Implementing cvtColor

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I need to convert RGB to LAB color space and this operation needs to be done parallelly for all pixels. The main issue is that I couldn't find a proper implementation from OpenCV's code. After trying out various other implementations, I couldn't get the expected results as the implementation needs to have the exact values(transformation matrix) as used by cvtColor.

Any way in which I can get the exact values as used by the cvtColor function, even a link to the actual implementation will be very useful. I myself found some implementations e.g here but this doesn't seem to be the exact one as I couldn't reproduce the desired results.