Direct conversion from YCbCr to CIE L * a * b *
You need to go through every step. YCbCr is often encoded over the video range (16-235 / 240 for 8 bits) and needs to be converted to XYZ using a specific definition of RGB video space (i.e. Rec709 for High Def) that includes canceling non-channel-RGB linearity, and then multiplying by the primary matrix RGB-> XYZ. Then you need to put a white point (usually D65, that is, in the definition of RGB space), apply another nonlinearity, and then another matrix to create L * a * b *. I doubt that much efficiency can be achieved by combining it all into one transformation.
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