VOC2012: PIL Image.open converts PNG to 2d array

I am working with VOC2012 dataset. The input image is in PNG format which has a shape of (375, 500, 4) when I use imageio to open the image. When I use PIL to open the image, then suddenly the shape becomes (500, 375). PNG images should have four dimensions on the last axis: r g b & alpha.

The image is obviously colored image, so it should have 3 dimensions (height, width, depth). PIL seems to suggest that it only has two dimensions: width & height.

Can PNG images be represented by a 2d array? Please help! So lost at the moment. Thanks!

from PIL import Image
from keras.preprocessing.image import img_to_array
import os, imageio
import numpy as np

root_path = '/Users/johnson/Downloads/'

imageio_img = imageio.imread(
    os.path.join(root_path, '2009_003193.png')
)

# (375, 500, 4)
print(imageio_img.shape)
# [  0 128 192 224 255]
print(np.unique(imageio_img))


PIL_img = Image.open(
    os.path.join(root_path, '2009_003193.png')
)

# (500, 375)
print(PIL_img.size)

PIL_img_to_array = img_to_array(PIL_img)
# (375, 500, 1)
print(PIL_img_to_array.shape)
# [  0.   2. 255.]
print(np.unique(PIL_img_to_array))

It's also quite magical that PIL seems to know how VOC2012 labels the data. PIL_image_to_array has a unique value of [0, 2, 255]. Conveniently, 2 denotes bicycle in VOC2012. 0 means background and 255 probably means the yellowish boundary around the bicycle. But from the first code snippet, I never passed the pascal classes to PIL for conversion.

def pascal_classes():
  classes = {'aeroplane' : 1,  'bicycle'   : 2,  'bird'        : 3,  'boat'         : 4,
             'bottle'    : 5,  'bus'       : 6,  'car'         : 7,  'cat'          : 8,
             'chair'     : 9,  'cow'       : 10, 'diningtable' : 11, 'dog'          : 12,
             'horse'     : 13, 'motorbike' : 14, 'person'      : 15, 'potted-plant' : 16,
             'sheep'     : 17, 'sofa'      : 18, 'train'       : 19, 'tv/monitor'   : 20}

  return classes

def pascal_palette():
  palette = {(  0,   0,   0) : 0 ,
             (128,   0,   0) : 1 ,
             (  0, 128,   0) : 2 ,
             (128, 128,   0) : 3 ,
             (  0,   0, 128) : 4 ,
             (128,   0, 128) : 5 ,
             (  0, 128, 128) : 6 ,
             (128, 128, 128) : 7 ,
             ( 64,   0,   0) : 8 ,
             (192,   0,   0) : 9 ,
             ( 64, 128,   0) : 10,
             (192, 128,   0) : 11,
             ( 64,   0, 128) : 12,
             (192,   0, 128) : 13,
             ( 64, 128, 128) : 14,
             (192, 128, 128) : 15,
             (  0,  64,   0) : 16,
             (128,  64,   0) : 17,
             (  0, 192,   0) : 18,
             (128, 192,   0) : 19,
             (  0,  64, 128) : 20 }

Your image is palletised, not RGB. Each pixel is represented by an 8-bit index into a palette. You can see this by looking at image.mode which shows up as P.

If you want an RGB image, use:

rgb = Image.open('bike.png').convert('RGB')

If you want and RGBA image with transparency, use:

RGBA = Image.open('bike.png').convert('RGBA')

However, there is no useful information in the alpha channel, so that seems pointless.

Regarding the pascal palette, you can get that via PIL like this:

im = Image.open('bike.png')                                                                 

p = im.getpalette()

for i in range (256): 
    print(p[3*i:3*i+3])

