为了更好地保留原彩色图像的局部对比度, 获得感知准确的灰度图像, 提出了一种轮廓波域内局部对比度增强的灰度化算法。首先在CIE Lab色彩空间中利用共轭梯度算法优化目标函数, 求取全局映射函数参数, 得到初步灰度化图像; 然后采用轮廓波变换对彩色图像和初步灰度化图像进行多尺度多方向分解, 利用局部色彩对比度比值对方向细节图像进行对比度增强, 应用轮廓波逆变换得到增强后的细节图像; 最后将初步灰度化图像与增强后的细节图像进行叠加, 得到灰度化图像。对COLOR250和adik图像集的实验结果表明, 本文提出的算法优于已有文献的一些典型灰度化算法, 能够有效保留原始图像的对比度与结构信息, 灰度化图像视觉感知自然, 主客观评价结果均为最优。

In order to better preserve the local contrast of the original color image and obtain the perceptual accurate grayscale image, a grayscale algorithm based on local contrast enhancement in contourlet transform domain is proposed. Firstly, in CIE Lab space, the conjugate gradient algorithm is used to optimize the objective function to get the global mapping function parameters, and preliminary grayscale image is obtained. Then, the multi-scale and multi-direction decomposition of color image and preliminary grayscale image are carried out by contourlet transform, local chromatic contrast ratio is used to enhance the contrast of directional detail image, and the enhanced detail image is obtained by inverse contourlet transform. Finally, the enhanced detail image is superimposed with the preliminary grayscale image to get the final grayscale image. Experiments on COLOR250 and adik database show that the proposed algorithm in this paper is superior to some typical algorithms in the existing literature, which can effectively preserve the contrast and structure information of the original image. The visual perception is natural for the grayscale image, whose subjectivity and objective evaluations are optimal.