# Improving DFT-Based Image Watermarking Using Particle Swarm Optimization Algorithm
## 1 Introduction  
Currently, most modern capturing equipment produces digital images in an easy and massive way. With the great growth of multimedia systems, the users have found novel ways to exchange and publish digital images without any restriction, mainly in social networks and on web sites. This behavior has increased the interest of scientific community in developing efficient methods to address copyright protection problems in recent decades [1, 2, 3, 4]. Digital watermarking is an emerging technique that has been widely used by multimedia systems for improving aspects of information security related to ownership authentication and copyright protection. Conventionally, over time digital watermarking has been classified mainly in visible and invisible modalities, respectively. In general terms, digital watermarking methods embed information (i.e., a watermark) to create a trustworthy link between the watermark and the hosting object. In the context of invisible watermarking, a technique of this modality is as efficient as the results of its evaluations regarding robustness and imperceptibility. Robustness is a measurement to evaluate if inserted binary data remains even after severe attacks are performed upon the watermarked image. Furthermore, in many applications, it is imperative to preserve almost the same visual quality as the original image to prevent any distortion perceptible by a naked eye. In this way, invisible watermarking is the most explored modality in scientific literature to find a solution to information security issues in digital images. This modality involves two stages. The first one embeds a signal called a \"watermark\" in an imperceptible manner, obtaining often high visual quality watermarked images. The second stage extracts and detects the watermark signal even though the protected images have experienced changes because of several image processing operations during its transmission or storage, either caused in an intentional or non-intentional manner, such as image compression, noise corruption, filtering, or geometric distortions, among others. The aim of invisible watermarking is detecting or extracting the watermark signal without any ambiguity, even if the image is severely distorted, i.e., the digital image watermarking algorithm should be as robust as possible against attacks that attempt to make difficult recovery tasks. The main capabilities of invisible watermarking are: (a) embed an invisible watermark pattern in the image content, (b) identify ownership, (c) high transparency image content, (d) high quality of watermarked image, (e) need for explicit extractor/detector, (f) high robustness, (g) often higher computational complexity and (h) versatility of media content. Conventionally, invisible watermarking is performed in spatial, frequency or hybrid domains, in conjunction with advanced image and/or signal processing techniques to improve, as far as possible, the performance in terms of imperceptibility, capacity and robustness [1; 2; 3; 4]. In this way, in recent years the scientific community has been focusing its attention in optimize the invisible watermarking methods via optimization algorithms, such as the particle swarm optimization (PSO) algorithm [5], among others. Motivated and inspired by this trend, this paper proposes an improved robust invisible watermarking algorithm in discrete Fourier transform (DFT) via direct sequence code division multiple access (DS-CDMA) [6; 7], optimizing the key operation parameters, particularly the amounts of bands and coefficients of frequency as well as the watermark strength factor, using the PSO algorithm in conjunction with visual information fidelity (VIF) and bit correct rate (BCR) criteria. Experimental results obtained in this research shows improved robustness against common signal processing and geometric distortions, preserving a high visual quality in color images.