[0, 0, 0]
[128, 0, 0]
[0, 128, 0]
[128, 128, 0]
[0, 0, 128]
[128, 0, 128]
[0, 128, 128]
[128, 128, 128]
[64, 0, 0]
[192, 0, 0]
[64, 128, 0]
[192, 128, 0]
[64, 0, 128]
[192, 0, 128]
[64, 128, 128]
[192, 128, 128]
[0, 64, 0]
[128, 64, 0]
[0, 192, 0]
[128, 192, 0]
[0, 64, 128]
[128, 64, 128]
[0, 192, 128]
[128, 192, 128]
[64, 64, 0]
[192, 64, 0]
[64, 192, 0]
[192, 192, 0]
[64, 64, 128]
[192, 64, 128]
[64, 192, 128]
[192, 192, 128]
[0, 0, 64]
[128, 0, 64]
[0, 128, 64]
[128, 128, 64]
[0, 0, 192]
[128, 0, 192]
[0, 128, 192]
[128, 128, 192]
[64, 0, 64]
[192, 0, 64]
[64, 128, 64]
[192, 128, 64]
[64, 0, 192]
[192, 0, 192]
[64, 128, 192]
[192, 128, 192]
[0, 64, 64]
[128, 64, 64]
[0, 192, 64]
[128, 192, 64]
[0, 64, 192]
[128, 64, 192]
[0, 192, 192]
[128, 192, 192]
[64, 64, 64]
[192, 64, 64]
[64, 192, 64]
[192, 192, 64]
[64, 64, 192]
[192, 64, 192]
[64, 192, 192]
[192, 192, 192]
[32, 0, 0]
[160, 0, 0]
[32, 128, 0]
[160, 128, 0]
[32, 0, 128]
[160, 0, 128]
[32, 128, 128]
[160, 128, 128]
[96, 0, 0]
[224, 0, 0]
[96, 128, 0]
[224, 128, 0]
[96, 0, 128]
[224, 0, 128]
[96, 128, 128]
[224, 128, 128]
[32, 64, 0]
[160, 64, 0]
[32, 192, 0]
[160, 192, 0]
[32, 64, 128]
[160, 64, 128]
[32, 192, 128]
[160, 192, 128]
[96, 64, 0]
[224, 64, 0]
[96, 192, 0]
[224, 192, 0]
[96, 64, 128]
[224, 64, 128]
[96, 192, 128]
[224, 192, 128]
[32, 0, 64]
[160, 0, 64]
[32, 128, 64]
[160, 128, 64]
[32, 0, 192]
[160, 0, 192]
[32, 128, 192]
[160, 128, 192]
[96, 0, 64]
[224, 0, 64]
[96, 128, 64]
[224, 128, 64]
[96, 0, 192]
[224, 0, 192]
[96, 128, 192]
[224, 128, 192]
[32, 64, 64]
[160, 64, 64]
[32, 192, 64]
[160, 192, 64]
[32, 64, 192]
[160, 64, 192]
[32, 192, 192]
[160, 192, 192]
[96, 64, 64]
[224, 64, 64]
[96, 192, 64]
[224, 192, 64]
[96, 64, 192]
[224, 64, 192]
[96, 192, 192]
[224, 192, 192]
[0, 32, 0]
[128, 32, 0]
[0, 160, 0]
[128, 160, 0]
[0, 32, 128]
[128, 32, 128]
[0, 160, 128]
[128, 160, 128]
[64, 32, 0]
[192, 32, 0]
[64, 160, 0]
[192, 160, 0]
[64, 32, 128]
[192, 32, 128]
[64, 160, 128]
[192, 160, 128]
[0, 96, 0]
[128, 96, 0]
[0, 224, 0]
[128, 224, 0]
[0, 96, 128]
[128, 96, 128]
[0, 224, 128]
[128, 224, 128]
[64, 96, 0]
[192, 96, 0]
[64, 224, 0]
[192, 224, 0]
[64, 96, 128]
[192, 96, 128]
[64, 224, 128]
[192, 224, 128]
[0, 32, 64]
[128, 32, 64]
[0, 160, 64]
[128, 160, 64]
[0, 32, 192]
[128, 32, 192]
[0, 160, 192]
[128, 160, 192]
[64, 32, 64]
[192, 32, 64]
[64, 160, 64]
[192, 160, 64]
[64, 32, 192]
[192, 32, 192]
[64, 160, 192]
[192, 160, 192]
[0, 96, 64]
[128, 96, 64]
[0, 224, 64]
[128, 224, 64]
[0, 96, 192]
[128, 96, 192]
[0, 224, 192]
[128, 224, 192]
[64, 96, 64]
[192, 96, 64]
[64, 224, 64]
[192, 224, 64]
[64, 96, 192]
[192, 96, 192]
[64, 224, 192]
[192, 224, 192]
[32, 32, 0]
[160, 32, 0]
[32, 160, 0]
[160, 160, 0]
[32, 32, 128]
[160, 32, 128]
[32, 160, 128]
[160, 160, 128]
[96, 32, 0]
[224, 32, 0]
[96, 160, 0]
[224, 160, 0]
[96, 32, 128]
[224, 32, 128]
[96, 160, 128]
[224, 160, 128]
[32, 96, 0]
[160, 96, 0]
[32, 224, 0]
[160, 224, 0]
[32, 96, 128]
[160, 96, 128]
[32, 224, 128]
[160, 224, 128]
[96, 96, 0]
[224, 96, 0]
[96, 224, 0]
[224, 224, 0]
[96, 96, 128]
[224, 96, 128]
[96, 224, 128]
[224, 224, 128]
[32, 32, 64]
[160, 32, 64]
[32, 160, 64]
[160, 160, 64]
[32, 32, 192]
[160, 32, 192]
[32, 160, 192]
[160, 160, 192]
[96, 32, 64]
[224, 32, 64]
[96, 160, 64]
[224, 160, 64]
[96, 32, 192]
[224, 32, 192]
[96, 160, 192]
[224, 160, 192]
[32, 96, 64]
[160, 96, 64]
[32, 224, 64]
[160, 224, 64]
[32, 96, 192]
[160, 96, 192]
[32, 224, 192]
[160, 224, 192]
[96, 96, 64]
[224, 96, 64]
[96, 224, 64]
[224, 224, 64]
[96, 96, 192]
[224, 96, 192]
[96, 224, 192]
[224, 224, 192]

Then, if you want to make the bicycle red, you can do:

# Load the image and make Numpy version
im = Image.open('bike.png') 
n = np.array(im)

# Make all pixels belonging to bike (2) into red (palette index 9)
n[n==2] = 9
# Make all pixels not red (9) into grey (palette index 7)
n[n!=9] = 7

# Convert back into PIL palettised image and re-apply original palette
r = Image.fromarray(n,mode='P') 
r.putpalette(im.getpalette()) 
r.save('result.png')