Question

我有两个光栅图像。一个具有值（img）和其他分段索引（分段）：

library(raster)

img=raster(t(matrix(c(1,3,1,2,1,11,11,10,NA,NA,2,12,13,14,2,3,1,2,2,1,2,13,NA,12,0,1,2,20,21,2,3,12,13,14,3,2,21,22,21,1,1,2,3,2,2,1,1,NA,NA,1,NA,NA,13,14,NA,NA,27,6,6,5,NA,NA,NA,12,22,28,7,8,5,6,NA,NA,23,24,22,5,NA,NA,5,8,1,2,1,1,2,5,NA,NA,NA,NA,1,2,1,2,2,7,6,5,NA,NA),10,10)))

segmentation=raster(t(matrix(c(1,1,1,1,1,4,4,4,4,4,1,7,7,7,1,5,5,5,5,5,1,7,7,7,1,5,5,6,6,5,1,7,7,7,1,5,6,6,6,5,1,1,1,1,1,5,5,5,5,5,9,9,9,8,8,3,3,3,3,3,9,9,9,8,8,3,3,3,3,3,8,8,8,8,8,3,3,3,3,3,2,2,2,2,2,3,3,3,10,10,2,2,2,2,2,3,3,3,10,10),10,10)))

我需要获取一个向量列表，其中列表的每一行代表包含NA值的段索引，每个向量包含图像值。

我能够使用for循环来完成它。但是，这使得我在使用更大的图像时处理速度非常慢。有一种方法可以在没有for循环或更优化的方法的情况下执行此操作吗？

segNumber = length(freq(segmentation)[,1]) #obtain the number of segments

NAPixels <- which(is.na(img[])) #pixels that are NA in img

segsWithNA <- vector() #initializing

segsWithNA <- unique(segmentation[NAPixels]) #segmentation index that contains NA

listOfSegmentValues <- list() #initializing

for (i in 1:length(segsWithNA)){ #For each segment that contains NA
    listOfSegmentValues[[i]] = which(segmentation[] == segsWithNA[i])
}

Answer 1

这是一个选项。一切都可以矢量化。不需要for-loop。

library(raster)

# Get the value as a vector, test if the value is NA
NA_value <- is.na(values(img))

# Label the vector with the segment number
names(NA_value) <- values(segmentation)

# Show segment number and the indices of NA
NA_which <- which(NA_value)

NA_which
# 4   4   7   5   5   9   9   8   3   9   9   9   8   8   3   3   3   3  10  10  10  10 
# 9  10  23  48  49  51  52  55  56  61  62  63  71  72  77  78  87  88  89  90  99 100

# Split to a list
split(NA_which, f = names(NA_which))
# $`10`
# 10  10  10  10 
# 89  90  99 100 
# 
# $`3`
# 3  3  3  3  3 
# 56 77 78 87 88 
# 
# $`4`
# 4  4 
# 9 10 
# 
# $`5`
# 5  5 
# 48 49 
# 
# $`7`
# 7 
# 23 
# 
# $`8`
# 8  8  8 
# 55 71 72 
# 
# $`9`
# 9  9  9  9  9 
# 51 52 61 62 63

从索引矢量和光栅图像创建矢量列表

1 个答案: