Batch resizing images

Using the `imager` package to resize multiple images in a batch

This was done for a set of field trip photos to be posted to my university's online class system. They were taken with a DSLR that produced 24 megapixel images (6000 × 4000), with file sizes about 6 megabytes. Rather than upload many large images, the following code was used to make smaller (1200 × 800) copies of each image which were written to new files to be uploaded.

First we load the imager package:

library(imager)

We then store the directory the images are stored in as a string, and create a vector of filenames in that directory:

idir <- "C:/Users/00028958/OneDrive - The University of Western Australia/My Documents/aaTeaching/ENVT3361 EnvAssess/Field ENVT3361/AFR-photos-2024/"
(flist <- list.files(idir))

##  [1] "DSC_0873.JPG" "DSC_0874.JPG" "DSC_0875.JPG" "DSC_0876.JPG" "DSC_0877.JPG" "DSC_0878.JPG"
##  [7] "DSC_0879.JPG" "DSC_0880.JPG" "DSC_0881.JPG" "DSC_0884.JPG" "DSC_0887.JPG" "DSC_0888.JPG"
## [13] "DSC_0889.JPG" "DSC_0890.JPG" "DSC_0891.JPG" "DSC_0892.JPG" "DSC_0893.JPG" "DSC_0894.JPG"
## [19] "DSC_0895.JPG" "DSC_0896.JPG" "DSC_0897.JPG" "DSC_0898.JPG" "DSC_0899.JPG" "DSC_0900.JPG"
## [25] "DSC_0901.JPG" "DSC_0902.JPG" "DSC_0903.JPG" "DSC_0904.JPG" "DSC_0905.JPG" "DSC_0906.JPG"
## [31] "DSC_0907.JPG" "DSC_0908.JPG" "DSC_0909.JPG" "DSC_0910.JPG" "DSC_0911.JPG" "DSC_0912.JPG"
## [37] "DSC_0913.JPG" "DSC_0914.JPG" "DSC_0915.JPG" "DSC_0916.JPG" "DSC_0917.JPG" "DSC_0918.JPG"
## [43] "DSC_0919.JPG" "DSC_0920.JPG" "DSC_0921.JPG" "DSC_0922.JPG" "DSC_0923.JPG" "DSC_0924.JPG"
## [49] "DSC_0925.JPG" "DSC_0926.JPG" "DSC_0927.JPG" "DSC_0928.JPG" "DSC_0929.JPG" "DSC_0930.JPG"
## [55] "DSC_0931.JPG" "DSC_0932.JPG" "DSC_0933.JPG" "DSC_0934.JPG" "DSC_0935.JPG" "DSC_0936.JPG"
## [61] "DSC_0937.JPG" "DSC_0938.JPG" "DSC_0939.JPG" "DSC_0940.JPG" "DSC_0941.JPG" "DSC_0942.JPG"
## [67] "DSC_0943.JPG" "DSC_0944.JPG" "DSC_0945.JPG" "DSC_0946.JPG" "DSC_0947.JPG" "DSC_0948.JPG"
## [73] "DSC_0949.JPG" "DSC_0950.JPG" "DSC_0951.JPG" "DSC_0952.JPG" "DSC_0953.JPG" "DSC_0954.JPG"
## [79] "DSC_0955.JPG" "DSC_0956.JPG" "DSC_0957.JPG" "DSC_0958.JPG" "DSC_0959.JPG" "DSC_0960.JPG"
## [85] "DSC_0961.JPG" "DSC_0962.JPG" "DSC_0963.JPG" "DSC_0964.JPG" "DSC_0965.JPG" "DSC_0966.JPG"
## [91] "DSC_0967.JPG" "DSC_0968.JPG"

We then use a simple loop to sequentially

load each large image using imager::load.image()
reduce the size of each loaded image to 20% of the original using imager::imresize() with the argument scale=0.2
write a new file to a new directory, using imager::save.image(), adding the string _s to the original file name (i.e. '_s' for 'small').
Note that we specify .jpg as the file extension which prompts the save.image() function to save as a JPEG file; this is done as the extension seems to be case-sensitive.

