The issue of pinned field image binarization for signature generation in the ownership verification of the protected image is investigated. The pinned field explores the texture information of the protected image and can be employed to enhance the watermark robustness. In the proposed method, four optimization schemes are utilized to determine the threshold values for transforming the pinned field into a binary feature image, which is then utilized to generate an effective signature image. Experimental results show that the utilization of optimization schemes can significantly improve the signature robustness from the previous method (Lee and Chang, Opt. Eng. 49 (9), 097005, 2010). While considering both the watermark retrieval rate and the computation speed, the genetic algorithm is strongly recommended. In addition, compared with Chang and Lin's scheme (J. Syst. Softw. 81 (7), 1118-1129, 2008), the proposed scheme also has better performance.