Numpy dot product

Question

I am having an image described with a Numpy array, i.e. each pixel is an array [r,g,b]. Now, I need to transform it to YUV using matrix multiplication and trying not to use loops.

self.yuv=self.rgb

self.yuv=dot([[   0.299,  0.587,    0.114  ],

              [-0.14713, -0.28886,  0.436  ],

              [   0.615, -0.51499, -0.10001]], 

             self.yuv[:,:])

I get the error - objects not aligned. I guess that's because of self. yuv[i,j] is not a perpendicular vector. 

Answer 1

Please be informed that your matrix has shape (3, 3) while your figure has shape (rows, cols, 3) and np.dot does "a sum-product over the last axis of a and the second-to-last of b."

You just need to reverse the order of the operands inside np.dot and then perform the transpose your conversion matrix:

rgb2yuv = np.array([[0.299, 0.587, 0.114],

                    [-0.14713, -0.28886, 0.436],

                    [0.615, -0.51499, -0.10001]])

self.yuv = np.dot(self.rgb, rgb2yuv.T)

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