Determine file image size in R

image_dimensions.R

jpeg_dimensions <- function(filename, bytes = 1024) {
  bytes <- readBin(filename, "raw", n = bytes)
  
  if (length(bytes) < 2 || bytes[[1]] != 0xFF || bytes[[2]] != 0xD8) {
    stop("Couldn't decode jpeg")
  }
  
  ff <- which(bytes == 0xFF)
  c0 <- which(bytes == 0xC0)
  
  sof0 <- head(ff[ff %in% (c0 - 1)], 1)
  
  if (length(sof0) == 0) {
    stop("Couldn't decode jpeg")
  }
  if (length(bytes) < sof0 + 9) {
    stop("Couldn't decode jpeg")
  }
  height_bytes <- bytes[(sof0 + 5):(sof0 + 6)]
  width_bytes <- bytes[(sof0 + 7):(sof0 + 8)]
  
  list(
    width = readBin(width_bytes, "integer", 1, 2, endian = "big"),
    height = readBin(height_bytes, "integer", 1, 2, endian = "big")
  )
}

png_dimensions <- function(filename) {
  bytes <- readBin(filename, "raw", n = 24)
  
  if (length(bytes) < 24) {
    stop("Couldn't decode png")
  }
  
  png_signature <- as.raw(c(0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A))
  
  if (!identical(bytes[1:8], png_signature)) {
    stop("Couldn't decode png")
  }
  if (!identical(bytes[13:16], charToRaw("IHDR"))) {
    stop("Couldn't decode png")
  }
  
  width_bytes <- bytes[17:20]
  height_bytes <- bytes[21:24]
  
  list(
    width = readBin(width_bytes, "integer", 1, 4, endian = "big"),
    height = readBin(height_bytes, "integer", 1, 4, endian = "big")
  )
}

gif_dimensions <- function(filename) {
  bytes <- readBin(filename, "raw", n = 10)
  
  if (length(bytes) < 10) {
    stop("Couldn't decode gif")
  }
  
  if (!identical(bytes[1:6], charToRaw("GIF89a")) && !identical(bytes[1:6], charToRaw("GIF87a"))) {
    stop("Couldn't decode gif")
  }
  
  width_bytes <- bytes[7:8]
  height_bytes <- bytes[9:10]
 
  list(
    width = readBin(width_bytes, "integer", 1, 2, endian = "little"),
    height = readBin(height_bytes, "integer", 1, 2, endian = "little")
  )
}