python - RGB matrix of an image

Question

Taking an image as input, how can I get the rgb matrix corresponding to it? I checked out the numpy.asarray function. Does that give me the rgb matrix or some other matrix?

Answer 1

Note that this answer is outdated as of 2018; scipy has deprecated imread, and you should switch to imageio.imread. See this transition doc about differences between the two. The code below should work with no changes if you just import the new library in place of the old, but I haven’t tested it.

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The simplest answer is to use the NumPy and SciPy wrappers around PIL. There's a great tutorial, but the basic idea is:

from scipy import misc
arr = misc.imread('lena.png') # 640x480x3 array
arr[20, 30] # 3-vector for a pixel
arr[20, 30, 1] # green value for a pixel

For a 640x480 RGB image, this will give you a 640x480x3 array of uint8.

Or you can just open the file with PIL (or, rather, Pillow; if you're still using PIL, this may not work, or may be very slow) and pass it straight to NumPy:

import numpy as np
from PIL import Image
img = Image.open('lena.png')
arr = np.array(img) # 640x480x4 array
arr[20, 30] # 4-vector, just like above

This will give you a 640x480x4 array of type uint8 (the 4th is alpha; PIL always loads PNG files as RGBA, even if they have no transparency; see img.getbands() if you're every unsure).

If you don't want to use NumPy at all, PIL's own PixelArray type is a more limited array:

arr = img.load()
arr[20, 30] # tuple of 4 ints

This gives you a 640x480 PixelAccess array?of RGBA 4-tuples.

Or you can just call getpixel on the image:

img.getpixel(20, 30) # tuple of 4 ints