This nim source converts binary of cifar10 to PNG. Some packages are needed, if you use this, please install them for yourself.

cifar10ConvertToPng.nim

import os
import streams
import sequtils
import nimPNG, math, tables, base64
import private.buffer
import progress

const 
  height: int = 32
  width: int = 32

type
  cifarFileNameArray = array[0..5, string]
  
  patchData = object
    id: uint8
    seq_r_values: seq[uint8]
    seq_g_values: seq[uint8]
    seq_b_values: seq[uint8]

  Image = ref object
    data: string
    width, height: int
    colorType: PNGcolorType
    bitDepth: int
    
  patchPngData = object
    id: uint8
    image: Image
  
proc checkCifarExist(cifar_file_array: cifarFileNameArray) : bool = 
  for file_name in cifar_file_array:
    if not existsFile(file_name):
      return false
  return true

proc loadCifar( cifar_file: string ): seq[patchData] =
  echo "|| ", cifar_file, " is being loaded ... "
  result = newSeq[patchData](10000)

  const num_data_in_row = 3073
  const max_row_num = 10000
  var row_idx : int = 0
  var col_idx : int = 0

  var fs = newFileStream(cifar_file, fmRead)
  var id : uint8
  var b = newSeq[uint8](1024)
  var g = newSeq[uint8](1024)
  var r = newSeq[uint8](1024)
  
  if not isNil(fs):
    while not fs.atEnd:
      let value : uint8 = fs.readUint8()
      
      if col_idx == 0:
        id = value
      elif col_idx <= 1024:
        b[col_idx-1] = value
      elif col_idx <= 2048:
        g[col_idx-1-1024] = value
      elif col_idx <= 3072:
        r[col_idx-1-1024-1024] = value
      else:
        echo "|| [ERROR] Ploblem in parsing"
        break

      col_idx = col_idx + 1

      if col_idx == num_data_in_row:
        col_idx = 0
        result[row_idx] = patchData(id: id, seq_b_values: b, seq_g_values: g, seq_r_values: r)
        row_idx = row_idx + 1

    fs.close()
    assert row_idx == max_row_num  

proc encodePNG(data: seq[patchData]) : seq[patchPngData] = 
  result = newSeq[patchPngData](len(data))

  for i in 0..len(data)-1:
    var image: Image
    new(image)
    image.width = width
    image.height = height
    image.colorType = LCT_RGBA
    image.bitDepth = 8
    image.data = newString(width * height * 4)
    for v in 0 .. height-1:
      for u in 0 .. width-1:
        image.data[v*width*4 + u*4 + 0] = chr(data[i].seq_b_values[v*width + u])
        image.data[v*width*4 + u*4 + 1] = chr(data[i].seq_g_values[v*width + u])
        image.data[v*width*4 + u*4 + 2] = chr(data[i].seq_r_values[v*width + u])
        image.data[v*width*4 + u*4 + 3] = chr(255)

    result[i].image = image
    result[i].id = data[i].id
  
# main
echo "||================================= Cifar-10 (Nim) ================================= "
let
  root_path : string = os.getAppDir()
var
  cifar_files: cifarFileNameArray
  seq_patch_data : seq[patchData] # -> nil

seq_patch_data = @[] # necessary initialization
  
cifar_files = [root_path&"/cifar10/data_batch_1.bin",
               root_path&"/cifar10/data_batch_2.bin",
               root_path&"/cifar10/data_batch_3.bin",
               root_path&"/cifar10/data_batch_4.bin",
               root_path&"/cifar10/data_batch_5.bin",
               root_path&"/cifar10/test_batch.bin"]

if not checkCifarExist(cifar_files):
  quit(1)

createDir(root_path & "/png/train")
createDir(root_path & "/png/test")
for i in 0 .. 9:
  createDir(root_path & "/png/train/" & intToStr(i,1))
  createDir(root_path & "/png/test/" & intToStr(i,1))

var train_data = newSeq[patchData](50000)
for i in 1 .. 5:
  train_data[(i-1)*10000 .. (i)*10000 - 1] = loadCifar(cifar_files[i-1])

var test_png_data = encodePNG(loadCifar(cifar_files[len(cifar_files)-1]))
var train_png_data = encodePNG(train_data)
var bar = newProgressBar(total = 60000)
bar.start()
for idx, png in train_png_data:
  discard savePNG32(root_path & "/png/train/" & intToStr(int(png.id)) & "/" & intToStr(idx) & ".png", 
                    png.image.data, png.image.width, png.image.height)
  bar.increment()

for idx, png in test_png_data:
  discard savePNG32(root_path & "/png/test/" & intToStr(int(png.id)) & "/" & intToStr(idx) & ".png", 
                    png.image.data, png.image.width, png.image.height)
  bar.increment()

bar.finish()
echo "|| Converting has been done ... "