# Image Watermarking between Conventional and Learning-Based Techniques: A Literature Review
## 4 Conventional Image Watermarking Schemes
### Spatial Domain  
It is easier to work with the spatial domain than with the frequency domain because no transformations are required. At the same time, the frequency domain is more robust since it allows obtaining an image representation that is invariant to slight modifications, resulting in better robustness of a watermark [65] but worse imperceptibility. In techniques based on the spatial domain, such as as the insertion domain, the mark is inserted by a direct modification of the pixels. These pixels are usually selected using a secret key or are based on a psycho-visual model. In the most commonly used designs, the values of the color channels, luminance signals, or brightness signals of the digital image are used in this situation [66], based on the logo, signature, or random bits defined by the owner. Many watermarking techniques are proposed in this regard, such as least significant bit (LSB) modification algorithms, intermediate significant bits (ISB) and patchwork algorithms, spread spectrums, and correlation-based algorithms. Because of their simplicity, the earliest digital watermarking algorithms were developed to work in this domain. [PERSON] et al. [67] proposed the first spatial watermarking method. It consists of inserting each element of the mark in the least significant bit of each pixel. Another method called the \"patchwork\" algorithm has been proposed by [68]. This algorithm also operates directly in the spatial domain. The watermark is concealed with a specific Gaussian distribution in the luminance values of the original image. To retrieve the watermark, correlation-based algorithms are mostly used in the spatial domain. In the following, we review several recent watermarking approaches based on the spatial domain that have attracted the attention of the research community in the last few decades because of their optimal balance among robustness, imperceptibility, and capacity, all of which are required for any watermarking technique. First, [PERSON] et al. [69] presented a digital watermarking method for camera-captured images. In this scheme, they embed a binary image with the same size as the host image, with each pixel in the host image used to carry a watermark bit. Several types of pixel values distortions and geometric distortions were investigated after the watermarked image was printed and captured by a digital camera. The method shows robustness against various types of distortions from the printing and camera-capturing processes. However, with the distortions introduced, reliable extraction was not achievable on certain levels, essentially because of camera position, camera settings, and lighting conditions. Desynchronization attacks produce a delay between the insertion and detection algorithms. As a result, the watermark often has difficulty surviving this type of attack, especially for color images. [PERSON] et al. [70] proposed a new robust color-image watermarking method based on a significant bit-plane histogram. Here, the watermark is embedded in the affine invariant local feature regions of the RGB plane. The scheme is invisible and robust against common signal-processing and desynchronization attacks. However, the method has a lower watermark capacity and a higher computation time, making it unsuitable for real-time applications. [PERSON] et al. [71] designed an effective fragile watermarking scheme for RGB image tamper detection and restoration. In this technique, three copies of the watermark are concealed over three components (red, green, and blue channels) of the host image, improving detection accuracy and restoration capabilities. The Bayer pattern is used to decrease the capacity of insertion by converting the host color image into a gray-level watermark. Moreover, the Torus automorphism permutation is adopted in order to scramble the gray-level watermark. However, security analysis in this work is inadequate, and the method must to be used in real application scenarios where security issues become a serious problem.