This process may take some time! – but it's better than resizing each file manually.

dir.create(paste0(idir,"resized/"))
for(i in 1:length(flist)){
  pic <- load.image(paste0(idir,flist[i]))
  pic <- imresize(pic, scale=0.2)
  save.image(pic,
             file = paste0(idir,"resized/",substr(flist[i],0,8),"_s",".jpg"))
}

Finally we can see that the new files have been written to the new subdirectory:

list.files(paste0(idir,"resized/"))

##  [1] "DSC_0873_s.jpg" "DSC_0874_s.jpg" "DSC_0875_s.jpg" "DSC_0876_s.jpg" "DSC_0877_s.jpg"
##  [6] "DSC_0878_s.jpg" "DSC_0879_s.jpg" "DSC_0880_s.jpg" "DSC_0881_s.jpg" "DSC_0884_s.jpg"
## [11] "DSC_0887_s.jpg" "DSC_0888_s.jpg" "DSC_0889_s.jpg" "DSC_0890_s.jpg" "DSC_0891_s.jpg"
## [16] "DSC_0892_s.jpg" "DSC_0893_s.jpg" "DSC_0894_s.jpg" "DSC_0895_s.jpg" "DSC_0896_s.jpg"
## [21] "DSC_0897_s.jpg" "DSC_0898_s.jpg" "DSC_0899_s.jpg" "DSC_0900_s.jpg" "DSC_0901_s.jpg"
## [26] "DSC_0902_s.jpg" "DSC_0903_s.jpg" "DSC_0904_s.jpg" "DSC_0905_s.jpg" "DSC_0906_s.jpg"
## [31] "DSC_0907_s.jpg" "DSC_0908_s.jpg" "DSC_0909_s.jpg" "DSC_0910_s.jpg" "DSC_0911_s.jpg"
## [36] "DSC_0912_s.jpg" "DSC_0913_s.jpg" "DSC_0914_s.jpg" "DSC_0915_s.jpg" "DSC_0916_s.jpg"
## [41] "DSC_0917_s.jpg" "DSC_0918_s.jpg" "DSC_0919_s.jpg" "DSC_0920_s.jpg" "DSC_0921_s.jpg"
## [46] "DSC_0922_s.jpg" "DSC_0923_s.jpg" "DSC_0924_s.jpg" "DSC_0925_s.jpg" "DSC_0926_s.jpg"
## [51] "DSC_0927_s.jpg" "DSC_0928_s.jpg" "DSC_0929_s.jpg" "DSC_0930_s.jpg" "DSC_0931_s.jpg"
## [56] "DSC_0932_s.jpg" "DSC_0933_s.jpg" "DSC_0934_s.jpg" "DSC_0935_s.jpg" "DSC_0936_s.jpg"
## [61] "DSC_0937_s.jpg" "DSC_0938_s.jpg" "DSC_0939_s.jpg" "DSC_0940_s.jpg" "DSC_0941_s.jpg"
## [66] "DSC_0942_s.jpg" "DSC_0943_s.jpg" "DSC_0944_s.jpg" "DSC_0945_s.jpg" "DSC_0946_s.jpg"
## [71] "DSC_0947_s.jpg" "DSC_0948_s.jpg" "DSC_0949_s.jpg" "DSC_0950_s.jpg" "DSC_0951_s.jpg"
## [76] "DSC_0952_s.jpg" "DSC_0953_s.jpg" "DSC_0954_s.jpg" "DSC_0955_s.jpg" "DSC_0956_s.jpg"
## [81] "DSC_0957_s.jpg" "DSC_0958_s.jpg" "DSC_0959_s.jpg" "DSC_0960_s.jpg" "DSC_0961_s.jpg"
## [86] "DSC_0962_s.jpg" "DSC_0963_s.jpg" "DSC_0964_s.jpg" "DSC_0965_s.jpg" "DSC_0966_s.jpg"
## [91] "DSC_0967_s.jpg" "DSC_0968_s.jpg"

Here's one of the resized photos:

student wearing chest waders sampling water from a seasonal wetland pond

Batch resizing images

Using the imager package to resize multiple images in a batch

Using the `imager` package to resize multiple images in a batch