  Saliency detection, finding the most important parts of an image, has become
increasingly popular in computer vision. In this paper, we introduce
Hierarchical Cellular Automata (HCA) -- a temporally evolving model to
intelligently detect salient objects. HCA consists of two main components:
Single-layer Cellular Automata (SCA) and Cuboid Cellular Automata (CCA). As an
unsupervised propagation mechanism, Single-layer Cellular Automata can exploit
the intrinsic relevance of similar regions through interactions with neighbors.
Low-level image features as well as high-level semantic information extracted
from deep neural networks are incorporated into the SCA to measure the
correlation between different image patches. With these hierarchical deep
features, an impact factor matrix and a coherence matrix are constructed to
balance the influences on each cell's next state. The saliency values of all
cells are iteratively updated according to a well-defined update rule.
Furthermore, we propose CCA to integrate multiple saliency maps generated by
SCA at different scales in a Bayesian framework. Therefore, single-layer
propagation and multi-layer integration are jointly modeled in our unified HCA.
Surprisingly, we find that the SCA can improve all existing methods that we
applied it to, resulting in a similar precision level regardless of the
original results. The CCA can act as an efficient pixel-wise aggregation
algorithm that can integrate state-of-the-art methods, resulting in even better
results. Extensive experiments on four challenging datasets demonstrate that
the proposed algorithm outperforms state-of-the-art conventional methods and is
competitive with deep learning based approaches.
