基于小波域边缘方向特征的SAR图象噪声抑制方法

Speckle Reduction for SAR Images Using Edge Directions in Wavelet Domain

给出了一种新的基于小波变换的合成孔径雷达 (SAR)图象斑点噪声抑制方法 .利用每一级小波分解得到的小波系数子带 HL和 L H,以及对原图进行水平方向旋转正负 4 5°扫描后得到的另外两个正交方向的小波系数子带 rc HL和 ra HL ,可以判断出对应点边缘方向性的强弱 ,通过设定方向性阈值 ,确定该点是否位于边缘上 ,进而对没有位于边缘的点进行平滑 ,达到保留图象边缘的同时 ,抑制斑点噪声的目的 .为解决对某些振荡型边缘的检测问题 ,还结合阈值法 ,对该方法做了改进 .实验表明 ,与小波域的硬阈值或软阈值去噪方法相比 ,此方法在有效地抑制斑点噪声的同时 ,更好地保留了 SAR图象中的边缘和纹理信息 .

A filter for speckle reduction in SAR image is proposed. On each level of wavelet decomposition, three images are used. One is the original image, and the two others are obtained by rotating the original image by 45° and -45° respectively, and so 12 subbands are gotten. In the 12 subbands, four subbands, HL subband and LH subband corresponding to the original image, and two HL subbands corresponding to the second and third image respectively, are used for edge detection, and the LL, HL, LH, HH subbands of the original image are used for synthesis. By using each point's four wavelet coefficients in the four subbands for edge detection, the edge direction property of the point on the original image is captured, and then the edges are detected by setting a proper threshold. And so, the speckle can be reduced while the edges being preserved well by setting the wavelet coefficients in the synthesis subbands corresponding to the points not on edges to zero but retain the wavelet coefficients in the synthesis subbands corresponding to the points on edges. For detection of some oscillating edges, the filter is improved by combining with the traditional threshold method. Simulations on synthetic images indicate that the new filter performs better than the traditional wavelet domain hard threshold or soft threshold method.