Hi, I was looking at the code in h323.cxx function "H323Connection::MergeCapabilities((unsigned sessionID, const H323Capability & local, H323Capability * remote)" OpalVideoFormat & remoteFormat = (OpalVideoFormat &)(remote->GetWritableMediaFormat()); const OpalMediaFormat & localFormat = local.GetMediaFormat(); if (remoteFormat.Merge(localFormat)) { This looks like it would call the "Merge" function of OpalVideoFormat but when debugging it uses the Merge function of the base class OpalMediaFormat. Looking through the code it seems that the class OpalVideoFormat is actually never instantiated. I was thinking of fixing this by making "GetWritableMediaFormat" virtual and reimplementing it in for example "H323VideoCapability" class to create a real OpalVideoFormat instance. I was wondering if anybody knows this code better and could tell if this is a good idea... Greetings Niek