From Wojciech Lewandowski, "I did few tests to see whether adding PreDraw callback would help us with SPI problems when using Viewer. Results were positive so I decided to give it a try and ask you to verify and maybe merge with existing codebase.

I added _preDrawCallback member and neccessary access methods plus modified osgUtil RenderStage.cpp to invoke it before all drawInner calls are made. I tried to maintain symmetry with postDrawCallback but you know better where is a proper place for this call ;-)
"

src/osg/Camera.cpp

@@ -52,6 +52,7 @@ Camera::Camera(const Camera& camera,const CopyOp& copyop):
     _renderTargetImplementation(camera._renderTargetImplementation),
     _renderTargetFallback(camera._renderTargetFallback),
     _bufferAttachmentMap(camera._bufferAttachmentMap),
+    _preDrawCallback(camera._preDrawCallback),
     _postDrawCallback(camera._postDrawCallback)
 {
     // need to copy/share graphics context?

src/osgUtil/RenderStage.cpp

@@ -864,6 +864,12 @@ void RenderStage::draw(osg::RenderInfo& renderInfo,RenderLeaf*& previous)
         
         if (!useThread) useContext->makeCurrent();
     }
+
+    if (_camera && _camera->getPreDrawCallback())
+    {
+        // if we have a camera with a post draw callback invoke it.
+        (*(_camera->getPreDrawCallback()))(*_camera);
+    }
     
     bool doCopyTexture = _texture.valid() ? 
                         (callingContext != useContext) :