Make shadow debug HUD thread-safe

* Double buffer the frustum uniforms.
* Don't mess with the debug geometry's StateSet.
* Change two-element vectors to arrays so the size is explicit.

components/sceneutil/mwshadowtechnique.cpp

@@ -3104,12 +3104,12 @@ SceneUtil::MWShadowTechnique::DebugHUD::DebugHUD(int numberOfShadowMapsPerLight)
     fragmentShader = new osg::Shader(osg::Shader::FRAGMENT, debugFrustumFragmentShaderSource);
     frustumProgram->addShader(fragmentShader);
 
-    for (int i = 0; i < 2; ++i)
+    for (auto& frustumGeometry : mFrustumGeometries)
     {
-        mFrustumGeometries.emplace_back(new osg::Geometry());
-        mFrustumGeometries[i]->setCullingActive(false);
+        frustumGeometry = new osg::Geometry();
+        frustumGeometry->setCullingActive(false);
 
-        mFrustumGeometries[i]->getOrCreateStateSet()->setAttributeAndModes(frustumProgram, osg::StateAttribute::ON);
+        frustumGeometry->getOrCreateStateSet()->setAttributeAndModes(frustumProgram, osg::StateAttribute::ON);
     }
 
     osg::ref_ptr<osg::DrawElementsUShort> frustumDrawElements = new osg::DrawElementsUShort(osg::PrimitiveSet::LINE_STRIP);
@@ -3145,12 +3145,14 @@ void SceneUtil::MWShadowTechnique::DebugHUD::draw(osg::ref_ptr<osg::Texture2D> t
     // It might be possible to change shadow settings at runtime
     if (shadowMapNumber > mDebugCameras.size())
         addAnotherShadowMap();
-
-    mFrustumUniforms[shadowMapNumber]->set(matrix);
     
-    osg::ref_ptr<osg::StateSet> stateSet = mDebugGeometry[shadowMapNumber]->getOrCreateStateSet();
+    osg::ref_ptr<osg::StateSet> stateSet = new osg::StateSet();
     stateSet->setTextureAttributeAndModes(sDebugTextureUnit, texture, osg::StateAttribute::ON);
 
+    auto frustumUniform = mFrustumUniforms[cv.getTraversalNumber() % 2][shadowMapNumber];
+    frustumUniform->set(matrix);
+    stateSet->addUniform(frustumUniform);
+
     // Some of these calls may be superfluous.
     unsigned int traversalMask = cv.getTraversalMask();
     cv.setTraversalMask(mDebugGeometry[shadowMapNumber]->getNodeMask());
@@ -3205,6 +3207,6 @@ void SceneUtil::MWShadowTechnique::DebugHUD::addAnotherShadowMap()
     mFrustumTransforms[shadowMapNumber]->setCullingActive(false);
     mDebugCameras[shadowMapNumber]->addChild(mFrustumTransforms[shadowMapNumber]);
 
-    mFrustumUniforms.push_back(new osg::Uniform(osg::Uniform::FLOAT_MAT4, "transform"));
-    mFrustumTransforms[shadowMapNumber]->getOrCreateStateSet()->addUniform(mFrustumUniforms[shadowMapNumber]);
+    for(auto& uniformVector : mFrustumUniforms)
+        uniformVector.push_back(new osg::Uniform(osg::Uniform::FLOAT_MAT4, "transform"));
 }

components/sceneutil/mwshadowtechnique.hpp

@@ -19,6 +19,7 @@
 #ifndef COMPONENTS_SCENEUTIL_MWSHADOWTECHNIQUE_H
 #define COMPONENTS_SCENEUTIL_MWSHADOWTECHNIQUE_H 1
 
+#include <array>
 #include <mutex>
 
 #include <osg/Camera>
@@ -282,8 +283,8 @@ namespace SceneUtil {
             osg::ref_ptr<osg::Program> mDebugProgram;
             std::vector<osg::ref_ptr<osg::Node>> mDebugGeometry;
             std::vector<osg::ref_ptr<osg::Group>> mFrustumTransforms;
-            std::vector<osg::ref_ptr<osg::Uniform>> mFrustumUniforms;
-            std::vector<osg::ref_ptr<osg::Geometry>> mFrustumGeometries;
+            std::array<std::vector<osg::ref_ptr<osg::Uniform>>, 2> mFrustumUniforms;
+            std::array<osg::ref_ptr<osg::Geometry>, 2> mFrustumGeometries;
         };
 
         osg::ref_ptr<DebugHUD>                  _debugHud;