ACES NPM round about calculation question

[eyeforcolor]

Reading through the google colab explanation of "how to derive the primaries and white point from an ACES Input Device Transform (IDT) matrix" I see the following code to calculate the NPM

NPM = matrix_RGB_to_RGB_to_normalised_primary_matrix( np.linalg.inv(M_IDT), colour.RGB_COLOURSPACES['ACES2065-1'].matrix_XYZ_to_RGB)

Wouldn't you get similar matrix by doing the below?
NPM = np.dot(colour.RGB_COLOURSPACES['ACES2065-1'].matrix_RGB_to_XYZ, M_IDT)

I checked the output and they do give exact same result, just wanted to understand if there is something I am missing when using the method from google colab.

Please let me know!

[KelSolaar]

Hello @eyeforcolor,

I don't have the document front of me so I'm not sure what the matrix_RGB_to_RGB_to_normalised_primary_matrix definition does, it is not a colour definition.

Maybe @aforsythe or @sdyer can help here!

Cheers,

Thomas

[nick-shaw]

Don't know about a colab, but there is a RGB_to_RGB_matrix_to_normalised_primary_matrix() function defined in this post.

@KelSolaar, you may want to fix whatever has gone wrong with the equation formatting in that post.

Or rather, the equations are ok. It's the inline maths that is coming out as unrendered LaTeX.

[eyeforcolor]

Apologies, here is the google collab link that has the code below.

def matrix_RGB_to_RGB_to_normalised_primary_matrix(matrix_RGB_to_RGB,
                                                   matrix_XYZ_to_RGB):
    M = np.dot(matrix_RGB_to_RGB, matrix_XYZ_to_RGB)
    M = np.linalg.inv(M)
    return M

This is linked from colour-science.org and is titled "colour_how-to-guide.ipynb".

[KevinW1]

I was looking at the same code recently and got matching results using your method as well, doing something similar to the following:

    CAM_TO_ACES, RGB_w = colour.matrix_idt(sensitivities, illuminant)

    # converts the rgb-to-rgb IDT into a rgb-to-xyz matrix
    # we have cam to aces, and aces to xyz, so we create cam to xyz
    ACES_TO_XYZ = colour.RGB_COLOURSPACES["ACES2065-1"].matrix_RGB_to_XYZ
    CAM_TO_XYZ = np.matmul(ACES_TO_XYZ, CAM_TO_ACES)  # NPM for cam

(fyi I am not a colour expert)