有效消除光晕现象和颜色保持的彩色图像增强算法

由于对图像中明暗突变区域的背景光照估计不准确,经典Retinex彩色图像增强算法易产生光晕现象且存在增强后图像细节信息减弱和颜色失真等不足.为此,结合人眼视觉特性提出一种彩色图像增强算法.首先利用人眼对图像结构特征及颜色信息的敏感特性,通过构造彩色双边滤波器来获取图像背景光照,以避免光照突变处产生光晕现象;其次依据人眼系统局部自适应调节特性,通过引入一个对比度调节函数自适应增强图像的细节信息,克服经典Retinex算法在整体对比度提高的同时局部对比度下降的不足;最后利用一种线性的颜色恢复算法恢复增强所得亮度图像的颜色信息.与MSRCR等彩色图像增强算法比较的实验结果表明,文中算法更有效,增强后的图像不仅细节清晰,而且色彩鲜艳、自然.     

改进的自动颜色均衡化图像对比度增强

提出了一种改进的自动颜色均衡化方法,用于灰度和彩色图像对比度的增强。通过考虑图像中颜色或灰度的空间分布,首先对图像进行了局部自适应滤波,其中使用了改进的相对亮度表观函数。而后对图像进行动态范围调整以得到最终结果。在对彩色图像处理时,通过将图像从RGB空间转至HSV空间,仅对亮度通道进行滤波以保持颜色不失真。实验结果表明,方法可有效增强图像的对比度,且不会引入明显噪声。     

基于模糊理论的低照度彩色图像增强算法

低照度彩色图像存在整体亮度低、对比度差、颜色偏暗和信噪比低等特点,传统图像增强算法对其增强效果非常有限。提出了一种基于模糊理论的低照度彩色图像增强算法,将三原色(red,green,blue,RGB)图像转换成色相饱和度(HSV)图像,以保证增强处理不引起图像的色彩失真。对亮度图像进行非线性变化,实现动态范围展宽;采用修正后的隶属度函数将图像映射到模糊平面,实现对比度增强。实验结果表明:该算法显著地提高了图像整体亮度和对比度,改善了低照度彩色图像的视觉效果。     

子图像加权的彩色图像对比度增强算法

为了更好地实现彩色图像的对比度增强,提出一种有效的基于亮度保持的子图像加权对比度增强算法.首先利用基于亮度保持的双直方图均衡(BBHE)算法定义了2个子图像;然后根据输入图像以及2个子图像的亮度均值,给出一种基于亮度保持的权重系数的计算方法;最后对2个子图像进行加权求和,得到输出图像.对具有不同对比度的彩色图像的实验结果表明,与其他算法相比,文中算法能够有效地实现对比度增强,并且不会出现局部过度增强以及细节丢失等噪声问题,使得输出图像显得更为自然.     

基于局部适应性的高动态范围图像显示方法

在高动态范围环境中，人眼依靠局部适应性也能够观察到细节变化。提出了一个基于区域信息的局部适应亮度计算方法来模拟局部适应性。使用区域生长法对图像进行分割，然后采用基于区域的双边滤波技术来计算每一像素的局部适应亮度，再联合色调映射算子获得可显示的低动态范围图像。实验结果显示，输出的图像避免了光晕，同时较好地保持了细节。     

基于边缘检测的Retinex图像增强算法

针对消除背景光照对图像的影响时出现的细节弱化、色彩失真的问题,提出了一种基于边缘检测的Retinex彩色图像增强算法。根据人类视觉特性从图像中提取亮度分量并检测边缘信息,在平滑点和边缘点处采用不同的模板估计背景光照,以避免强边缘处的光晕现象;通过调整图像中的背景光照比例提升局部对比度,并根据反射图像直方图自适应调整全局对比度;利用R、G、B通道及亮度分量的等价变换进行色彩恢复,以保证增强前后图像色调一致。实验结果表明,增强后的图像标准差提升了19.34%,信息熵增大了13.18%。     

基于变分的图像增强算法和伪彩色映射

提出了一种新的基于变分方法的灰度图像增强算法。将灰度图像的对比度场进行线性放大作为目标图像的对比度场，在图像显示和打印的动态范围约束条件下。利用变分方法求解出增强的图像，使其具有最接近于目标对比度场的对比度。若把目标对比度进一步作动态范围压缩后，将此方法应用到彩色图像的亮度分量中去(色度和饱和度不变)。并根据人类视觉感知的统计特性将结果图进行伪彩色映射，得到视觉效果更佳的彩色图像。这种增强方法不需要人为选定待增强图像区域。实验结果表明了该图像增强方法的有效性。当图像中含有极其微弱的有用的局部对比度信息时，效果尤佳。     

