models support flipped normals on double sided geometry

libraries/entities-renderer/src/paintStroke.slf

@@ -19,6 +19,8 @@
 <@include paintStroke.slh@>
 <$declarePolyLineBuffers()$>
 
+<@include CullFace.slh@>
+
 LAYOUT(binding=0) uniform sampler2D _texture;
 
 <@if not HIFI_USE_FORWARD@>
@@ -35,9 +37,9 @@ void main(void) {
 
 <@if not HIFI_USE_FORWARD@>
     <@if HIFI_USE_TRANSLUCENT@>
-        packDeferredFragmentTranslucent((2.0 * float(gl_FrontFacing) - 1.0) * _normalWS, texel.a, texel.rgb, DEFAULT_ROUGHNESS);
+        packDeferredFragmentTranslucent(evalFrontOrBackFaceNormal(_normalWS), texel.a, texel.rgb, DEFAULT_ROUGHNESS);
     <@else@>
-        packDeferredFragmentUnlit((2.0 * float(gl_FrontFacing) - 1.0) * _normalWS, texel.a, texel.rgb);
+        packDeferredFragmentUnlit(evalFrontOrBackFaceNormal(_normalWS), texel.a, texel.rgb);
     <@endif@>
 <@else@>
     _fragColor0 = texel;

libraries/graphics/src/graphics/Material.slh

@@ -13,7 +13,6 @@
 
 <@include graphics/ShaderConstants.h@>
 
-
 const int MAX_TEXCOORDS = 2;
 
 struct TexMapArray { 

libraries/render-utils/src/CullFace.slh

@@ -0,0 +1,20 @@
+<!
+//  CullFace.slh
+//  libraries/render-utils/src
+//
+//  Created by HifiExperiments on 4/16/2020.
+//  Copyright 2020 Vircadia
+//
+//  Distributed under the Apache License, Version 2.0.
+//  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
+!>
+<@if not CULL_FACE_SLH@>
+<@def CULL_FACE_SLH@>
+
+// NOTE: this calculation happens once per fragment.  this could be optimized by creating different shaders (via defines)
+// for front, back, and double-sided.  for front/back-only triangles, this will simplify to always 1 or always -1
+vec3 evalFrontOrBackFaceNormal(vec3 normal) {
+    return (2.0 * float(gl_FrontFacing) - 1.0) * normal;
+}
+
+<@endif@>

libraries/render-utils/src/model.slf

@@ -13,6 +13,7 @@
 <@include graphics/Material.slh@>
 <@include graphics/MaterialTextures.slh@>
 <@include render-utils/ShaderConstants.h@>
+<@include CullFace.slh@>
 
 <@if not HIFI_USE_SHADOW@>
     <@if HIFI_USE_FORWARD or HIFI_USE_TRANSLUCENT@>
@@ -129,7 +130,7 @@ void main(void) {
             _fragColor0 = vec4(albedo * isUnlitEnabled(), opacity);
     <@else@>
             packDeferredFragmentUnlit(
-                normalize(_normalWS),
+                evalFrontOrBackFaceNormal(normalize(_normalWS)),
                 opacity,
                 albedo * isUnlitEnabled());
     <@endif@>
@@ -195,7 +196,7 @@ void main(void) {
     <@else@>
             vec3 fragNormalWS = _normalWS;
     <@endif@>
-        fragNormalWS = normalize(fragNormalWS);
+        fragNormalWS = evalFrontOrBackFaceNormal(normalize(fragNormalWS));
 
     <@if HIFI_USE_FORWARD@>
         TransformCamera cam = getTransformCamera();

libraries/render-utils/src/simple.slf

@@ -13,6 +13,7 @@
 <@include gpu/Color.slh@>
 <@include DefaultMaterials.slh@>
 <@include render-utils/ShaderConstants.h@>
+<@include CullFace.slh@>
 
 <@if HIFI_USE_FORWARD or HIFI_USE_TRANSLUCENT@>
     <@if not HIFI_USE_UNLIT@>
@@ -94,7 +95,7 @@ void main(void) {
             1.0,
             DEFAULT_OCCLUSION,
             fragPosition,
-            normalize(_normalWS) * (2.0 * float(gl_FrontFacing) - 1.0),
+            evalFrontOrBackFaceNormal(normalize(_normalWS)),
             texel.rgb,
             fresnel,
             metallic,
@@ -111,7 +112,7 @@ void main(void) {
             1.0,
             DEFAULT_OCCLUSION,
             fragPosition,
-            normalize(_normalWS) * (2.0 * float(gl_FrontFacing) - 1.0),
+            evalFrontOrBackFaceNormal(normalize(_normalWS)),
             texel.rgb,
             fresnel,
             metallic,
@@ -119,7 +120,7 @@ void main(void) {
             texel.a);
     <@else@>
         packDeferredFragment(
-            normalize(_normalWS) * (2.0 * float(gl_FrontFacing) - 1.0),
+            evalFrontOrBackFaceNormal(normalize(_normalWS)),
             1.0,
             texel.rgb,
             DEFAULT_ROUGHNESS,
@@ -141,7 +142,7 @@ void main(void) {
             , texel.a);
     <@else@>
         packDeferredFragmentUnlit(
-            normalize(_normalWS) * (2.0 * float(gl_FrontFacing) - 1.0),
+            evalFrontOrBackFaceNormal(normalize(_normalWS)),
             1.0,
             texel.rgb
         <@if HIFI_USE_FADE@>

libraries/render-utils/src/simple_procedural.slf

@@ -24,6 +24,7 @@
 <@include gpu/Transform.slh@>
 <$declareStandardCameraTransform()$>
 
+<@include CullFace.slh@>
 <@include render-utils/ShaderConstants.h@>
 
 layout(location=RENDER_UTILS_ATTR_POSITION_MS) in vec4 _positionMS;
@@ -66,7 +67,7 @@ float getProceduralFragmentWithPosition(inout ProceduralFragmentWithPosition pro
 
 #line 2030
 void main(void) {
-    vec3 normal = normalize(_normalWS.xyz) * (2.0 * float(gl_FrontFacing) - 1.0);
+    vec3 normal = evalFrontOrBackFaceNormal(normalize(_normalWS.xyz));
     vec3 diffuse = _color.rgb;
     vec3 fresnel = DEFAULT_FRESNEL;
     float roughness = DEFAULT_ROUGHNESS;