检测二维地块中的倾斜 [英] Detecting dips in a 2D plot

问题描述

我需要自动检测2D绘图中的倾斜，如下图中用红色圆圈标记的区域。我只对"主"倾斜感兴趣，这意味着倾斜必须跨越x轴的最小长度。凹陷的数量是未知的，即不同的地块将包含不同的凹陷数量。有什么想法吗？

更新：

按照要求，下面是样本数据，并尝试按照VINES的建议使用中值滤波对其进行平滑处理。

看起来我现在需要一种健壮的方法来近似每个点的导数，这样就可以忽略数据中保留的小点。是否有标准方法？

y <- c(0.9943,0.9917,0.9879,0.9831,0.9553,0.9316,0.9208,0.9119,0.8857,0.7951,0.7605,0.8074,0.7342,0.6374,0.6035,0.5331,0.4781,0.4825,0.4825,0.4879,0.5374,0.4600,0.3668,0.3456,0.4282,0.3578,0.3630,0.3399,0.3578,0.4116,0.3762,0.3668,0.4420,0.4749,0.4556,0.4458,0.5084,0.5043,0.5043,0.5331,0.4781,0.5623,0.6604,0.5900,0.5084,0.5802,0.5802,0.6174,0.6124,0.6374,0.6827,0.6906,0.7034,0.7418,0.7817,0.8311,0.8001,0.7912,0.7912,0.7540,0.7951,0.7817,0.7644,0.7912,0.8311,0.8311,0.7912,0.7688,0.7418,0.7232,0.7147,0.6906,0.6715,0.6681,0.6374,0.6516,0.6650,0.6604,0.6124,0.6334,0.6374,0.5514,0.5514,0.5412,0.5514,0.5374,0.5473,0.4825,0.5084,0.5126,0.5229,0.5126,0.5043,0.4379,0.4781,0.4600,0.4781,0.3806,0.4078,0.3096,0.3263,0.3399,0.3184,0.2820,0.2167,0.2122,0.2080,0.2558,0.2255,0.1921,0.1766,0.1732,0.1205,0.1732,0.0723,0.0701,0.0405,0.0643,0.0771,0.1018,0.0587,0.0884,0.0884,0.1240,0.1088,0.0554,0.0607,0.0441,0.0387,0.0490,0.0478,0.0231,0.0414,0.0297,0.0701,0.0502,0.0567,0.0405,0.0363,0.0464,0.0701,0.0832,0.0991,0.1322,0.1998,0.3146,0.3146,0.3184,0.3578,0.3311,0.3184,0.4203,0.3578,0.3578,0.3578,0.4282,0.5084,0.5802,0.5667,0.5473,0.5514,0.5331,0.4749,0.4037,0.4116,0.4203,0.3184,0.4037,0.4037,0.4282,0.4513,0.4749,0.4116,0.4825,0.4918,0.4879,0.4918,0.4825,0.4245,0.4333,0.4651,0.4879,0.5412,0.5802,0.5126,0.4458,0.5374,0.4600,0.4600,0.4600,0.4600,0.3992,0.4879,0.4282,0.4333,0.3668,0.3005,0.3096,0.3847,0.3939,0.3630,0.3359,0.2292,0.2292,0.2748,0.3399,0.2963,0.2963,0.2385,0.2531,0.1805,0.2531,0.2786,0.3456,0.3399,0.3491,0.4037,0.3885,0.3806,0.2748,0.2700,0.2657,0.2963,0.2865,0.2167,0.2080,0.1844,0.2041,0.1602,0.1416,0.2041,0.1958,0.1018,0.0744,0.0677,0.0909,0.0789,0.0723,0.0660,0.1322,0.1532,0.1060,0.1018,0.1060,0.1150,0.0789,0.1266,0.0965,0.1732,0.1766,0.1766,0.1805,0.2820,0.3096,0.2602,0.2080,0.2333,0.2385,0.2385,0.2432,0.1602,0.2122,0.2385,0.2333,0.2558,0.2432,0.2292,0.2209,0.2483,0.2531,0.2432,0.2432,0.2432,0.2432,0.3053,0.3630,0.3578,0.3630,0.3668,0.3263,0.3992,0.4037,0.4556,0.4703,0.5173,0.6219,0.6412,0.7275,0.6984,0.6756,0.7079,0.7192,0.7342,0.7458,0.7501,0.7540,0.7605,0.7605,0.7342,0.7912,0.7951,0.8036,0.8074,0.8074,0.8118,0.7951,0.8118,0.8242,0.8488,0.8650,0.8488,0.8311,0.8424,0.7912,0.7951,0.8001,0.8001,0.7458,0.7192,0.6984,0.6412,0.6516,0.5900,0.5802,0.5802,0.5762,0.5623,0.5374,0.4556,0.4556,0.4333,0.3762,0.3456,0.4037,0.3311,0.3263,0.3311,0.3717,0.3762,0.3717,0.3668,0.3491,0.4203,0.4037,0.4149,0.4037,0.3992,0.4078,0.4651,0.4967,0.5229,0.5802,0.5802,0.5846,0.6293,0.6412,0.6374,0.6604,0.7317,0.7034,0.7573,0.7573,0.7573,0.7772,0.7605,0.8036,0.7951,0.7817,0.7869,0.7724,0.7869,0.7869,0.7951,0.7644,0.7912,0.7275,0.7342,0.7275,0.6984,0.7342,0.7605,0.7418,0.7418,0.7275,0.7573,0.7724,0.8118,0.8521,0.8823,0.8984,0.9119,0.9316,0.9512)

yy <- runmed(y, 41)
plot(y, type="l", ylim=c(0,1), ylab="", xlab="", lwd=0.5)
points(yy, col="blue", type="l", lwd=2)

推荐答案

编辑：函数根据需要将区域剥离为只包含最低部分。

实际上，使用平均数比使用中位数容易。这使您可以找到实际值持续低于平均值的区域。中值不够平滑，无法轻松应用。

执行此操作的一个示例函数为：

FindLowRegion <- function(x,n=length(x)/4,tol=length(x)/20,p=0.5){
    nx <- length(x)
    n <-  2*(n %/% 2) + 1
    # smooth out based on means
    sx <- rowMeans(embed(c(rep(NA,n/2),x,rep(NA,n/2)),n),na.rm=T)
    # find which series are far from the mean
    rlesx <- rle((sx-x)>0)
    # construct start and end of regions
    int <- embed(cumsum(c(1,rlesx$lengths)),2)
    # which regions fulfill requirements
    id <- rlesx$value & rlesx$length > tol
    # Cut regions to be in general smaller than median
    regions <-
    apply(int[id,],1,function(i){
        i <- min(i):max(i)
        tmp <- x[i]
        id <- which(tmp < quantile(tmp,p))
        id <- min(id):max(id)
        i[id]            
    })
    # return
    unlist(regions)
}

其中

• n确定使用多少值来计算移动平均值
• tol确定连续多少个值应该低于谈论低区域的运行平均值，
• p确定用于将区域剥离到其最低部分的截止值(作为分位数)。当p=1时，将显示完整的下部区域。

函数进行了调整以处理您所显示的数据，但是可能需要稍微调整数字以处理其他数据。

此函数返回一组索引，用于查找低区域。用y矢量进行说明：

Lows <- FindLowRegion(y)

newx <- seq_along(y)
newy <- ifelse(newx %in% Lows,y,NA)
plot(y, col="blue", type="l", lwd=2)
lines(newx,newy,col="red",lwd="3")

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