一种基于视觉特性的彩色图像增强算法

为了增强图像暗区域部分,基于人类视觉系统的全局和局部自适应调节原理,提出一种彩色图像增强方法。该方法主要包括全局自适应亮度调节、局部对比度增强和颜色恢复3个部分。全局亮度调节采用直方图非线性自适应拉伸来增强暗区域的亮度;局部对比度增强利用当前点与区域像素之间的关系,调节当前点的亮度,以增强图像局部对比度;通过一种自适应的非线性颜色恢复算法恢复图像色彩。通过大量图像对比实验分析表明,本文方法可以自适应有效快速地实现图像增强。     

一种颜色保持的彩色图像增强新算法

分析了Retinex理论及其典型彩色图像增强算法,在此基础上提出一种颜色保持的彩色图像增强新算法。利用非线性sigmoid传输函数多尺度增强亮度分量,并进行局部对比度增强后获得亮度增益曲面,最后利用亮度增益曲面对原彩色图像RGB三颜色分量同比增强,保证了色调恒定不失真。几种算法实验结果的比较和分析。表明,这种算法是有效的。     

基于自适应邻域对比度增强的直方图均衡算法

图像增强技术可分为局部增强和整体增强两种.在分析文字图像褪化因素及特点的基础上,提出一种基于自适应邻域对比度增强的直方图均衡法,它不仅能在增大整幅文字图像动态范围的同时增强局部对比度,去除扩散,而且能抑制一定的背景噪声.在此基础上,用灰度拉伸技术进一步加大文字图像的对比度,去除背景噪声,改善了褪化文字的视觉效果.     

基于人眼视觉感知驱动的井下图像增强算法

为解决传统的对比度增强方法在对井下图像进行处理时不能兼顾压缩动态范围、调整亮度以及增强图像对比度等问题,提出一种基于人眼视觉感知特性的井下图像对比度增强算法。首先根据人眼亮度掩蔽特性对图像进行区域划分,然后基于非线性亮度映射模型,对图像的不同区域进行不同尺度的非线性调整,最后再将亮度调整后的不同区域组合成新的图像。实验结果表明,该方法能有效增强井下低照度图像的对比度,提升图像的视觉效果。     

A framework for inverse tone mapping

In recent years many tone mapping operators (TMOs) have been presented in order to display high dynamic range images (HDRI) on typical display devices. TMOs compress the luminance range while trying to maintain contrast. The inverse of tone mapping, inverse tone mapping, expands a low dynamic range image (LDRI) into an HDRI. HDRIs contain a broader range of physical values that can be perceived by the human visual system. We propose a new framework that approximates a solution to this problem. Our framework uses importance sampling of light sources to find the areas considered to be of high luminance and subsequently applies density estimation to generate an expand map in order to extend the range in the high luminance areas using an inverse tone mapping operator. The majority of today’s media is stored in the low dynamic range. Inverse tone mapping operators (iTMOs) could thus potentially revive all of this content for use in high dynamic range display and image based lighting (IBL). Moreover, we show another application that benefits quick capture of HDRIs for use in IBL.     

Physiological inverse tone mapping based on retina response

The mismatch between the Low Dynamic Range (LDR) content and the High Dynamic Range (HDR) display arouses the research on inverse tone mapping algorithms. In this paper, we present a physiological inverse tone mapping algorithm inspired by the property of the Human Visual System (HVS). It first imitates the retina response and deduce it to be local adaptive; then estimates local adaptation luminance at each point in the image; finally, the LDR image and local luminance are applied to the inversed local retina response to reconstruct the dynamic range of the original scene. The good performance and high-visual quality were validated by operating on 40 test images. Comparison results with several existing inverse tone mapping methods prove the conciseness and efficiency of the proposed algorithm.     

Luminance adaptation transform based on brightness functions for LDR image reproduction

Tone mapping algorithms are used for image processing to reduce the dynamic range of an image to be displayed on low dynamic range (LDR) devices. The Retinex, which was developed using multi-scale and luminance-based methods, is one of the tone mapping algorithms for dynamic range compression, color constancy and color rendition. Retinex algorithms still have drawbacks, such as lower contrast and desaturation. This paper proposes a multi-scale luminance adaptation transform (MLAT) based on visual brightness functions for the enhancement of contrast and saturation of rendered images. In addition, the proposed algorithm was used to estimate the minimum and maximum luminance and a visual gamma function for local adapted viewing conditions. MLAT showed enhanced contrast and better color representation than the conventional methods in the objective evaluations (CIEDE200 and VCM).     

