3.1.1. Color Feature

The color feature is a global feature that describes the surface properties of the scene corresponding to the image or image area, and is also the most direct visual feature in the physical characteristics of the image. Compared with various other image features, the color feature has two obvious advantages: one is stability, low sensitivity to various changes in the image such as translation, scaling, rotation, etc., and strong robustness; the second is that its complicated calculation degree is low. The pixel values in the image are converted, and the corresponding numerical expression is used to obtain the image characteristics [16]. Because of the stability and simple calculation, the color feature has become a widely used image feature.

Shown in Figure 2, the color histogram is widely used in many image retrieval systems. It describes the proportion of different colors in the entire image, and does not pay attention to the spatial location of each color; that is, it cannot describe the objects or objects in the image.

**Figure 2.** Color histogram of an image (RGB channel).

The color histogram can be defined as the joint probability density function of the three color channels (RGB) in the image:

$$h\_{\mathcal{R},\mathcal{G},\mathcal{B}}(a,b,c) = N \bullet P(R=a,\mathcal{G}=b,B=c) \tag{1}$$

where *R*, *G*, and *B* represent the RGB color channels of the image, *N* indicates the number of image pixels, *P* represents the probability density function, and *h* represents a histogram function, defined as a four-dimensional eigenvector *H*(*HR*, *HG*, *HB*, μ). The first three dimensions *H*R, *H*G, and *H*<sup>B</sup> correspond to the three color channels, and the last dimension μ indicates the proportion of the color in the entire image.

#### 3.1.2. Texture Feature

Texture is a visual feature that reflects homogeneous phenomena in an image, and it reflects the surface structure organization and arrangement properties of an object surface with slow or periodic changes. Texture is a pattern produced by the gray or color of the target image in space in a certain form [17]. From the perspective of texture, the image can be roughly divided into three cases: first, the gray distribution has a certain periodicity (even if the gray change is random, it also has certain statistical characteristics, and may be in a larger area repeatedly); second, the basic components that make up the sequence are regular rather than random; third, the texture of each part in the texture area shows roughly the same size, structure, and image, and is uniformly distributed as a whole.

The Fourier power spectrum method is used to measure the texture characteristics of the image. Let the texture image be *f(x, y)*; its Fourier transform can be expressed by Equation (2).

$$\mathbf{F}(\mathbf{u}, \mathbf{v}) = \int \int\_{-\infty}^{\infty} \mathbf{f}(\mathbf{x}, \mathbf{y}) \exp\{-\mathbf{j}2\pi(\mu\mathbf{x} + vy)\} \mathrm{d}\mathbf{x} d\mathbf{y} \tag{2}$$

The definition of the power spectrum of the two-dimensional Fourier transform is shown in Equation (3):

$$\|F\|^2 = F\mathcal{F}^\*\tag{3}$$

where *F*\* stands for the conjugate of *F*. The power spectrum |*F*| <sup>2</sup> reflects the nature of the entire image. If the Fourier transform is expressed in polar form, i.e., *F*(*r*, θ) form, then the energy on the circle *r* from the origin is

$$\Phi\_r = \int\_0^{2\pi} [\mathbf{F}(r, \theta)]^2 \mathbf{d}\theta. \tag{4}$$

From research on the energy in the small fan-shaped region in the angle θ direction, the law of this energy changing with the angle can be obtained by Equation (5):

$$\Phi\_{\theta} = \int\_{0}^{\infty} |F(r, \theta)|^{2} \mathrm{d}r. \tag{5}$$

When a texture image runs along θ and there are many lines, edges, etc., in the direction θ + <sup>π</sup> <sup>2</sup> , i.e., in a right-angle direction to θ, the energy is concentrated. If the texture does not show directionality, there is no directionality in the power spectrum. Therefore, the |*F*| <sup>2</sup> value reflects the directionality of the texture.
