Fix medium layer base weight in testrender

src/testrender/shading.cpp

@@ -1319,7 +1319,7 @@ process_medium_closure(const ShaderGlobalsType& sg, float path_roughness,
                                        0.f, 1.f));
             closure                   = params->top;
             ptr_stack[stack_idx]      = params->base;
-            weight_stack[stack_idx++] = weight * base_w;
+            weight_stack[stack_idx++] = base_w;
             break;
         }
         case MX_ANISOTROPIC_VDF_ID: {

testsuite/render-mx-medium-vdf/medium_layer_weight.osl

@@ -0,0 +1,16 @@
+// Copyright Contributors to the Open Shading Language project.
+// SPDX-License-Identifier: BSD-3-Clause
+// https://github.com/AcademySoftwareFoundation/OpenShadingLanguage
+
+surface medium_layer_weight(int layered = 0)
+{
+    closure color medium
+        = transparent() + anisotropic_vdf(color(0), color(2, 4, 8), 0);
+    // Keep the layer closure while making its top response negligible.
+    closure color negligible_top = 1.0e-6 * reflection(N, 1.5);
+
+    if (layered)
+        Ci = 0.5 * layer(negligible_top, medium);
+    else
+        Ci = 0.5 * medium;
+}

testsuite/render-mx-medium-vdf/run.py

@@ -7,5 +7,7 @@
 failthresh = 0.01
 failpercent = 1
 
-outputs = [ "out.exr" ]
+outputs = [ "out.exr", "out_layer_weight.exr" ]
 command = testrender("-v -r 98 98 -aa 32 scene.xml out.exr")
+command += testrender(
+    "-r 160 80 -aa 32 scene_layer_weight.xml out_layer_weight.exr")

testsuite/render-mx-medium-vdf/scene_layer_weight.xml

@@ -0,0 +1,23 @@
+<World>
+    <Camera eye="0, 0, 10" look_at="0,0,0" fov="30" />
+
+    <ShaderGroup is_light="yes">
+        param color sky 1 1 1;
+        param color horizon 1 1 1;
+        param color sun 0 0 0;
+        shader envmap layer1;
+    </ShaderGroup>
+    <Background resolution="1024" />
+
+    <ShaderGroup>
+        param int layered 0;
+        shader medium_layer_weight layer1;
+    </ShaderGroup>
+    <Sphere center="-1.1,0,0" radius="1" />
+
+    <ShaderGroup>
+        param int layered 1;
+        shader medium_layer_weight layer1;
+    </ShaderGroup>
+    <Sphere center="1.1,0,0" radius="1" />
+</World>