Dependence of perceived display brightness on peripheral vision luminance and field of view: A comparison study

Considerable research effort has been devoted to enhancing perceived display brightness by improving the display luminance level. However, studies have shown that the human visual system (HVS) is influenced not only by physical luminance but also by other factors, such as adaptation to the surrounding luminance. Therefore, enhancing the maximum display luminance alone cannot be an optimal solution. In this study, we investigated the effects of the peripheral vision luminance (PVL) level and field of view (FOV) on the display brightness perceived by the observer, by using Stevens' brightness function, irrespective of the effect of adaptation to surround luminance. The analysis of the experimental results revealed that the perceived display brightness is inversely related to the level of peripheral vision luminance on the display. It was also found that, whereas increase in FOV results in an increase in the absolute brightness value, it reduces the effect of PVL on the perceived display brightness. The findings of this study will serve as useful basic data for optimizing display specifications with respect to image signals.     

Calibration of diagnostic monitors: Theoretical determination of optimal luminance settings

Abstract— Common practice today is to calibrate diagnostic monitors according to the gray‐scale standard display function (GSDF) described in DICOM part 14. However, the GSDF is based on the assumption of variable adaptation of the human‐visual system (HVS). It is well known that the HVS adapts to the average quantity of light falling on the retina, so‐called fixed adaptation. For the luminance setting of a monitor, the effect of fixed adaptation is of interest. The wider the luminance range of the monitor, the larger the number of available just‐noticeable differences (JNDs). However, at the same time, the sensitivity of the HVS to the average contrast change is decreased since it occurs at a luminance level further away from the adaptation luminance. A computer program was therefore written which takes the effect of the fixed adaptation into account by determining the number of effective JNDs for a given luminance setting of a monitor. The probability of detecting each change in presentation value is then calculated from the distribution of effective JNDs. Based on the assumptions implemented in the program, it is shown that for monitors calibrated according to the GSDF, the optimal luminance setting for visualizing the image information in the best possible way is to use the entire available luminance range.     

A real-time image processor with combining dynamic contrast ratio enhancement and inverse gamma correction for PDP

Inverse gamma correction must be performed before displaying the received video signal because alternating current plasma display panel (AC PDP) has a linear output luminance response to a digital-valued input. At the same time contrast ratio enhancement is necessary for improving the image quality of display devices. The histogram equalization (HE) is an important contrast ratio enhancement method. But sometimes HE can produce unrealistic effects in images. In this paper, a new method of combining dynamic contrast ratio enhancement and inverse gamma correction for AC PDP is proposed. The dynamic contrast ratio enhancement and the inverse gamma correction are realized simultaneously in the proposed method. Furthermore the over-enhancement caused by the traditional HE can be avoided. A real-time image processor with the proposed method was designed and implemented. Simulations and experimental results on a 50-in. AC PDP show that the image quality of AC PDP can be improved obviously.     

Qualified viewing space determination of near‐eye and head‐up displays

A method for determining the extents of a qualified viewing space (QVS) based on repeatable and reproducible luminance measurements of augmented and virtual reality near‐eye displays is described. This QVS mapping can also use other display performance metrics such as (1) Michelson contrast, (2) modulation transfer function, or (3) color as boundary condition parameters. We describe the use of a 4‐mm‐diameter entrance pupil, 1° to 2° field of view tele‐spectroradiometer, to determine the luminance and color uniformity of the virtual image. A 1‐mm‐diameter entrance pupil is used to map the QVS boundaries based on luminance at the center of the virtual image. The luminance measurement results from a pair of binocular augmented reality display glasses in three separate eye relief planes of the QVS of both eyes are presented. The data are further reduced to provide a perimeter profile of the QVS for the 50% of peak luminance boundary points in each eye relief plane.     

Luminance and luminance contrast requirements for legibility of self-luminous displays in aircraft cockpits

A series of studies was conducted to investigate the effects of display background luminance, display contrast ratio and adaptation luminance on the legibility of self-luminous displays in aircraft cockpits. The studies were designed to avoid the typical confounding of these three variables, and employed luminance levels that are representative of operational conditions. The data show that with typical display symbols and luminances under low and moderate ambient illumination, contrast ratio is the major determinant of legibility; that increasing contrast ratios beyond as little as 1·4:1 may provide no significant improvements in legibility; and that, beyond their contribution to contrast ratio, the absolute luminances of the symbol and the background have no significant effect on legibility. Given the high ambient illumination typical of a sunny day, however, the pilot is adapted to luminances of up to 32 264 cd/m² (10 000 fL). Under these conditions, increases in contrast ratio of up to 30:1 are shown to improve legibility. When the adaptation luminance is more than 100 times that of the display background luminance, symbols on the display cannot be immediately identified, and the absolute luminances of the adapting field and the display background contribute to recovery time apart from their ratio relationship. Hyperbolic functions are applied to describe the multiplicative effects of these variables on legibility.