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Grayscale images were calculated from truecolour RGB images (Fig. 2) with weighting factors of [0.2989, 0.5870, 0.1140] for [R, G, B] channels respectively and then gamma compressed, as described by Equation (A.1) of the Supplementary Information. The low MSE (<0.1%) and high SSIM metrics (>95%) indicate good agreement between theoretical predictions and experiment data.