R Under development (unstable) (2024-07-13 r86895 ucrt) -- "Unsuffered Consequences"
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> suppressPackageStartupMessages(library(stars))
> m = matrix(1:121, 11, 11)
> m
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11]
 [1,]    1   12   23   34   45   56   67   78   89   100   111
 [2,]    2   13   24   35   46   57   68   79   90   101   112
 [3,]    3   14   25   36   47   58   69   80   91   102   113
 [4,]    4   15   26   37   48   59   70   81   92   103   114
 [5,]    5   16   27   38   49   60   71   82   93   104   115
 [6,]    6   17   28   39   50   61   72   83   94   105   116
 [7,]    7   18   29   40   51   62   73   84   95   106   117
 [8,]    8   19   30   41   52   63   74   85   96   107   118
 [9,]    9   20   31   42   53   64   75   86   97   108   119
[10,]   10   21   32   43   54   65   76   87   98   109   120
[11,]   11   22   33   44   55   66   77   88   99   110   121
> s = st_as_stars(m)
> st_downsample(s, 1)
stars object with 2 dimensions and 1 attribute
attribute(s):
    Min. 1st Qu. Median Mean 3rd Qu. Max.
A1     1    28.5     61   61    93.5  121
dimension(s):
   from to offset delta point x/y
X1    1  6      0     2 FALSE [x]
X2    1  6      0     2 FALSE [y]
> st_downsample(s, 1)[[1]]
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1   23   45   67   89  111
[2,]    3   25   47   69   91  113
[3,]    5   27   49   71   93  115
[4,]    7   29   51   73   95  117
[5,]    9   31   53   75   97  119
[6,]   11   33   55   77   99  121
> st_downsample(s, 1, offset = 1)
stars object with 2 dimensions and 1 attribute
attribute(s):
    Min. 1st Qu. Median Mean 3rd Qu. Max.
A1    13      37     61   61      85  109
dimension(s):
   from to offset delta point x/y
X1    1  5      1     2 FALSE [x]
X2    1  5      1     2 FALSE [y]
> st_downsample(s, 1, offset = 1)[[1]]
     [,1] [,2] [,3] [,4] [,5]
[1,]   13   35   57   79  101
[2,]   15   37   59   81  103
[3,]   17   39   61   83  105
[4,]   19   41   63   85  107
[5,]   21   43   65   87  109
> st_downsample(s, 1, FUN = mean)
stars object with 2 dimensions and 1 attribute
attribute(s):
    Min. 1st Qu. Median Mean 3rd Qu. Max.
A1     7      31     55   55      79  103
dimension(s):
   from to offset delta point x/y
X1    1  5      0     2 FALSE [x]
X2    1  5      0     2 FALSE [y]
> st_downsample(s, 1, FUN = mean)[[1]]
     [,1] [,2] [,3] [,4] [,5]
[1,]    7   29   51   73   95
[2,]    9   31   53   75   97
[3,]   11   33   55   77   99
[4,]   13   35   57   79  101
[5,]   15   37   59   81  103
> st_downsample(s, 1, offset = 1, FUN = mean)
stars object with 2 dimensions and 1 attribute
attribute(s):
    Min. 1st Qu. Median Mean 3rd Qu. Max.
A1    19      43     67   67      91  115
dimension(s):
   from to offset delta point x/y
X1    1  5      1     2 FALSE [x]
X2    1  5      1     2 FALSE [y]
> st_downsample(s, 1, offset = c(0,1), FUN = mean)[[1]]
     [,1] [,2] [,3] [,4] [,5]
[1,]   18   40   62   84  106
[2,]   20   42   64   86  108
[3,]   22   44   66   88  110
[4,]   24   46   68   90  112
[5,]   26   48   70   92  114
> st_downsample(s, 1, offset = c(0,1))
stars object with 2 dimensions and 1 attribute
attribute(s):
    Min. 1st Qu. Median Mean 3rd Qu. Max.
A1    12    36.5     61   61    85.5  110
dimension(s):
   from to offset delta point x/y
X1    1  6      0     2 FALSE [x]
X2    1  5      1     2 FALSE [y]
> st_downsample(s, 1, offset = c(0,1))[[1]]
     [,1] [,2] [,3] [,4] [,5]
[1,]   12   34   56   78  100
[2,]   14   36   58   80  102
[3,]   16   38   60   82  104
[4,]   18   40   62   84  106
[5,]   20   42   64   86  108
[6,]   22   44   66   88  110
> 
> proc.time()
   user  system elapsed 
   0.59    0.09    0.68