From 0351120f0763b88677d20326cdcb31e2cc05e1fc Mon Sep 17 00:00:00 2001
From: Sean Lilley <lilleyse@gmail.com>
Date: Wed, 3 Mar 2021 21:09:07 -0500
Subject: [PATCH] Premultiplied alpha to fix colors for globe translucency

---
 Source/Scene/Vector3DTileClampedPolylines.js       | 2 +-
 Source/Shaders/Vector3DTileClampedPolylinesFS.glsl | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Source/Scene/Vector3DTileClampedPolylines.js b/Source/Scene/Vector3DTileClampedPolylines.js
index cad5b034e96a..a8074171d86a 100644
--- a/Source/Scene/Vector3DTileClampedPolylines.js
+++ b/Source/Scene/Vector3DTileClampedPolylines.js
@@ -478,7 +478,7 @@ function getRenderState(mask3DTiles) {
     cull: {
       enabled: true, // prevent double-draw. Geometry is "inverted" (reversed winding order) so we're drawing backfaces.
     },
-    blending: BlendingState.ALPHA_BLEND,
+    blending: BlendingState.PRE_MULTIPLIED_ALPHA_BLEND,
     depthMask: false,
     stencilTest: {
       enabled: mask3DTiles,
diff --git a/Source/Shaders/Vector3DTileClampedPolylinesFS.glsl b/Source/Shaders/Vector3DTileClampedPolylinesFS.glsl
index 444edf3e0c5c..2bb70a206ffb 100644
--- a/Source/Shaders/Vector3DTileClampedPolylinesFS.glsl
+++ b/Source/Shaders/Vector3DTileClampedPolylinesFS.glsl
@@ -48,5 +48,8 @@ void main()
     }
     gl_FragColor = u_highlightColor;
 
+    // Premultiply alpha. Required for classification primitives on translucent globe.
+    gl_FragColor.rgb *= gl_FragColor.a;
+
     czm_writeDepthClamp();
 }
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