Color Modulating Conditions for Invisible Information Hiding in 60 Hz Video by Use of Temporal Color Fusion of Human Visual System

When two colors are presented alternately on a display at 60 Hz, humans perceive their temporally fused color while a camera captures each color. This is the principle of the color modulation-based hidden image embedding technique. In this technique, a color pair that is alternately displayed at 60 Hz is generated to be point-symmetrical with respect to the original color in a color space while keeping luminance constant. When a linear color space such as the CIE XYZ or CIE xyY color space is employed, no color distortion occurs. However, complex calculations are required for each pixel to obtain a modulation amount that suppresses flicker. If a perceptually uniform color space such as the CIELAB, CIELUV, or ITP color space is employed and its Euclidean color difference formulas are used as a parameter for the amount of modulation, the generation of color pairs becomes easier. In order to find the appropriate color modulation amount to prevent flicker, subjective evaluation experiments were carried out for various perceptually uniform color spaces. The experiments showed that flicker is imperceptible or not annoying if the color differences between the original and each color in the color pair are less than 20 in the respective color difference formula. The temporally fused color shifts from the original color due to the use of a nonlinear color space, and the amount of shift increases with the modulation amount. However, it is suggested that this shift is perceptible but not annoying under modulation conditions where flicker is imperceptible or not annoying.