Focusing attention at the level of objects and other salient image regions.
Also addressing the data issue, but in a completely different way, Vittorio Ferrari gave an interesting keynote at the ImageNet and COCO Visual Recognition Workshop on learning object detectors without bounding boxes. Suppose you have a fixed budget for data annotations. How should you spend it? It turns out that it is much more efficient to use human’s to verify bounding boxes, rather than to draw them.
In recent years, we have seen binary codes used successfully for both low-level image descriptors (e.g. FREAK, BRIEF) and high-level semantic image representations (e.g. binary hashing for image retrieval). Binary codes have the advantage of maximising the information carried by each bit, achieving greater memory efficiency over floating point representations that waste bits describing the nth decimal place. So it’s really exciting to see binary codes finding their way into the intermediate layers of a neural net. In XNOR-Net: ImageNet Classification Using Binary Convolutional Neural Networks, both the convolutional filters and the feature representations are binary, resulting in massive memory and computation savings. While the full binary XNOR-Net does sacrifice accuracy, it seems to be a very promising research direction.
At a high level, it seems like a lot of papers were presenting new or modified deep learning architectures, demonstrating improved performance for particular tasks. My question is, which of these ideas will turn out to flexible, elegant and reusable designs? I look forward to the day when some of the giants in the field will catalog all this accumulated wisdom and experience into a concise book on Deep Learning Design Patterns, similar to the original classic book on reusable software engineering by the ‘Gang of Four’.
For example: when should we use a neural attention mechanism? Or a memory network? What are the consequences and trade-offs of using it? What are the alternatives? I think the answers are still a few years away.
Having spent a good week listening to ECCV presentations and reading papers, I can’t help but feel we have moved way beyond the capacity of current formats — block diagrams, supported by equations and text — to adequately describe the complex deep learning architectures that people are using now. For want of a better term, I would say the deep learning community is missing a Network Description Language (NDL) — a standard language to describe the structure and training / inference procedure of neural network architectures. Standardization may seem boring, but I suspect it would be incredibily handy if networks could be accurately described, and ultimately, designed, without being tied to a particular software framework such as Caffe or TensorFlow. Code releases are good, but given the number of different deep learning frameworks around — including proprietary ones — code releases are only part of the answer. I’m sure it’s no accident that the digital circuit design ecosystem revolves around two standard Hardware Description Languages, with a bunch of tools to synthesize to different implementation technologies. I bet we could borrow a lot of good ideas from existing data-flow languages such as VHDL and Verilog. There are already tools to convert nets between different deep learning frameworks, but no standard network description.
Edit: I hear you saying, isn’t Caffe’s protobuf network specification equivalent to a Network Description Language? The answer is no. Caffe’s protobuf format references Caffe layers, so it is tied to the Caffe implementation. If I change the ‘Convolution’ layer in the Caffe source, the protobuf now means something different. The other issue with this approach is that there is no way to specify a new type of layer or network component in protobuf. Also, protobuf is way too verbose. For example, the 152-layer Residual Net is nearly 7000 lines. A good Network Description Language needs variables and loops to specify repeated structure in a much more concise way. I wonder if the big players like Facebook, Microsoft and Google would ever work together on a Network Description Language?
What else is missing from deep learning?