CS294-9 :: Fall 2003 vic and NAÏVE K. Mayer-Patel

CS294-9 :: Fall 2003 vic: Overview/Motivation Apply ALF principles to video conferencing Framework for composing applications Extensible UI Compression scheme

CS294-9 :: Fall 2003 Tools vs. Toolkits Composable tools better than API-based toolkits. –Small, highly optimized functional units. –Glued together with scripting. Application logic. –Video conferencing vic - video tool vat - audio tool conference bus - coordination and control

CS294-9 :: Fall 2003 Software Architecture C++ classes that passed where possible ADU’s Tcl used as scripting language to glue them together in various ways. See figure in paper.

CS294-9 :: Fall 2003 Conference Bus Key for scripting application semantics that cut across media types. –Synchronization –Floor control Typed messages broadcast onto communication channel –Used local loopback multicast –Tools responded to messages in well known and possibly configurable ways. –Application logic implemented as just another participant in this conversation.

CS294-9 :: Fall 2003 Are they right? Discussion question: –Do composable tools embody ALF principles better than toolkits?

CS294-9 :: Fall 2003 Conditional Replenishment Key to the compression scheme. Compare blocks to last time block was sent. –Uses subset of pixels for comparison. Why? Send only on “signficant” change. –Need a threshold for significant. Send some blocks if they get old enough regardless if there was change or not.

CS294-9 :: Fall 2003 CR Features Robustness to loss Localizes error Avoids persistent error Balances compression with CPU load –CR done in pixel domain before transform coding. –Doesn’t require decode operation on sender-side. –Are these design decisions still relevant?

CS294-9 :: Fall 2003 Intra-H.261 Adapted existing H.261 standard –All blocks intra coded –CR implemented via skipped blocks –RTP packetization standard already defined. –Intra-H.261 was technically still valid syntactically as a subset of H.261

CS294-9 :: Fall 2003 Intra-H.261 as ALF Decoder state checkpointed in RTP payload header. –Not mentioned in paper, but important to creating TDU independence Dependency chains limited to missing block updates. –Localizes error. CR timeout algorithm ensured eventual consistency. Although compression weakens, performance under loss improves.

CS294-9 :: Fall 2003 Weaknesses What are the papers main weaknesses?

CS294-9 :: Fall 2003 NAÏVE: Overview/Motivation Design new video codec with loss in mind. –Broadcast –Any subset of packets should work. –Support instantaneous changes in send rate. –Graceful degradation of quality given loss. –Rapid joins.

CS294-9 :: Fall 2003 Design Principles Globalness –Every packet reconstructs entire frame. –Every packet adds value to any previous packets from same frame. Independence –No packet depends on any other. Precludes inter-frame coding altogether.

CS294-9 :: Fall 2003 Algorithm Consider each image to be a multi-res pyramid. Each pixel at each resolution level given a confidence weight. Start with some set of samples at highest resolution. Push to create approximations of lower resolution levels. Pull to reconstruct representation of higher resolution levels.

CS294-9 :: Fall 2003 Sampling Break image into blocks. Each packet contains at least one sample from every block. Cn contain more samples from some blocks. Sample can come from levels 0, 1, or 2. Sampling pattern generated randomly. –Source has to communicate random number seed to dest in some manner. What are our choices?

CS294-9 :: Fall 2003 Temporal Locality Samples from past frames reused in reconstruction of subsequent frames. Special code word to avoid this (i.e., flush old samples). What does this imply about source content? In other words, what are they assuming?

CS294-9 :: Fall 2003 Performance Reconstruction results are impressive. Compression suffers –Nothing for free. SNR maintained well under loss.

CS294-9 :: Fall 2003 Weaknesses What are the main weaknesses of the paper?