Deep learning has been widely used in recent years for printed circuit board (PCB) defect detection because of its excellent performance. However, deep-learning-based approaches often suffer from the over-fitting problem due to the lack of sufficient training data in real applications. Meanwhile, these approaches still have some challenges to detect these defects with small sizes and irregular shapes. To address these problems, this article has developed a novel deep Siamese semantic segmentation network which integrates the similarity measurement of the Siamese network with the encoder–decoder semantic segmentation network for PCB welding defect detection. This network includes two encoders sharing weighted values, a decoder, and some correlation modules, in which the decoder integrates deep features from two decoders with their feature difference computed by some correlation modules via skipping connections to recover spatial information on multiple output layers, and thus this proposed network can perform PCB welding small defect semantic segmentation. Moreover, via these correlation modules, this proposed network can pay more attention to semantic difference and further alleviate the over-fitting issue because of insufficient defect samples. Finally, we propose a combined loss function which combines the weighted cross-entropy loss, the Lovasz softmax loss, and the weighted precision–recall loss for network training to further improve small defect segmentation and recall improvement. Experimental results demonstrate that the proposed network can be trained on limited training images and achieve high efficiency and outstanding effects for PCB welding small defect segmentation.

Histogram equalisation has been widely used for image enhancement because of its simple implementation and satisfactory performance. However, traditional histogram equalisation uniformly redistributes an entire histogram or multiple piecewise histograms with the same equalisation strategy, which may produce unnatural artefacts, over‐enhancement or under‐enhancement in wide dynamic range dark image enhancement. This study proposes an adaptive extended piecewise histogram equalisation algorithm (AEPHE) for dark image enhancement. First, an original histogram is divided into a group of extended piecewise histograms. Then, an adaptive histogram equalisation, which balances intensity preservation and contrast boosting, is further developed and respectively applied to these extended piecewise histograms. The final histogram for image enhancement is produced by a weighted fusion of these equalised histograms. The experimental results indicate that AEPHE is superior to multiple state‐of‐the‐art algorithms.

Single image defogging algorithms based on prior assumptions or constraints have captured much attention because of their simplicity and practicality. However, they still have some challenges to deal with foggy images captured under weather conditions where these assumptions or constraints may not be effective or efficient enough. In this paper, we aim to develop a novel image defogging algorithm by directly predicting the fog density of recovered images rather than adopting prior assumptions or constraints. In order to achieve this goal, two specific steps are introduced. First, we adopt three fog-relevant statistical features derived from foggy images, and further develop a simple fog density evaluator (SFDE) by creating a linear combination of these fog-relevant features. This proposed evaluator can efficiently perceive the fog density of a single image without reference to a corresponding fog-free image and has a low computational load compared with an existing method. Second, a physics-based mathematical relationship between the transmission and the fog density score of the recovered image is developed via SFDE, thus image defogging can be posed as a minimization problem on the fog density score of the recovered image. As a result, two optimal transmission models, called an Optimal Transmission model via SFDE (OTSFDE) and a Simpler Optimal Transmission models via SFDE (SOTSFDE), are present to determine the key transmission map for efficient fog removal. Compared to OTSFDE, SOTSFDE has low computational complexity with slight performance degradation. Experimental results demonstrate that the proposed algorithms can effectively remove fog and are not confined by any assumptions or constraints, both quantitatively and qualitatively, compared with some existing algorithms.

Deep learning has been widely used in recent years for surface defect detection because of its excellent performance. However, current deep-learning-based approaches still remain a challenging problem in sawn lumber defect inspection because different lumber defects often keep similar texture and color since different growing environment, surface stains, etc. Meanwhile, the same defect often shows different characteristics. Furthermore, lumber defects have ambiguous boundaries or regions, and large-scale variations in size and shape. To address these problems, we have developed a deep Gaussian attention network via Deeplabv3+ for lumber surface defect segmentation. This network introduces an attention network via a transformer to capture the long-distance dependence for global information extraction, which can efficiently improve mis-segmentation since different lumber defects are very similar in some local regions. Furthermore, we introduce a Gaussian module into the channel attention module and positional attention module, respectively, to reassign and reactivate the minor semantic features for hard example mining so that all regions of defects can be activated except the key regions for efficient semantic segmentation. Finally, the activated global information and local information in astrous spatial pyramid pooling(ASPP) are integrated to achieve efficient feature extraction. Experimental results demonstrate the proposed network can efficiently address the ambiguous defect regions and irregular sizes and shapes of sawn lumber surface defect segmentation.