对比度
对比度：Contrast is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other
objects and the background.视觉对比就是使物体与背景或其他物体相区别的不同，包括亮度和灰度。
Because the human visual system is more sensitive to contrast than absolute luminance, we can perceive the world similarly regardless
of the huge changes in illumination over the day or from place to place.人类视觉体系对差别更加敏感，相对于纯粹亮度。
This tells us that the humanvisual-system is most sensitive in detecting contrast differences occurring at 4 cycles
per degree, i.e. a t this sapcial frequency humans can detect lower contrast differences than at any other spatial frequency.视觉系统在周期为一度视角，频率是4（重复出现的物体个数）最为敏感。
Contrast is also the difference between the color or shading of the printed material on a document and the background on which it is printed, for example i
n optical character recognition.
对比度不同的计算公式：
Weber contrast
The Weber contrast is defined aswith I and Ib representing the luminance of the features and the background luminance, respectively. It is commonly used in cases where small features are present on a large uniform background, i.e. the average luminance is approximately equal to the background luminance.
用于小物体在大的背景下（空白比较多），主要进行色彩对比。
Michelson contrast
It is commonly used for patterns where both bright and dark features are equivalent and take up similar fractions of the area. The Michelson contrast is defined as
RMS contrast
(RMS) contrast does not depend on the spatial frequency content or the spatial distribution of contrast in the image.
RMS contrast is defined as the standard deviation of the pixel intensities:
where intensities Iij are the i-th j-th element of the two dimensional image of size M by N.is the average
intensity of all pixel values in the image. The image I is assumed to have its pixel intensities normalized
in the range [0,1].
利用方差来解决对比度问题
with Imax and Imin representing the highest and lowest luminance. The denominator represents twice the average of the luminance