Segmentation of unevenly illuminated line scanned images

Purpose ‐ Though many segmentation methods have been published, few of them are developed especially for line scanned images. An ill-illuminated line scanned (IILS) image tends to have a uniform intensity distribution in column direction while non-uniform intensity distribution in the row direction. So, it is improper to segment IILS images using either a pixed threshold or threshold surface. In view of this, the purpose of this paper is to develop a segmentation method that is suitable for segmented IILS images. Design/methodology/approach ‐ To obtain satisfactory segmentation results, the illumination variation across the column of a line scanned image was taken into account and a column-based segmentation method was developed. The method first calculates each column's standard deviation. Then a threshold value is automatically assigned to each column based on the derived values. Finally, by assembling each columns threshold value, a so-called threshold line is formed. The method is threshold-line segmentation method based on standard deviation (TLSTD). Findings ‐ The developed threshold-line-based segmentation method is compared with Otsu's fixed threshold segmentation method and Niblack's threshold-surface-based segmentation method. The results show that the threshold-line-based segmentation method is more suitable for segmenting IILS images. Research limitations/implications ‐ Despite TLSTD outperforming Otsu's and Nilblack's segmentation methods, there are some limitations to it. The most obvious one is that the predetermined allowable deviation has influences on the integrality of the extracted flaws. Besides, since the proposed method is designed specifically for segmenting images captured by line scan cameras with a slant line light source, it is suitable for segmenting the kind of images only. In other words, the method shows no advantages in segment area scanned images. Practical implications ‐ Generally, the approach is useful in automated visual inspection where line scan cameras are employed. Originality/value ‐ The merit of the proposed method is that the slant of the line light source is now allowed. In other words, even if a grabbed line scanned image is unevenly illuminated, the proposed segmentation method is still able to successfully detect desired flaws.