineFS, false, undefined);
 
         var vs = new ShaderSource({
-            defines : ['VECTOR_TILE'],
+            defines : ['VECTOR_TILE', !FeatureDetection.isInternetExplorer() ? 'CLIP_POLYLINE' : ''],
             sources : [PolylineCommon, vsSource]
         });
         var fs = new ShaderSource({

Source/Shaders/PolylineCommon.glsl

@@ -76,11 +76,25 @@ vec4 getPolylineWindowCoordinatesEC(vec4 positionEC, vec4 prevEC, vec4 nextEC, f
     float expandWidth = width * 0.5;
     vec2 direction;
 
+#ifdef CLIP_POLYLINE
+    if (clipped)
+    {
+        if (prevEC.z - positionEC.z < 0.0)
+        {
+            direction = vec2(prevWC.y, -prevWC.x);
+        }
+        else
+        {
+            direction = vec2(-prevWC.y, prevWC.x);
+        }
+    }
+    else
+#endif
     if (czm_equalsEpsilon(prevEC.xyz - positionEC.xyz, vec3(0.0), czm_epsilon1) || czm_equalsEpsilon(prevWC, -nextWC, czm_epsilon1))
     {
         direction = vec2(-nextWC.y, nextWC.x);
     }
-    else if (czm_equalsEpsilon(nextEC.xyz - positionEC.xyz, vec3(0.0), czm_epsilon1) || clipped)
+    else if (czm_equalsEpsilon(nextEC.xyz - positionEC.xyz, vec3(0.0), czm_epsilon1))
     {
         direction = vec2(prevWC.y, -prevWC.x);
     }