Error indexing a NumPy image array in Python

Question

I've written a program using Python 2.7, Numpy and OpenCV to grab a photo from my webcam and give the rgb value of every pixel. After running the code on a 640x480 pixel photo:

for x in range(638):
    for y in range(478):
        red, green, blue = image[x, y]
        print(red, green, blue)

I get the error message:

red, green, blue = image[x, y]
IndexError: index 480 is out of bounds for axis 0 with size 480

Does anybody know why this is?

it's [y,x] in numpy and opencv. also, please, never iterate over pixels like that, it's horrible slow, error-prone, and totally defeats the purpose of a high-level library — berak
– berak, Commented Apr 18, 2015 at 9:03

YXD · Accepted Answer · 2015-04-18 13:27:26Z

1

The short answer is a 640 x 480 image has shape (480, 640, n_channels). If you change your code to image[y, x] you will not get this error. It might be easier to understand if you write the code as:

for row in range(image.shape[0]):
    for col in range(image.shape[1]):
      r, g, b = image[row, col]

Here's a tutorial on indexing image data which shows you how to do a few operations efficiently and has some details on indexing conventions.

answered Apr 18, 2015 at 13:27

YXD

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