CV HE

[junxnone]

Histogram Equalization 直方图均衡

Reference

• 直方图均衡化
• 1974 Image enhancement techniques for cockpit displays
• 1987 Adaptive Histogram Equalization and Its Variations
• Histogram Equalization - opencv
• Histogram equalization - wikipedia
• 直方图均衡化原理
• 原理公式推导
• Contrast Stretching
• Histogram Equalization
• Local Histogram Equalization - scikit-image

Brief

• 图像直方图均匀分布
• 增强图像 - 更好的对比度
• CDF - cumulative distribution function - 累积分布函数
• HE - Histogram Equalization
• AHE - Adaptive Hisogram Equalization
• CLAHE - Contrast Limited Adaptive Hitogram Equalization

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Contrast Stretching

• 8 bit image 最小值 Vmin = 0 Vmax = 255
• image 中 Pixel 最小值 Pmin, 最大值 Pmax
• cutoff fraction - 小于 Pmin = histogram_peak * cutoff_fraction 的值忽略掉

Process	Image	Histogram
Original
Contrast Stretching Pmin=79 Pmax=136
Histogram Equalizing
Histogram Equalizing + Contrast Stretching cutoff_fraction = 0.03
Histogram Equalizing + Contrast Stretching cutoff_fraction = 0.125

Pout = (Pin - Pmin)(Vmax-Vmin)/(Pmax-Pmin) + Vmin

HE - Histogram Equalization

• 单调非线性

公式	Description
	Image A(x,y) => Image B(x,y)
	对应区间内像素点总数不变
	A0 像素点个数 L 灰度阶数

Input Image DA => Output Image DB 映射图

AHE - Adaptive Hisogram Equalization

• 自适应直方图均衡化
• 考虑到局部图像区域 - WxW 窗口
• LAHE - Local Area HE

CLAHE - Contrast Limited Adaptive Hitogram Equalization

• 限制对比度自适应直方图均衡化
• 将直方图中超过阈值的部分平均分配到各灰度级