diff --git a/resources/shaders/110/gouraud.fs b/resources/shaders/110/gouraud.fs
index 6f354ff9a6..7c90ee25d5 100644
--- a/resources/shaders/110/gouraud.fs
+++ b/resources/shaders/110/gouraud.fs
@@ -54,6 +54,11 @@ varying vec4 world_pos;
 varying float world_normal_z;
 varying vec3 eye_normal;
 
+vec3 getBackfaceColor(vec3 fill) {
+    float brightness = 0.2126 * fill.r + 0.7152 * fill.g + 0.0722 * fill.b;
+    return (brightness > 0.75) ? vec3(0.11, 0.165, 0.208) : vec3(0.988, 0.988, 0.988);
+}
+
 void main()
 {
     if (any(lessThan(clipping_planes_dots, ZERO)))
@@ -95,8 +100,11 @@ void main()
 	}
 	color.rgb = (any(lessThan(pv_check_min, ZERO)) || any(greaterThan(pv_check_max, ZERO))) ? mix(color.rgb, ZERO, 0.3333) : color.rgb;
 	
+    // Orca: add backface outline
+    if (!gl_FrontFacing)
+        gl_FragColor = vec4(getBackfaceColor(color.rgb), 1.0);
     //BBS: add outline_color
-    if (is_outline)
+    else if (is_outline)
         gl_FragColor = uniform_color;
 #ifdef ENABLE_ENVIRONMENT_MAP
     else if (use_environment_tex)
diff --git a/resources/shaders/140/gouraud.fs b/resources/shaders/140/gouraud.fs
index 84bce5c035..ece05fc985 100644
--- a/resources/shaders/140/gouraud.fs
+++ b/resources/shaders/140/gouraud.fs
@@ -56,6 +56,11 @@ in vec3 eye_normal;
 
 out vec4 out_color;
 
+vec3 getBackfaceColor(vec3 fill) {
+    float brightness = 0.2126 * fill.r + 0.7152 * fill.g + 0.0722 * fill.b;
+    return (brightness > 0.75) ? vec3(0.11, 0.165, 0.208) : vec3(0.988, 0.988, 0.988);
+}
+
 void main()
 {
     if (any(lessThan(clipping_planes_dots, ZERO)))
@@ -97,8 +102,11 @@ void main()
 	}
 	color.rgb = (any(lessThan(pv_check_min, ZERO)) || any(greaterThan(pv_check_max, ZERO))) ? mix(color.rgb, ZERO, 0.3333) : color.rgb;
 	
+    // Orca: add backface outline
+    if (!gl_FrontFacing)
+        out_color = vec4(getBackfaceColor(color.rgb), 1.0);
     //BBS: add outline_color
-    if (is_outline)
+    else if (is_outline)
         out_color = uniform_color;
 #ifdef ENABLE_ENVIRONMENT_MAP
     else if (use_environment_tex)
diff --git a/src/slic3r/GUI/3DScene.cpp b/src/slic3r/GUI/3DScene.cpp
index f407a999a6..0d00c8ccd9 100644
--- a/src/slic3r/GUI/3DScene.cpp
+++ b/src/slic3r/GUI/3DScene.cpp
@@ -472,10 +472,6 @@ void GLVolume::render_with_outline(const Transform3d &view_model_matrix)
 //BBS add render for simple case
 void GLVolume::simple_render(GLShaderProgram* shader, ModelObjectPtrs& model_objects, std::vector<ColorRGBA> extruder_colors)
 {
-    if (this->is_left_handed())
-        glFrontFace(GL_CW);
-    glsafe(::glCullFace(GL_BACK));
-
     bool color_volume = false;
     ModelObject* model_object = nullptr;
     ModelVolume* model_volume = nullptr;
@@ -504,6 +500,7 @@ void GLVolume::simple_render(GLShaderProgram* shader, ModelObjectPtrs& model_obj
         }
     } while (0);
 
+    auto r = [&]() {
     if (color_volume && !picking) {
         // when force_transparent, we need to keep the alpha
         if (force_native_color && render_color.is_transparent()) {
@@ -545,6 +542,29 @@ void GLVolume::simple_render(GLShaderProgram* shader, ModelObjectPtrs& model_obj
         else
             model.render(this->tverts_range);
     }
+    };
+
+    if (this->is_left_handed())
+        glFrontFace(GL_CW);
+
+    // Render front faces
+    glsafe(::glCullFace(GL_BACK));
+    r();
+
+    // Then render back faces in line mode to add an outline
+    GLboolean cull_face = GL_FALSE;
+    ::glGetBooleanv(GL_CULL_FACE, &cull_face);
+    glsafe(::glEnable(GL_CULL_FACE));
+    glsafe(::glCullFace(GL_FRONT));
+    glsafe(::glPolygonMode(GL_BACK, GL_LINE));
+    r();
+
+    // Reset mode
+    glsafe(::glPolygonMode(GL_BACK, GL_FILL));
+    glsafe(::glCullFace(GL_BACK));
+    if (!cull_face)
+        glsafe(::glDisable(GL_CULL_FACE));
+
     if (this->is_left_handed())
         glFrontFace(GL_CCW);
 }
@@ -854,6 +874,7 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType type, bool disab
     }
 
     glsafe(::glCullFace(GL_BACK));
+    glsafe(::glEnable(GL_CULL_FACE));
     if (disable_cullface)
         glsafe(::glDisable(GL_CULL_FACE));
 
diff --git a/src/slic3r/GUI/GCodeViewer.cpp b/src/slic3r/GUI/GCodeViewer.cpp
index aedc729bc0..2ef664c2b2 100644
--- a/src/slic3r/GUI/GCodeViewer.cpp
+++ b/src/slic3r/GUI/GCodeViewer.cpp
@@ -1247,6 +1247,7 @@ void GCodeViewer::render(int canvas_width, int canvas_height, int right_margin)
 
     glsafe(::glEnable(GL_DEPTH_TEST));
     render_shells();
+    glsafe(::glClear(GL_DEPTH_BUFFER_BIT)); // This makes sure toolpaths are on top of shells
     render_toolpaths();
     float legend_height = 0.0f;
     render_legend(legend_height, canvas_width, canvas_height, right_margin);
@@ -4030,7 +4031,7 @@ void GCodeViewer::render_shells()
     if (shader == nullptr)
         return;
 
-    glsafe(::glDepthMask(GL_FALSE));
+    //glsafe(::glDepthMask(GL_FALSE));
 
     shader->start_using();
     shader->set_uniform("emission_factor", 0.1f);
@@ -4039,7 +4040,7 @@ void GCodeViewer::render_shells()
     shader->set_uniform("emission_factor", 0.0f);
     shader->stop_using();
 
-    glsafe(::glDepthMask(GL_TRUE));
+    //glsafe(::glDepthMask(GL_TRUE));
 }
 
 //BBS