re-use bzzts waterplane overlay for debug; makes it easier to test. To use this, set the env OPENMW_WATER_CULLING_DEBUG=1; You will see blue rectangles corresponding to water outlines. Once there are no more outlines, water is culled. You can further see this by pressing F3 3 times to check the the amount of quads. Before culling it should be around 1600, after culling it should drop to 0.

components/terrain/chunkmanager.cpp

@@ -227,6 +227,8 @@ osg::ref_ptr<osg::Node> ChunkManager::createChunk(float chunkSize, const osg::Ve
     transform->addChild(geometry);
     transform->getBound();
 
+    geometry->setupWaterBoundingBox(-1, chunkSize * mStorage->getCellWorldSize() / numVerts);
+
     if (mSceneManager->getIncrementalCompileOperation())
     {
         mSceneManager->getIncrementalCompileOperation()->add(geometry);

components/terrain/quadtreeworld.cpp

@@ -1,6 +1,8 @@
 #include "quadtreeworld.hpp"
 
 #include <osgUtil/CullVisitor>
+#include <osg/ShapeDrawable>
+#include <osg/PolygonMode>
 
 #include <limits>
 #include <sstream>
@@ -13,6 +15,7 @@
 #include "viewdata.hpp"
 #include "chunkmanager.hpp"
 #include "compositemaprenderer.hpp"
+#include "terraindrawable.hpp"
 
 namespace
 {
@@ -323,9 +326,8 @@ void updateWaterCullingView(HeightCullCallback* callback, ViewData* vd, osgUtil:
     for (unsigned int i=0; i<vd->getNumEntries(); ++i)
     {
         ViewData::Entry& entry = vd->getEntry(i);
-        osg::BoundingBox bb = static_cast<osg::Drawable*>(entry.mRenderingNode->asGroup()->getChild(0))->getBoundingBox();
-        float minZ = bb._min.z();
-        if (minZ > highZ)
+        osg::BoundingBox bb = static_cast<TerrainDrawable*>(entry.mRenderingNode->asGroup()->getChild(0))->getWaterBoundingBox();
+        if (!bb.valid())
             continue;
         osg::Vec3f ofs (entry.mNode->getCenter().x()*cellworldsize, entry.mNode->getCenter().y()*cellworldsize, 0.f);
         bb._min += ofs; bb._max += ofs;
@@ -333,8 +335,30 @@ void updateWaterCullingView(HeightCullCallback* callback, ViewData* vd, osgUtil:
         bb._max.z() = highZ;
         if (cv->isCulled(bb))
             continue;
-        lowZ = minZ;
-        break;
+        lowZ = bb._min.z();
+
+        static bool debug = getenv("OPENMW_WATER_CULLING_DEBUG") != nullptr;
+        if (!debug)
+            break;
+        osg::Box* b = new osg::Box;
+        b->set(bb.center(), bb._max - bb.center());
+        osg::ShapeDrawable* drw = new osg::ShapeDrawable(b);
+        static osg::ref_ptr<osg::StateSet> stateset = nullptr;
+        if (!stateset)
+        {
+            stateset = new osg::StateSet;
+            stateset->setMode(GL_CULL_FACE, osg::StateAttribute::OFF);
+            stateset->setMode(GL_DEPTH_TEST, osg::StateAttribute::OFF);
+            stateset->setAttributeAndModes(new osg::PolygonMode(osg::PolygonMode::FRONT_AND_BACK, osg::PolygonMode::LINE), osg::StateAttribute::ON);
+            osg::Material* m = new osg::Material;
+            m->setEmission(osg::Material::FRONT_AND_BACK, osg::Vec4f(0,0,1,1));
+            m->setDiffuse(osg::Material::FRONT_AND_BACK, osg::Vec4f(0,0,0,1));
+            m->setAmbient(osg::Material::FRONT_AND_BACK, osg::Vec4f(0,0,0,1));
+            stateset->setAttributeAndModes(m, osg::StateAttribute::ON);
+            stateset->setRenderBinDetails(100,"RenderBin");
+        }
+        drw->setStateSet(stateset);
+        drw->accept(*cv);
     }
     callback->setLowZ(lowZ);
     cv->popCurrentMask();

components/terrain/terraindrawable.cpp

@@ -10,6 +10,16 @@
 namespace Terrain
 {
 
+TerrainDrawable::TerrainDrawable()
+{
+
+}
+
+TerrainDrawable::~TerrainDrawable()
+{
+
+}
+
 TerrainDrawable::TerrainDrawable(const TerrainDrawable &copy, const osg::CopyOp &copyop)
     : osg::Geometry(copy, copyop)
     , mPasses(copy.mPasses)
@@ -118,6 +128,25 @@ void TerrainDrawable::setLightListCallback(SceneUtil::LightListCallback *lightLi
     mLightListCallback = lightListCallback;
 }
 
+void TerrainDrawable::setupWaterBoundingBox(float waterheight, float margin)
+{
+    osg::Vec3Array* vertices = static_cast<osg::Vec3Array*>(getVertexArray());
+    for (unsigned int i=0; i<vertices->size(); ++i)
+    {
+        const osg::Vec3f& vertex = (*vertices)[i];
+        if (vertex.z() <= waterheight)
+            mWaterBoundingBox.expandBy(vertex);
+    }
+    if (mWaterBoundingBox.valid())
+    {
+        const osg::BoundingBox& bb = getBoundingBox();
+        mWaterBoundingBox.xMin() = std::max(bb.xMin(), mWaterBoundingBox.xMin() - margin);
+        mWaterBoundingBox.yMin() = std::max(bb.yMin(), mWaterBoundingBox.yMin() - margin);
+        mWaterBoundingBox.xMax() = std::min(bb.xMax(), mWaterBoundingBox.xMax() + margin);
+        mWaterBoundingBox.xMax() = std::min(bb.xMax(), mWaterBoundingBox.xMax() + margin);
+    }
+}
+
 void TerrainDrawable::compileGLObjects(osg::RenderInfo &renderInfo) const
 {
     for (PassVector::const_iterator it = mPasses.begin(); it != mPasses.end(); ++it)

components/terrain/terraindrawable.hpp

@@ -36,8 +36,8 @@ namespace Terrain
         virtual const char* className() const { return "TerrainDrawable"; }
         virtual const char* libraryName() const { return "Terrain"; }
 
-        TerrainDrawable() = default;
-        ~TerrainDrawable() = default;
+        TerrainDrawable();
+        ~TerrainDrawable(); // has to be defined in the cpp file because we only forward declared some members.
         TerrainDrawable(const TerrainDrawable& copy, const osg::CopyOp& copyop);
 
         virtual void accept(osg::NodeVisitor &nv);
@@ -52,10 +52,14 @@ namespace Terrain
 
         virtual void compileGLObjects(osg::RenderInfo& renderInfo) const;
 
+        void setupWaterBoundingBox(float waterheight, float margin);
+        const osg::BoundingBox& getWaterBoundingBox() const { return mWaterBoundingBox; }
+
         void setCompositeMap(CompositeMap* map) { mCompositeMap = map; }
         void setCompositeMapRenderer(CompositeMapRenderer* renderer) { mCompositeMapRenderer = renderer; }
 
     private:
+        osg::BoundingBox mWaterBoundingBox;
         PassVector mPasses;
 
         osg::ref_ptr<osg::ClusterCullingCallback> mClusterCullingCallback;