Apply transformation matrix to pixels in OpenCV image

Question

I want to change the color basis of an image from RGB to something else. I have a matrix M that I want to apply to each pixel's RGB, which we can define as x_ij.

I am currently iterating over each pixel of the NumPy image and calculating Mx_ij manually. I can't even vectorize it over the rows, because the RGB is a 1x3 instead of a 3x1 array.

Is there a better way to do this? Maybe a function in OpenCV or NumPy?

http://docs.opencv.org/modules/imgproc/doc/miscellaneous_transformations.html#cv2.cvtColor — berak, Feb 27 '14 at 22:15
@berak I want to do a custom transform, not one of the standard ones with cvtColor — wennho, Feb 27 '14 at 22:27

score 2 · Accepted Answer · answered Feb 27 '14 at 22:16

Can't remember the canonical way to do this (possibly avoiding the transposes) but this should work:

import numpy as np

M = np.random.random_sample((3, 3))

rgb = np.random.random_sample((5, 4, 3))

slow_result = np.zeros_like(rgb)
for i in range(rgb.shape[0]):
    for j in range(rgb.shape[1]):
        slow_result[i, j, :] = np.dot(M, rgb[i, j, :])

# faster method
rgb_reshaped = rgb.reshape((rgb.shape[0] * rgb.shape[1], rgb.shape[2]))
result = np.dot(M, rgb_reshaped.T).T.reshape(rgb.shape)

print np.allclose(slow_result, result)

If it's a transformation between standard colorspaces then you should use Scikit Image:

http://scikit-image.org/docs/dev/api/skimage.color.html

Apply transformation matrix to pixels in OpenCV image

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