Incorporating Cluster Information into Multispectral Image Edge Detection
Uthai Sangthongpraw, Yuttapong Rangsanseri, and Punya Thitimajshima
Department of Telecommunications Engineeing,
Faculty of Engineering King Mongkut's Institute of Technology Ladkrabang,
Bangkok 10520, Thailand
E-mail : kryuttha@kmitl.ac.th, ktpunya@kmitl.ac.th
Abstract
A multispectral image edge detection algorithm is proposed in this paper based on the idea that uses global spectral information to guide local gradient computation. The image is first segmented into a small number of cluster by using a clustering algorithm. Edges are then treated as transitions from one cluster to another. Edge detection is performed by calculating gradient magnitudes separately on each channel. The best linear projection which maximizes the inter-cluster distance will be used to recombine the gradient magnitudes in individual channels. The algorithm has been tested on JERS-1/OPS images, and the experimental results are included.
1. Introduction
Edge detection plays very important roles in many image interpretation systems. For this reason, several edge detection algorithms have been proposed [1] [2]. JERS-1 satellite has optical sensor (OPS), which observes in seven spectral bands including three Visible and Near-Infrared (VNIR) bands. The interpretation of these VNIR images is usually done by associating the three bands with the three additive color primaries, RGB, in the display device. Edge detection in these images can be also performed by using general algorithms of the color image [3]. In some of many color edge detection algorithms, the RGB space is first transformed into other spaces in which the coordinates match the perceptual attributes of colors, e.g. HSI space, YC1C2 space. In these spaces, components are relatively independent to each other. Traditional edge detectors can be adopted in each channel and the results are recombined in ways that make use of both color and luminance information.
In this paper, we present a method for edge detection in multispectral images by incorporating cluster information. In fact, edge pixels tend to occur in the boundary where the neighboring pixels belong to different cluster, Our method is studied for the case of three band images, thus usually referred as color images.
2. Two alternative approaches
The monochrome version of a color image can be obtained by extracting the luminance component for each pixel. The linear combination of the R,G, and B components is usually used. Edge can be obtained by using the traditional edge detector. An alternative linearly for each pixel. The above two methods can be explained as :
Figure 1: Block diagram of the proposed algorithm.
I' = 0.31'r + 0.59I'g + 0.11I'b (1)
= (0.31'r + 0.59I'g + 0.11I'b) (2)
Where I'
r, I'
g, I'
b and I'
r, I'
g, I'
b denote the gradients and the intensities in each color channel respectively. We can see that this process can be viewed as the projection of multi-dimensional gradient vectors to scalar values [0.3, 0.59, 0.11], some gradient vectors with significant magnitudes can be projected to small values if the projection direction is not at the same direction of the gradient vectors. If several possible projection vectors are derived for a given multispectral JERS-1 image by statistic analysis, Then for each pixel, the gradient vector is projected in a way that results the maximum edge value. The spectral information can be used efficiently in this way.
3 The proposed method
Clustering information is applied in this paper to find the appropriate projection vector. The information in every bands can be use efficiently in this algorithm. The block diagram of this method is shown in Figure 1.
3.1 Sobel operator
Generally, the computation of gradient image is use Sobel operator [2]. The gradient is computed by convolution of input image and 3x3 dimentsion mask, denote Sx and Sy. The gradient in x and y directions are computed. The Sobel mask Sx and Sy are shown below.
Figure 2: Sobel mask
The gradient magnitude (M) can be calculated from the following equation :
